Help understanding HSERIN/HSEROUT

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  1. #1

    Default Help understanding HSERIN/HSEROUT

    Hello all,

    I am trying to use the EUSART peripheral in the PIC16F688. I am using the 8Mhz internal oscillator and used Mister_es picMultiCalc to obtain the defines. I need help trying to understand how HSERIN works.

    From what I understand, EUSART is a hardware peripheral embedded in the pic similar to hardware PWM. I know that in PWM I can specify a duty cycle and it can run in the background while I do other things in software. Is HSERIN/HSEROUT the same concept?

    If I send a series of data bytes i.e.
    HSEROUT [byte1, byte2, byte3]
    how will the data be sent? Will it send the complete message first then proceed with the code, or will it initialize the transmission, the SW proceeds, and then finishes the transmission in the background?

    Same question with HSERIN. How does it receive data packets? I know RCIE enables UART interrupt flag and RCIF sets the interrupt flag. How is it interrupted?
    i.e an On-Change-Interrupt fires if a pin changes state, a timer interrupt fires at a timer overflow, or a comparator interrupt fires when it reaches a certain voltage. How would a HSERIN interrupt occur? Does it have some kind of buffer or something?

    Thank you all for any input. Hopefully I get enough information so I can begin playing around with this peripheral.

  2. #2
    Join Date
    Jul 2003


    HSEROUT operates in a continuous loop until the last byte is sent.

    First it tests the TXIF flag to see if the buffer is empty, when the buffer is empty, it then
    loads the outbound byte into TXREG, and returns for the next byte until all bytes are sent.

    After loading the last byte into TXREG and returning, depending on the baud rate, the PIC
    can normally execute several thousand instructions while the last byte is still in the process
    of being transmitted by the hardware UART. But it's still a continuous software loop until it
    reaches the last byte, and passes it to the TXREG.

    HSERIN is also a continuous software loop until all bytes have been received. RCIF is set
    when the stop bit is received, and the new byte is transferred from the RSR register into

    RCIF will remain set until all bytes have been read from RCREG, then RCIF is cleared in

    A receive interrupt only happens if you have the receive interrupt enabled. RCIF gets set
    regardless, and can only be cleared by reading RCREG, and emptying the FIFO buffer.

    tech at

  3. #3

    Default hserin vs serin2

    Great! thank you for your reply,

    let me get this straight.
    -TXREG and RCREG is the buffer where a byte is stored whether it is going to be sent or received
    -TXIF and RCIF determines whether the buffer is empty or full for tx and rx.
    Also depending on whether i have an interrupt enabled (TXIE or RCIE) it will fire but both get set regardless.
    -HSERIN/HSEROUT does not run in the background

    my question is now, how does this compare to serin2/serout2. what does UART have special that i can't do in software with serin2/serout2 (which i can use any pin by the way)?

    if i want to synch a message with a character will i not use:
    HSERIN [wait("a"), dat]
    but if i use this the SW will be hung at this line waiting for that character, where as if i use
    serin2[wait("a"), dat]
    is the same.

  4. #4
    Join Date
    Jul 2003


    It doesn't really matter if you're using HSERIN or SERIN2 with the WAIT option. Both are
    going to stall and WAIT for the character you're WAITing for before moving on.

    The hardware UART on most PICs has a 2-byte FIFO first-in, first-out buffer that will
    buffer up to 2-bytes. This doesn't require any software over-head. These 2-bytes of data
    are automatically stashed in the buffer, in the background, by the hardware UART.

    Just enable the UART for continuous receive, test RCIF for new characters, check for over-
    runs, etc.

    Below is a simple example.
    ' ** // PIC16F877A, 20MHz crystal
    ' ** // Buffers up to 80 characters using the hardware
    ' ** // USART. Buffering ends when the EOM "~" character
    ' ** // is received.
    ' ** // Uses RCIF bit polling for new characters
    DEFINE  OSC 20
    SPBRG = 129             ' 20MHz=129, 4MHz=25 to set baud rate at 9600
    RCSTA = $90	        ' Enable USART receive
    TXSTA = $24	        ' Set USART parameters
    GP      VAR byte        ' GP variable
    BytesIn VAR byte[80]    ' Up to 80 bytes
    ByteCnt VAR byte        ' Indicates number of bytes in buffer
    EOM     CON "~"         ' EOM = "End Of Message" marker
    OERR    VAR RCSTA.1     ' Alias USART over-run bit
    CREN    VAR RCSTA.4     ' Alias USART continuous receive enable bit
    RCIF    VAR PIR1.5      ' Alias USART received character interrupt flag bit
    INTCON  = 0	        ' Disable interrupts
    ByteCnt = 0             ' Zero counter
    ADCON1 = 7              ' A/D off, all digital
        IF RCIF THEN            ' If RCIF=1 there's a new character in RCREG
         BytesIn[ByteCnt]=RCREG ' Yes. Store new char in array
         IF BytesIn[ByteCnt]=EOM THEN OutMsg ' End of message?
         ByteCnt=ByteCnt+1      ' No. Increment array index pointer
         IF ByteCnt >=80 THEN OutMsg ' Test for software buffer over-run
        IF !OERR THEN  Main     ' If OERR=0 no over-run
        CREN = 0                ' Over-run so disable receive
        CREN = 1                ' Re-enable & clear OERR "over-run" flag
        GOTO Main
        HSEROUT [13,10,dec ByteCnt," bytes received",13,10]
        HSEROUT [str BytesIn\ByteCnt,13,10] ' Echo characters received
        FOR GP=0 to ByteCnt   ' Clear array bytes 0 to ByteCnt
         BytesIn[GP]=0        '
        NEXT                  '
        ByteCnt=0             ' Reset index pointer back to first element
        WHILE RCIF            ' Trash anything received after the EOM marker
         GP=RCREG             ' by reading RCREG until RCIF is cleared.
        GOTO Main
    With HSERIN or SERIN2 and the WAIT modifier, you would be stuck in a continuous loop
    waiting for your special character to show up. Of course, you can also use the TIMEOUT
    LABEL options to exit HSERIN or SERIN2, but if there's noise on the RX pin, the timeout
    period gets reset, and you're stuck again in a nasty continuous loop.

    Even if the timout/label option works, you're still wasting a boat-load of instruction cycles
    doing absolutely nothing.

    By just monitoring flag bits, and doing a little house-keeping, you have tons of time to do
    other things while the hardware UART buffers inbound data for you.

    That's the big difference. Even a command like HSERIN, that uses a hardware peripheral
    like the UART, can slow things down. Learning how to use the hardware by itself opens up
    a lot of options you don't have when using a software command to control the hardware.

    Another example is HPWM. You have limitations with the HPWM command that you don't
    when you learn how to setup/use the hardware yourself.

    tech at

  5. #5


    sorry for the long delay, but had to take care of other things.

    i see .... you implemented a recieve routine without HSERIN, by polling RCIF flag bit to see if it set, and if it did set, it emptied its register RCREG into an arrouy of bytes. Now, i'm guessing this can be done by interrupts also by enabling the RCIE bit and now we don't need to poll the flag bit, rather we just do whatever we need in code, then when the buffer fills the flag will be set, and we go to the interrupt routine, then we check for overruns etc. it makes sense.

    is this the same way with transmitting a byte. if we store a byte into TXREG will that automatically be sent serially, (the moment it gets stored to TXREG) or how does it transmit? i'm guessing the TXIF flag is enabled when it finishes sending a byte correct?

    I was able to get the PIC16F688 to talk to my PC (hyperterminal) via serial communication with its hardware USART using HSEROUT/HSERIN. I tried it at different baud rates and it worked perfectly! The only thing that i didn't like is that to do this i needed the MAX232 IC for voltage leveling, i was unable to communicate directly from the PIC. Is there any way of accomplishing this? i'v searched and i know it can be done, but it didn't work for me. any ideas?

    by the way thank you bruce for your responses, they've been very helpful

  6. #6
    Join Date
    Aug 2006
    SWITZERLAND (french part)

    Default HSERIN using EUSART interrupt RCIF (PIR1.5)

    Quote Originally Posted by Bruce View Post
    By just monitoring flag bits and doing a little house-keeping, you have tons of time to do other things while the hardware UART buffers inbound data for you...
    Hello there,

    I've been spending some hours on searching information about using HSERIN with its interrupt, RCIF (PIR1.5), and found an interresting info in one of Bruce's posts. It wasn't about the interrupt but he explained and other way around very clearly.

    My project is about a 3G monitoring system and receiving SMS messages is crucial (= it is crucial not to miss any incoming SMS).

    Why do I search for unsing interrupts? Because, IMO, it's the only way to make sure I will not miss any incoming SMS.

    Am I right or not, I don't know and maybe one of you can tell. But in the doubt, I'd like to give the best chances to my program not to oversee any SMS.

    Bruce states that there are tons of time to do whatever you need but in my case, where among others I'm controlling four latching relays, setting them and checking their states takes at least 0.5 seconds added to the rest.

    I never had the case up to now but I imagine I could receive to SMS within 0.5 seconds (but, as said, I'm not sure).

    Anyway, I have posted here under just a piece of my full code where the principle of the RX EUSART interrupt should be tested. Unfortunately, it doesn't work and I can't find why.

    ' ====== FUSES 16F690 =============================================================================
    @ ERRORLEVEL -306
    ' ====== REGISTERS =================================================================================
    OSCCON      = %01110000 ' Internal RC set to 8Mhz - Register not to be used with XTal
    OPTION_REG  = %10000000 ' PORT A&B Pull-Ups (look WPUA & WPUB)
    ADCON0      = %00000000 ' A/D Module
    ANSEL       = %00000000 ' Select analog inputs Channels 0 to 7
    ANSELH      = %00000000 ' Select analog inputs Channels 8 to 11
    ' ====== EUSART SETTINGS ==========================================================================
    ' Interrupts registers
    PIE1.5      = 1         ' Enable interrupt on EUSART
    INTCON.7    = 1         ' PEIE peripheral interrupt
    INTCON.6    = 1         ' GIE global interrupt
    ' Hardware EUSART registers (RX = PORTB.5, TX = PORTB.7)
    RCSTA       = %10010000 ' Enable serial port & continuous receive
    TXSTA       = %00100000 ' Enable transmit, BRGH = 0
    SPBRG       = 12        ' 9600 Baud @ 8MHz, 0.16%
    ' ====== DEFINES ==================================================================================
    Define OSC 8
    DEFINE HSER_CLROERR 1   ' Clear overflow automatically
    DEFINE NO_CLRWDT 1      ' Forces manual use of CLRWDT
    ' ====== VARIABLES ================================================================================
    LED1        VAR PORTC.4
    LED2        VAR PORTC.5
    Counter     VAR BYTE    ' ...a counter
    RxBuffer    VAR BYTE(18)' maximum SMS length (in characters)
    ' ====== PROGRAM ==================================================================================
        TOGGLE LED1
        PAUSE 1000
        ' Do whatever needs to be done and go back to waiting loop
        LED2 = 1
    ' ====== ISR ======================================================================================
        WHILE PIR1.5
            RxBuffer(Counter) = RCREG
            Counter = Counter +1

  7. #7
    Join Date
    Nov 2003

    Default Re: Help understanding HSERIN/HSEROUT

    I think there is no point in hunting down the incoming SMS as they are available in the GSM module.

    You can once your MCU wants or is available to ask for the new messages, and select the most recent or decide from the list which one you want to read, deleted etc. Your GSM module may receive also messages from the Network operator, messages by error from someone that typed wrong number or ads. So you better check the list and see who sent it, then process it.

    In this case you do not need Interrupts. Maybe if you want to raise a flag that a new SMS has arrived, but anyway, it is not necessary.

    Also On Interrupts are slow by default! I think I never used them.


  8. #8
    Join Date
    May 2013

    Default Re: Help understanding HSERIN/HSEROUT

    My project is about a 3G monitoring system and receiving SMS messages is crucial (= it is crucial not to miss any incoming SMS).

    the problem as i see it is that you are mixing up receiving a SMS message notification with receiving serial data without losing characters.

    the question really is
    how can i receive serial data as a background process with out data loss ?
    how can i signal foreground process that a message has been received and not lose messages?

    a workable solution is a interrupt driven ringbuffer for serial reception
    that signals the foreground with a flag on completion of notification message.

    the ringbuffer must be large enough to allow processing without data loss
    the chip must have adequate speed and resources [not a 690, more like 16f1829]
    hserin is not in the solution , its not flexible enough to be useful for this sort of task

    you will find ringbuffer examples on both forums
    This is more entertaining than Free to Air TV

  9. #9
    Join Date
    Nov 2003

    Default Re: Help understanding HSERIN/HSEROUT


    I think what you propose is the best solution and ultimately, Roger may follow it.

    But it may be too complicated at the moment to have such an advanced data handling. Or not?


  10. #10
    Join Date
    May 2013

    Default Re: Help understanding HSERIN/HSEROUT

    its like trying to remove the head off a motor with a shifting spanner.Name:  shifter.jpg
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    anyone who is serious about these things has a socket wrench in their toolkit
    Name:  socket.jpg
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    with proper tools things are easy, if your serious you make the investment.
    not only that when you have some isr driven serial tools in your kit all sorts of
    tasks become possible opening up a whole new vista
    This is more entertaining than Free to Air TV

  11. #11
    Join Date
    Nov 2003

    Default Re: Help understanding HSERIN/HSEROUT

    Totally agree, but Roger is still using On Interrupts...!


  12. #12
    Join Date
    Aug 2006
    SWITZERLAND (french part)

    Default Re: Help understanding HSERIN/HSEROUT


    I don't need to stick to interrupts; I just thought this would be a good idea....

    The thing is, I don't know the right (or best) "tool" to use and I appreciate your help and comments to take the correct path for my project

    "Ring buffer" is new to me so I'll first have to spend some hours of reading and searching to get more info. My monitor prototype is already in the field and every immediate improvement, the one I may have the skills to make immediately, is the always the best.

    So, because of my lack of knowledge (but it is getting better every day), these are the tools I will rely on for now:

    Name:  002304.jpg
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    Quote Originally Posted by Ioannis View Post
    I think there is no point in hunting down the incoming SMS as they are available in the GSM module.
    Having a routine checking the received message list every 5 seconds or so will allow me to start to improve the reliability of my monitor asap.

    Richard is right: I'm mixing up things and tracking incoming serial data is actually not that crucial. For any reason, I was focusing on the wrong target....

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