Serial Comms and Crystals


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  1. #1
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    Default Serial Comms and Crystals

    Hi Guys and Gals

    Some years ago, with the help of DT and Henrik I worked on a 4 channel thermostat project which I'm now looking at upgrading to incorporate some new sensors and options. One of the problems that I'm still trying to resolve is the serial communications.

    I've used PIC Multi Calc to give me the settings and defines for the serial port settings on both cases. I've also written a PC application in Liberty Basic to read and update the settings, but also tried communications by sending the commands via the terminal in MCS.

    OK - I'm using an 18F4580 with an OSC val of 48 in both cases. Inserting a 12Mhz crystal in the development board with the USART settings to
    Code:
    'DEFINE HSER_RCSTA 90h ' Enable serial port & continuous receive
    'DEFINE HSER_TXSTA 20h ' Enable transmit, BRGH = 0
    'DEFINE HSER_CLROERR 1 ' Clear overflow automatically
    'DEFINE HSER_SPBRG 25  ' 115200 Baud @ 48MHz, 0.16%
    'SPBRGH = 0
    'BAUDCON.3 = 1         ' Enable 16 bit baudrate generator
    Compile and load into the PIC and launch the application, which first checks the comm ports available and then scans for a response from the PIC by sending a "C" to the PIC which then responds with "R" and then on confirmation that it found the controller launches the application and sends "R again which instructs the PIC to send the data stream. This works fine when the 12 Mhz crystal is in place. I can also launch the serial monitor in MCS and replicate receiving the data stream.

    But as I really wanted to use a 20 Mhz crystal as it make other processes run faster which in a PID thermostat is my preference, I get "no controller found" when the PIC is polled from the application. In the serial monitor it registers connection with the com port (I'm using a USB/ Serial module from Mikroelectronika) but again fails to respond to the manual TX codes. Here's the revised USART settings that the multicalc application gives
    Code:
    DEFINE HSER_RCSTA 90h ' Enable serial port & continuous receive
    DEFINE HSER_TXSTA 24h ' Enable transmit, BRGH = 1
    DEFINE HSER_SPBRG 25  ' 115200 Baud @ 0.16%
    DEFINE HSER_CLROERR 1 ' Clear overflow automatically
    
    RCSTA = $90 ' Enable serial port & continuous receive
    TXSTA = $24 ' Enable transmit, BRGH = 1
    SPBRG = 25  ' 115200 Baud @ 0.16%
    Now I'm assuming that the reason it works with the 12 Mhz xtal is that the 48 OSC value is divisible by 4 to equal the crystal value. Where as using the 20 Mhz xtal, the OSC value is not directly divisible. So I changed the OSC value to 40, recalculated the USART settings for a 40 OSC and tried that, but to no avail.

    Any suggestions on how to get this working with a 20 Mhz xtal

  2. #2
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    Default Re: Serial Comms and Crystals

    Hi,
    The fact that you're using a 12MHz x-tal and DEFINE OSC 48 tells me that you're using the PLL in the chip to increase the frequency from 12 to 48MHz. This by itself seems a bit strange since the maximum operating frequency of the 18F4580 is 40MHz so if you really ARE running at 48MHz you're operating the chip outside of it specifications.

    Dropping in a 20MHz x-tal instead of the 12MHz means the PIC is trying to run at 80MHz - twice as fast as it's maximum rated speed.

    If it really does use a 12MHz x-tal with the PLL enabled - thus running at 48MHz - and it's doing it reliably then that is as fast as you're going to get.

    If it's actually running at 12MHz then what you want is to drop in a 10MHz x-tal and enable the PLL. Then you DEFINE OSC 40.

    Please remember that DEFINE OSC never ever changes the speed which the PIC runs at. It only informs the compiler what speed YOU intend to run the PIC at.

    /Henrik.

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    Default Re: Serial Comms and Crystals

    Cheers Henrik,

    10Mhz xtal, OSC set to 40 with HS PPL in the config and it works fine. I thought I was possibly on the right track, but didn't realise that I was in effect overclocking the PIC using the 12 and 20 Mhz crystals.

    Now to look for a cheap I2C humidity sensor. The DTH11 that I was playing with uses pause statements which slow the rest of the code down.

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    Default Re: Serial Comms and Crystals

    Quote Originally Posted by Scampy View Post
    ...

    Now to look for a cheap I2C humidity sensor. The DTH11 that I was playing with uses pause statements which slow the rest of the code down.
    Don't you mean "you" use PAUSE in your code? (I'm not familiar with that sensor)

    You could use DT INT instead to generate an interval.

    Robert

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    Default Re: Serial Comms and Crystals

    The code does include DT's INT file.

    I've found an indepth data sheet which states that each two way communication process should take 4ms, so why it seems that the code runs slow might of been down to the use of the wrong crystals ??

    Basically connection is via a single pin. This pin is kept high by pull up resistor. To get data, the PIC needs to make the pin go low for a minimum of 18ms then hight again and waits 20-40us for the response. The sensor then sends a response by taking the pin low for 80us and then back up high for a further 80us and then sends the data, which is a 40bit stream of high pulses with a duration of 20-28us to represent a 0 or 70us for a 1. Each pulse is preceded by a low pulse of 50us, with a final 50us low at the end of the data stream.

    I've seen examples using pulseIN command, with pause xxUS to trick the sensor into sending data, but don't know if interrupts could be used here ?

    here's the data sheet http://www.micropik.com/PDF/dht11.pdf

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    Default Re: Serial Comms and Crystals


  7. #7
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    Default Re: Serial Comms and Crystals

    You could use an HIH 4030. It outputs a voltage that you read with the ADC - as fast as you like.

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    Default Re: Serial Comms and Crystals

    Quote Originally Posted by richard View Post
    Thanks for the link. It states that it was for a 64mhz clock speed. Any idea what I need to change to make it run at 40 mhz ?

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    Default Re: Serial Comms and Crystals

    this should be close

    Code:
    if ((tmr6 <75)||(tmr6>85))then goto badread
    would become
    Code:
    if ((tmr6 <45)||(tmr6>55))then goto badread
    and
    Code:
    if ((tmr6>20)&&(tmr6<80)) then     ' noise ?
               if (tmr6 >50 ) then
    would become

    Code:
     if ((tmr6>12)&&(tmr6<50 then     ' noise ?
               if (tmr6 >31 ) then
    Last edited by richard; - 24th March 2015 at 10:47. Reason: and added

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    Default Re: Serial Comms and Crystals

    Richard, thanks for the suggestions.

    I'm in the process of editing the code (to reflect the pins I'm using and that I don't have a serial LCD), and came up with an error, presumably as I'm using 2.60 version rather than PBP 3 related to tmr6 and t6con statements. Any ideas

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    Default Re: Serial Comms and Crystals

    pic 18F4580 has no timer6 try using tmr2 or rework the timer counts for timer3 with a 8:1 prescale

  12. #12
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    Default Re: Serial Comms and Crystals

    Richard,

    need some additional help. The following code compiled, but displayed RH 0.0 and tmr 0 on the LCD. I changed the values for badread and noise as suggested, but still have the same result. As mentioned I'm using a 10 mhz crystal with HS_ppl enabled to run the pic at 40 Mhz. Could you point me in the right direction as to what I'm doing wrong ?

    Code:
    ; config settings 18F4580, 10mhz crystal HSPPL
    
    ASM  
      __CONFIG    _CONFIG1H, _OSC_HSPLL_1H
      __CONFIG    _CONFIG2L, _PWRT_ON_2L  
      __CONFIG    _CONFIG2H, _WDT_ON_2H & _WDTPS_512_2H
      __CONFIG    _CONFIG3H, _PBADEN_OFF_3H
      __CONFIG    _CONFIG4L, _LVP_OFF_4L & _XINST_OFF_4L
    ENDASM
    
    DEFINE  OSC 40
    ADCON1 = $0F
    clear
    INCLUDE "alldigital.pbp"
    CMCON = 7 
    TRISA = %11001111
    
    DEFINE LCD_DREG  PORTB           ' LCD Data port
    DEFINE LCD_DBIT  0               ' starting Data bit (0 or 4)
    DEFINE LCD_EREG  PORTB           ' LCD Enable port
    DEFINE LCD_EBIT  5               '     Enable bit  (on EasyPIC 5 LCD)
    DEFINE LCD_RSREG PORTB           ' LCD Register Select port
    DEFINE LCD_RSBIT 4               '     Register Select bit   (on EasyPIC 5 LCD)
    DEFINE LCD_BITS  4               ' LCD bus size (4 or 8 bits)
    DEFINE LCD_LINES 4               ' number of lines on LCD
    DEFINE LCD_COMMANDUS 2000        ' Command delay time in us 
    DEFINE LCD_DATAUS 50             ' Data delay time in us 
    
    ASM       
    hum          = _dht+3    
    tempa         = _dht+1
    ENDASM 
    
    T1CON = %00000001                               ; free-running, 1:1 prescaler
    
    dht_data var PORTA.1
    dht_dir var TRISA.1
    dht var byte[5]
    
    tempa var word EXT
    bits var byte
    crc   var byte
    hum  var word EXT 
    
    main:
    gosub read_dht
    LCDOut $FE,$94,"rh ",#hum/10,".",dec1 hum//10
    pause 4000
    goto main
        
    read_dht:
         for bits=4 to 0 step-1 
           dht[bits]=0 
         next
        
         tmr2=0
         dht_dir=0
         dht_data=0
         pauseus 1000                   'start it up
         dht_data=1
         pauseus 30
         dht_dir=1
         pauseus 40                    'wait till middle of response pulse window
         if ( dht_data)then goto badread   'no response then  give up
         while (!dht_data)
         wend 
         t2con=6
         while ( dht_data) 
         wend 
         t2con=0
         if ((tmr2 <75)||(tmr2>85))then goto badread      ' confirm presence ?
         for  bits =39 to 0  step -1
             tmr2=0
             while (!dht_data) 
             wend 
             t2con=6
             while ( dht_data) 
             wend 
             t2con=0
             if ((tmr2>20)&&(tmr2<80)) then     ' noise ?
               if (tmr2 >50 ) then
                  dht.0[bits] =  1 
               endif   
               
             else
              goto badread          ' noise ?
             endif
         next 
         crc=0 
         for bits=1 to 4         
           crc=crc+ dht[bits]
           
         next
         if crc !=  dht[0] then goto badcrc    'crc
    
    return
      
    badread:
    LCDOut $FE,$94 +8,"tmr6 ",#tmr2
        for bits=4 to 0 step-1 
           dht[bits]=0 
        next
    return
    badcrc:
        for bits=4 to 0 step-1 
    LCDOut $FE,$94 +16,hex dht[bits],"," 
        next  
    return

  13. #13
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    Default Re: Serial Comms and Crystals

    you did not make the changes I suggested

    ; config settings 18F4580, 10mhz crystal HSPPL

    ASM
    __CONFIG _CONFIG1H, _OSC_HSPLL_1H
    __CONFIG _CONFIG2L, _PWRT_ON_2L
    __CONFIG _CONFIG2H, _WDT_ON_2H & _WDTPS_512_2H
    __CONFIG _CONFIG3H, _PBADEN_OFF_3H
    __CONFIG _CONFIG4L, _LVP_OFF_4L & _XINST_OFF_4L
    ENDASM

    DEFINE OSC 40
    ADCON1 = $0F
    clear
    INCLUDE "alldigital.pbp"
    CMCON = 7
    TRISA = %11001111

    DEFINE LCD_DREG PORTB ' LCD Data port
    DEFINE LCD_DBIT 0 ' starting Data bit (0 or 4)
    DEFINE LCD_EREG PORTB ' LCD Enable port
    DEFINE LCD_EBIT 5 ' Enable bit (on EasyPIC 5 LCD)
    DEFINE LCD_RSREG PORTB ' LCD Register Select port
    DEFINE LCD_RSBIT 4 ' Register Select bit (on EasyPIC 5 LCD)
    DEFINE LCD_BITS 4 ' LCD bus size (4 or 8 bits)
    DEFINE LCD_LINES 4 ' number of lines on LCD
    DEFINE LCD_COMMANDUS 2000 ' Command delay time in us
    DEFINE LCD_DATAUS 50 ' Data delay time in us

    ASM
    hum = _dht+3
    tempa = _dht+1
    ENDASM

    T1CON = %00000001 ; free-running, 1:1 prescaler

    dht_data var PORTA.1
    dht_dir var TRISA.1
    dht var byte[5]

    tempa var word EXT
    bits var byte
    crc var byte
    hum var word EXT

    main:
    gosub read_dht
    LCDOut $FE,$94,"rh ",#hum/10,".",dec1 hum//10
    pause 4000
    goto main

    read_dht:
    for bits=4 to 0 step-1
    dht[bits]=0
    next

    tmr2=0
    dht_dir=0
    dht_data=0
    pauseus 1000 'start it up
    dht_data=1
    pauseus 30
    dht_dir=1
    pauseus 40 'wait till middle of response pulse window
    if ( dht_data)then goto badread 'no response then give up
    while (!dht_data)
    wend
    t2con=6
    while ( dht_data)
    wend
    t2con=0
    if ((tmr2 <45)||(tmr2>55))then goto badread ' confirm presence ?
    for bits =39 to 0 step -1
    tmr2=0
    while (!dht_data)
    wend
    t2con=6
    while ( dht_data)
    wend
    t2con=0
    if ((tmr2>12)&&(tmr2<50)) then ' noise ?
    if (tmr2 >31 ) then

    dht.0[bits] = 1
    endif

    else
    goto badread ' noise ?
    endif
    next
    crc=0
    for bits=1 to 4
    crc=crc+ dht[bits]

    next
    if crc != dht[0] then goto badcrc 'crc

    return

    badread:
    LCDOut $FE,$94 +8,"tmr6 ",#tmr2
    for bits=4 to 0 step-1
    dht[bits]=0
    next
    return
    badcrc:
    for bits=4 to 0 step-1
    LCDOut $FE,$94 +16,hex dht[bits],","
    next
    return

  14. #14
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    Default Re: Serial Comms and Crystals

    if you don't have a oscilloscope or logic analyser add some more diagnostics to the program


    Code:
     ; config settings 18F4580, 10mhz crystal HSPPL
    
     ASM 
     __CONFIG _CONFIG1H, _OSC_HSPLL_1H
     __CONFIG _CONFIG2L, _PWRT_ON_2L 
     __CONFIG _CONFIG2H, _WDT_ON_2H & _WDTPS_512_2H
     __CONFIG _CONFIG3H, _PBADEN_OFF_3H
     __CONFIG _CONFIG4L, _LVP_OFF_4L & _XINST_OFF_4L
     ENDASM
    
     DEFINE OSC 40
     ADCON1 = $0F
     clear
     INCLUDE "alldigital.pbp"
     CMCON = 7 
     TRISA = %11001111
    
     DEFINE LCD_DREG PORTB ' LCD Data port
     DEFINE LCD_DBIT 0 ' starting Data bit (0 or 4)
     DEFINE LCD_EREG PORTB ' LCD Enable port
     DEFINE LCD_EBIT 5 ' Enable bit (on EasyPIC 5 LCD)
     DEFINE LCD_RSREG PORTB ' LCD Register Select port
     DEFINE LCD_RSBIT 4 ' Register Select bit (on EasyPIC 5 LCD)
     DEFINE LCD_BITS 4 ' LCD bus size (4 or 8 bits)
     DEFINE LCD_LINES 4 ' number of lines on LCD
     DEFINE LCD_COMMANDUS 2000 ' Command delay time in us 
     DEFINE LCD_DATAUS 50 ' Data delay time in us 
    
     ASM 
     hum = _dht+3 
     tempa = _dht+1
     ENDASM 
    
     T1CON = %00000001 ; free-running, 1:1 prescaler
    
     dht_data var PORTA.1
     dht_dir var TRISA.1
     dht var byte[5]
    
     tempa var word EXT
     bits var byte
     crc var byte
     hum var word EXT 
    
     main:
     gosub read_dht
     LCDOut $FE,$94,"rh ",#hum/10,".",dec1 hum//10
     pause 4000
     goto main
    
     read_dht:
     for bits=4 to 0 step-1 
     dht[bits]=0 
     next
    
     tmr2=0
     dht_dir=0
     dht_data=0
     pauseus 1000 'start it up
     dht_data=1
     pauseus 30
     dht_dir=1
     pauseus 40 'wait till middle of response pulse window
     if ( dht_data)then goto badresponce 'no response then give up
     while (!dht_data)
     wend 
     t2con=6
     while ( dht_data) 
     wend 
     t2con=0
     if ((tmr2 <45)||(tmr2>55))then goto badpresence ' confirm presence ?
     for bits =39 to 0 step -1
     tmr2=0
     while (!dht_data) 
     wend 
     t2con=6
     while ( dht_data) 
     wend 
     t2con=0
     if ((tmr2>12)&&(tmr2<50)) then ' noise ?
     if (tmr2 >31 ) then
     dht.0[bits] = 1 
     endif 
    
     else
     goto badread ' noise ?
     endif
     next 
     crc=0 
     for bits=1 to 4 
     crc=crc+ dht[bits]
    
     next
     if crc != dht[0] then goto badcrc 'crc
    
     return
    
     badread:
     LCDOut $FE,$94 +8,"tmr2 ",#tmr2
     
     next
     return
     badcrc:
     for bits=4 to 0 step-1 
     LCDOut $FE,$94 +16,hex dht[bits],"," 
    pause 5000
     next 
     return
    badresponse:
     LCDOut $FE,$94 +8,"nr "
     return
    
    badpresence:
     LCDOut $FE,$94 +8,"pr",#tmr2
     return

  15. #15
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    Default Re: Serial Comms and Crystals

    Quote Originally Posted by richard View Post
    you did not make the changes I suggested
    Quote Originally Posted by Scampy View Post
    I changed the values for badread and noise as suggested, but still have the same result.
    Sorry Richard, I must of confused you by posting the code with the values in the original code.

    I'll take a look at your suggested code tomorrow and let you know - cheers for your help

    EDIT - loaded the code as is and the LCD displays rh 0.0 nr
    I assume that it's not getting a response from the sensor ? I have checked the connections and they appear correct (+ve to 5v, GND to ground, and signal to PORT A pin 1) this is connected via a header to the EasyPIC5 board with the 10K pull up resistors enabled (also tried without) I have a Hantek PC scope which I can use to test. I'm guessing I should see pin A1 going from high to low for the initial handshake between PIC and sensor ??
    Last edited by Scampy; - 25th March 2015 at 00:39. Reason: update

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    Default Re: Serial Comms and Crystals

    expected dht response to start signal
    Attached Images Attached Images  

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    Default Re: Serial Comms and Crystals

    OK seems to of been a hardware issue. Replaced the sensor and now get a value on the lcd. The program is giving a reading of 870.4 - how would I turn this into a percentage

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    Default Re: Serial Comms and Crystals

    Just to get a sense of the raw data collected, perhaps you can change your "main" subroutine to the follow and post the results output on the LCD?

    Code:
    main:
     gosub read_dht
     'LCDOut $FE,$94,"rh ",#hum/10,".",dec1 hum//10
     lcdout $FE, $94, hex2 dht(0), " ", hex2 dht(1), " ",hex2 dht(2), " ", hex2 dht(3), " ", hex2 dht(4)
     pause 4000
     goto main
    Regards,
    TABSoft

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    Default Re: Serial Comms and Crystals

    LCDOut $FE,$94,"rh ",#hum/10,".",dec1 hum//10 this gives a value of 870.4

    lcdout $FE, $94, hex2 dht(0), " ", hex2 dht(1), " ",hex2 dht(2), " ", hex2 dht(3), " ", hex2 dht(4) produces 38 00 16 00 22

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    Default Re: Serial Comms and Crystals

    In one of the other examples I've tested it has the following statement

    humid=dht[31]*128+dht[30]*64+dht[29]*32+dht[28]*16+dht[27]*8+dht[26]*4+dht[25]*2+dht[24]*1

    Using
    lcdout $FE,$C0,"Humidity = ",#humid,"% " gives the humidity as a percentage.

    Just trying to apply this to Richards code to get the same result

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    Default Re: Serial Comms and Crystals

    Not using the DHT11 sensor, what is the temperature around your test rig?
    Regards,
    TABSoft

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    Default Re: Serial Comms and Crystals

    As I read the datasheet for the DHT11 a few of things jump out.
    1. The resolution for the Humidity is 1%
    2. The resolution for the Temperature is 1 degC.

    The datastream is 40 bits long.
    8bit integral RH data + 8bit decimal RH data + 8bit integral T data + 8bit decimal T data + 8bit check sum.
    The algorithm you are implementing stores the bitstream into the dht array like this.
    dht[4] = integral RH data
    dht[3] = decimal RH data
    dht[2] = integral T data
    dht[1] = decimal T data
    dht[0] = checksum

    So the Hex data values you showed earlier "38 00 16 00 22" should be this
    dht[0] = checksum = $38 (56 decimal) $00 + $16 + $00 + $22
    dht[1] = decimal T data = $00 (this is the decimal portion of the Temperature, which will always be 0)
    dht[2] = integral T data = $16 (22 decimal)
    dht[3] = decimal RH data = 00 (this is the decimal portion of the Humidty, which will always be 0)
    dht[4] = integral RH data = $22 (34 decimal)

    I believe you do not need to perform any calculations to convert the RH data to a percentage.
    That is what is giving you the strange results.
    The calculation "#hum/10" is taking the hum variable and dividing it by 10.
    The issue as I see it is that using the hum variable is not necessary and giving you the incorrect values.
    PBP treats word variables as Little Endian numbers meaning the low order byte is stored first then the high order byte.
    So when hum = _dht+3 and hum is declared as a word, PBP is looking at dht[3] & dht[4] as the word variable hum.
    This would be $00 $22, but since we are looking at Little Endian numbers PBP will use dht[4] as the high byte and dht[3] as the low byte.
    This makes hum = $2200 (8704 dec), which will result in 870 decimal when you divide 8704 by 10 and a result of 4 with 8704 // 10.
    Making your output equal 870.4 as you say.

    For the RH percentage you simply need to display the value in dht[4], something like this.
    lcdout $FE, $94, dec dht[4], "%"
    You do not need the ".",dec1 stuff since the DHT11 does not report a decimal place for RH or Temperature because of the resolution of the sensor.

    As for the Temperature, if you are going to just use it as a Celsius number, there is no need for any calculation or conversion either.
    Just simply display the value in dht[2]. E.g. lcdout $FE, $94, dec dht[2].

    Hope this helps.
    Regards,
    TABSoft

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    Default Re: Serial Comms and Crystals

    looks like the dht11 only has a 8 bit range for rh an t so the display needs to be
    LCDOut $FE,$94,"rh ",#hum/100
    or
    LCDOut $FE,$94,"rh ",#dht[4]

    my fault I stopped reading the dht11 data sheet when I saw the 40 bit timings were the same as a dht22 , I have no dht11 to test with

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    Default Re: Serial Comms and Crystals

    just an observation but the fineoffset wh1081 weather station that I have uses a dht11 and its readings vary greatly from those of the dht22 right next to it . somehow I think the dht11 is not very accurate

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    Default Re: Serial Comms and Crystals

    Richard,

    I don't think #hum/100 will give the correct result.
    Since I. This example hum=8704 ($2200), 8704/100=87 ($57).

    The correct result should be 34 ($22).
    He just needs to use dht[4]
    Or
    To use a division calculation with hum it should be #hum/256.
    This will right shift 8704 ($2200) 8 bits resulting in 34 ($22).
    Regards,
    TABSoft

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    Default Re: Serial Comms and Crystals

    Guys thanks for your continued input.

    using the #hum/100 gives 87% as the value, which to me seems rather humid, especially when the indoor humidity according to my maplins weather station says it's 51%. As a comparison I tried the following code which uses the pause and pulsein statements to read the chip

    Code:
    ASM 
      __CONFIG    _CONFIG1H, _OSC_HS_1H
      __CONFIG    _CONFIG2L, _PWRT_ON_2L  
      __CONFIG    _CONFIG2H, _WDT_ON_2H & _WDTPS_512_2H
      __CONFIG    _CONFIG3H, _MCLRE_ON_3H & _LPT1OSC_OFF_3H & _PBADEN_OFF_3H  
      __CONFIG    _CONFIG4L, _LVP_OFF_4L & _XINST_OFF_4L
    ENDASM
    
    
    '*******************************************************************************
    ' LCD (20 x 4) set up
    '*******************************************************************************
    
    DEFINE LCD_DREG  PORTB                  ' LCD Data port
    DEFINE LCD_DBIT  0                      ' starting Data bit (0 or 4)
    DEFINE LCD_EREG  PORTB                  ' LCD Enable port
    DEFINE LCD_EBIT  5                      '     Enable bit  (on EasyPIC 5 LCD)
    DEFINE LCD_RSREG PORTB                  ' LCD Register Select port
    DEFINE LCD_RSBIT 4                      '     Register Select bit   (on EasyPIC 5 LCD)
    DEFINE LCD_BITS  4                      ' LCD bus size (4 or 8 bits)
    DEFINE LCD_LINES 4                      ' number of lines on LCD
    DEFINE LCD_COMMANDUS 2000               ' Command delay time in us 
    DEFINE LCD_DATAUS 50                    ' Data delay time in us 
    
    '*******************************************************************************
    ' Defines Statements
    '*******************************************************************************
    
    DEFINE  OSC 20                          ' 18F4520 / 18F2520, 20mhz crystal
    ADCON1 = $0F
    clear    
    
    '*******************************************************************************
    'Analog and Comparator settings
    '*******************************************************************************
    
    ADCON0 = %00000000                      'AD converter module disabled
    ADCON1 = %00001111                      'All Digital
    ADCON2 = %00000000
    CMCON = 7                               'Disable Comparators
    
    '*******************************************************************************
    'Port and Register settings  (interrupts)
    '*******************************************************************************
        
    TRISA  = %00010111              
    TRISB  = %00000011               
    T0CON  = %11000111
    
    T1CON = %00000001                               ; free-running, 1:1 prescaler
    TMR1H = %11111111
    TMR1L = %11111011
    
    
    '*******************************************************************************
    
    LCDOUT $FE,1:FLAGS=0:PAUSE 250:LCDOUT $FE,1:PAUSE 250 ' Initialize LCD
    
    '*******************************************************************************
    
    dht var byte[32]
    humidite var byte
    haut var byte
    bas var byte
    temp var byte
    x var byte
    dht11 var portA.1
    
    '*******************************************************************************
    start:
    TRISA.1 = 0 '
    high dht11
    pause 2000 ' wait 2 sec
    low dht11 : pause 18' send 20ms low
    high dht11 : pauseus 30 ' send 40us hi
    TRISA.1 = 0
    PulsIn PORTA.1, 1, haut
    if haut < 15 then goto start
    for x = 31 to 0 step-1
    PulsIn PORTA.1, 1, dht[x] ' 1
    next x
    For x = 31 to 0 step-1
    if dht(x)>=9 and dht(x)<=21 then dht(x)=0 'if pulsewidth between 20 and 40uS then read as '0'
    if dht(x)>=29 and dht(x)<=41 then dht(x)=1 'if pulsewidth between 60 and 80uS then read as '1'
    next x
    humidite=dht[31]*128+dht[30]*64+dht[29]*32+dht[28]*16+dht[27]*8+dht[26]*4+dht[25]*2+dht[24]*1
    temp=dht[15]*128+ dht[14]*64+dht[13]*32+dht[12]*16+dht[11]*8+dht[10]*4+dht[9]*2+dht[8]*1
    lcdout  $FE,1 
    lcdout  $FE,$C0,"Humidite = ",#humidite,"% "
    lcdout  $FE,$80,"Temperature = ",#temp,$DF,"C"
    goto start
    This gives 174c in our living room, with the humidity fluctuating between three values of 24%, 144% and 152%. Clearly there is something wrong with the hardware. I'm using a new solderless breadboard so I'll look at hard wiring the sensor and see if that helps, even though the connections between the Easypic board and sensor are next to each other

  27. #27
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    Default Re: Serial Comms and Crystals

    Quote Originally Posted by Tabsoft View Post
    Richard,

    I don't think #hum/100 will give the correct result.
    Since I. This example hum=8704 ($2200), 8704/100=87 ($57).

    The correct result should be 34 ($22).
    He just needs to use dht[4]
    Or
    To use a division calculation with hum it should be #hum/256.
    This will right shift 8704 ($2200) 8 bits resulting in 34 ($22).
    Dividing hum by 256 gives a reading of 35 on the LCD. The values for dht(1) to (5) are 39 00 16 00 23

    I'm still not satisfied the sensor is working right, and this is the second one I've tried

  28. #28
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    Default Re: Serial Comms and Crystals

    Hummm

    Well I've now soldered the wires direct to the sensor, and plugged the wires directly into the header on the EasyPIC5 board.

    Dht(0) - 4 now reads 38 00 19 00 1F with a hum/256 value of 34.

    whilst this seems to of resolved the issues with the breadboard, 34% humidity seems rather low, and quite a gap from the 50% shown on the weatherstation

  29. #29
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    Default Re: Serial Comms and Crystals

    brain fade due to lack of coffee divide 256 is of course correct

    attached the blue is a dht11 the red a dht22 in the same room
    Attached Images Attached Images  

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    Default Re: Serial Comms and Crystals

    As Richard has stated, the DHT11 doesn't seem to be too accurate in his experience.

    As far as your readings go 39 00 16 23

    This is 35% RH and 22c Temp.

    Just use dht[4] for RH and dht[2] for Temp.
    Regards,
    TABSoft

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    Default Re: Serial Comms and Crystals

    Thanks guys - I'll look at getting a DH22 as I need something more precise given that this will be monitoring the humidity in my snakes enclosure.

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    Default Re: Serial Comms and Crystals

    Hi Richard,

    I was wondering if you could help me understand your use of Timer2 in this project during the read loop for the 40 bits from the DHT11.

    In this section of code.....
    " t2con=6
    while ( dht_data)
    wend
    t2con=0
    if ((tmr2>12)&&(tmr2<50)) then ' noise ?
    if (tmr2 >31 ) then
    dht.0[bits] = 1
    endif
    else
    goto badread ' noise ?
    endif"

    I see that Timer2 is turned on with the Prescaler set to 16.
    Then after the data line drops to 0, Timer2 is stopped.

    Then the TMR2 register is tested for its count value.
    If the count is less than 12 or greater than 50, then it's a bad read.
    Otherwise if the count is greater than 31 and less than 50 the bit is a 1.
    Is this correct?

    If the Fosc is 40Mhz and Timer2 counts at Fosc/4 and the Timer2 Prescaler is 16.
    Then
    Would the test "((tmr2>12)&&(tmr2<50))" equate to
    If count is greater than 19.2us and less than 80us?

    And then would the test "(tmr2 >31 )" equate to
    If count is greater than 49.6us?

    Which I think the test logic would translate to...
    If the count is less than 19.2us or greater than 80us then it's a bad read.
    Otherwise if the count is greater than 49.6us and less than 80us the bit is a 1.

    I think this is right, but please let me know if not.
    Just trying to understand how it is implemented.

    Thanks
    Regards,
    TABSoft

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    Default Re: Serial Comms and Crystals

    Which I think the test logic would translate to...
    If the count is less than 19.2us or greater than 80us then it's a bad read.
    Otherwise if the count is greater than 49.6us and less than 80us the bit is a 1.
    that's it although a '0' is 26-28uS and a 1 is 70us according to the data sheet I have added some tolerance (this helps if interrupts are involved) if the count is outside this range then its not right and the process is abandoned , this test is not strictly necessary and could be omitted

    the 1/0 test is basically centered in between the 1 and the 0 times ie 50 is halfway between 30 and 70 once again to add some tolerance . if less than halfway its a 0 otherwise a 1


    there is some risk in this method in that hangs are possible if the dht responds in an unforseen manner . in my production version the tmrx int is enabled and if it is triggered then the process is also aborted to recover from such a sensor hang .

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    Default Re: Serial Comms and Crystals

    Tabsoft / Richard,

    I've incorporated the code below into my main program and whilst it runs fine, it does flash up a fair few "bad read" errors. Is there anything I could tweak to reduce these occurancies

    Code:
    read_dht:
     for bits=4 to 0 step-1 
     dht[bits]=0 
     next
     
     tmr2=0
     dht_dir=0
     dht_data=0
     pause 18 'start it up
     dht_data=1
     pauseus 30
     dht_dir=1
     pauseus 40 'wait till middle of response pulse window
     if ( dht_data)then goto badresponse 'no response then give up
     while (!dht_data)
     wend 
     t2con=6
     while ( dht_data) 
     wend 
     t2con=0
     if ((tmr2 <45)||(tmr2>55))then goto badpresence ' confirm presence ?
     for bits =39 to 0 step -1
     tmr2=0
     while (!dht_data) 
     wend 
     t2con=6
     while ( dht_data) 
     wend 
     t2con=0
     if ((tmr2>12)&&(tmr2<50)) then ' noise ?
     if (tmr2 >31 ) then
     dht.0[bits] = 1 
    
     endif 
    
     else
     goto badread ' noise ?
     endif
     next 
     crc=0 
     for bits=1 to 4 
     crc=crc+ dht[bits]
     next
     if crc != dht[0] then goto badcrc 'crc
    
     return
    
     badread:
     LCDOut $FE,$94 +8,"Bad read"
     pause 1000
     LCDOut $FE,$94 +8,"        "
     ' next
     return
     badcrc:
     for bits=4 to 0 step-1 
     LCDOut $FE,$94 +16,hex dht[bits],"," 
    pause 1000
     next 
     return
    badresponse:
     LCDOut $FE,$94+8,"No Response"
     return
    
    badpresence:
     LCDOut $FE,$d4,"pr",#tmr2
     return

  35. #35
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    Default Re: Serial Comms and Crystals

    does your program use interrupts ? if so how long does the isr take to execute ?
    the tolerance here is 10uS + or - so an interrupt can easily cause the timing tolerance to be exceeded
    possible solutions
    1. disable interrupts during dht read
    2. increase tolerance
    3. ignore bad reading and retry
    4. remove the noise test
    Code:
    t2con=0
     
    if ((tmr2>12)&&(tmr2<50)) then ' noise ?
     if (tmr2 >31 ) then
     dht.0[bits] = 1 
    
     endif 
    
     else
     goto badread ' noise ?
    
     endif
    5. use a faster chip

  36. #36
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    Default Re: Serial Comms and Crystals

    To get a better understanding what is going on, why don't you make a small change for debugging.

    Instead of this...
    Code:
    for bits =39 to 0 step -1
        tmr2=0
        while (!dht_data) 
        wend 
        t2con=6
        while ( dht_data) 
        wend 
        t2con=0
        if ((tmr2>12)&&(tmr2<50)) then ' noise ?
            if (tmr2 >31 ) then
                dht.0[bits] = 1 
            
            endif 
        
        else
            goto badread ' noise ?
        endif
    next
    Do this instead...
    Code:
     
     for bits =39 to 0 step -1
         tmr2=0
         while (!dht_data) 
         wend 
         t2con=6
         while ( dht_data) 
         wend 
         t2con=0
         if tmr2 <= 12 then goto tooshort
         if tmr2 => 50 then goto toolong
         if (tmr2 >31 ) then
            dht.0[bits] = 1 
         endif 
    
     next
    Then create the 2 subs, tooshort and toolong and have each print a different error message.
    Regards,
    TABSoft

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    Default Re: Serial Comms and Crystals

    TI and Silicon Labs have RH/Temperature devices with an I2C interface in the $5.00 single part range: Si7013, Si7020, Si7021 2%-4% accuracy TI HDC1000 and HDC1008 similar accuracy specification. Both are available at Digikey. Honeywell also has a device, HIH6130, but it is a bit more expensive, at $14.
    Tim Barr

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    Default Re: Serial Comms and Crystals

    Guys, thanks for the additional replies,

    @Tim, I'll check out those suggestions... cheers

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    Default Re: Serial Comms and Crystals

    malcom
    before you go too far down this track you might like to research some other devices or at least have a way to calibrate these things. I now have three sensors in the same room 2x dht22, 1 dht11. the dht 11 reads 60% the dht22's read 46.3 and 49.5 right now @19.6C (tis as close as they seem to get) . real hum is anyone's guess , the readings tend to converge @ around 20C but diverge wildly below 10C and above 30C. there is some info on the web about bedding these sensors in or "reforming them" . not sure how fussy reptiles are , I do know how fussy the cold blooded half of the human race is

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    Default Re: Serial Comms and Crystals

    Richard,

    Thanks for the suggestion. I've been having some issues and have to agree that these sensors are not really suitable where precision is required. I added tabsofts suggestion, and I got as many short errors as I did Longs... I've also got several not connected errors. I think I'll look at something that uses a standard communications interface such as those suggested by Tim.

    The humidity requirements of the snakes I keep are not that critical, but this probe is intended to be used to monitor the humidity in the container containing eggs in an incubator, where the humidity needs to be kept at a set range so they don't dry out.

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