16 bit conversion - AM2302

[Scampy]

Hi guys,

Following on from the thread when using a DHT11 sensor for humidity readings I've received my SHT11 / AM2302 sensor from E-bay.

My current routine for reading the DHT11 is

read_dht:

TRISA.1 = 0 '
high dht_data
low dht_data : pause 18' send 18ms low
high dht_data : pauseus 30 ' send 30us hi
TRISA.1 = 0
PulsIn PORTA.1, 1, dht
if dht < 9 then goto badresponse 
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
hum=dht[31]*128+dht[30]*64+dht[29]*32+dht[28]*16+dht[27]*8+dht[26]*4+dht[25]*2+dht[24]*1
return

The sensor uses the same 40 bit data stream as the DHT, but has twice the resolution. The data sheet (http://www.adafruit.com/datasheets/D...r%20AM2302.pdf) suggests that you convert the 16 bit decimal to decimal and then divide that by 10 to get the reading. As the sensor sends RH then Temp then checksum, what would be the simplest way of reading the first 16 bits rather then doing the multiply by 128, 64, 32, 16 etc

[Tabsoft]

You could possibly do something like this...

hum var word
j var byte
x var byte

read_dht:

TRISA.1 = 0 '
high dht_data
low dht_data : pause 18' send 18ms low
high dht_data : pauseus 30 ' send 30us hi
TRISA.1 = 0
PulsIn PORTA.1, 1, dht
if dht < 9 then goto badresponse 
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

'Stuff values into hum word variable based on pulsewidth value in dht byte arrary
j = 15
for x = 31 to 16 step - 1
    if dht(x)>=9 and dht(x)<=21 then hum.0[j] = 0
    if dht(x)>=29 and dht(x)<=41 then hum.0[j] = 1
    j = j-1    
next x

[Scampy]

Thanks Tabsoft,

I'll give that a try. I did stumble across a similar thread (http://www.picbasic.co.uk/forum/showthread.php?t=17867) and amended the code to

read_dht:

TRISA.1 = 0 '
high dht_data
low dht_data : pause 18' send 18ms low
high dht_data : pauseus 30 ' send 30us hi
TRISA.1 = 0
PulsIn PORTA.1, 1, dht
if dht < 9 then goto badresponse 
for x = 0 to 31 
PulsIn PORTA.1, 1, dht(x) ' 1
next x
For x = 0 to 31  
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
hum=32768*dht(1)+16384*dht(2)+8192*dht(3)+4096*dht(4)+2048*dht(5)+1024*dht(6)+512*dht(7)+256*dht(8)+128*dht(9)+64*dht(10)+32*dht(11)+16*dht(12)+8*dht(13)+4*dht(14)+2*dht(15)+1*dht(16)           

return

with an amended display line, but get random digits all over the range from 6453% to 17% !!

I'll give your example a try a and will get back

[Scampy]

tabsoft,

Using your code and the line

LCDOut $FE,$80+9,"RH ",dec hum

The LCD is displaying 6473, 6730, 6729, 6473, 6729 so I would assume that there is the odd misread now and again.

What divider should I use of the variable hum to get a realistic figure, and is there any tweaks to the values in the for/next loop to clearly define the 1's and 0's to make the reading of the sensor more stable. I'm using an 18F4580 running at 40 Mhz (10mhz crystal with the HS_ppl option enabled)

EDIT:

For info using

LCDOut $FE,$80+9,"RH ",#hum

the values jump around 18714, 18970,18970,18714,

[Scampy]

I'm assuming that for 100% humidity the value of hum would be 65535 and 0% would be 0 ?

If so then using DEC hum/256 gives a reading of 73, 74 on the LCD - which seems a tad to high for the living room. The cheap simple dial hydrometer in the reptile enclosures read 37% and the sensor built into the weather station which is 18" away from the development board displaying 42%

[Scampy]

OK, just to check things I got my Arduino out and downloaded a sketch and library file. here's the results



which is close enough to the readings of both hydrometers in the reptile tanks.

Now I don't understand the C style of these arduino sketches, but the link to the website is http://www.electroschematics.com/112...orial-library/ - maybe it might help in working out why this is such a pain to do in PBP

[Tabsoft]

Give this a try.

hum var word
j var byte
x var byte


read_dht:

TRISA.1 = 0 ' Make pin an input
high dht_data   '250ms High
pause 250

low dht_data    '20ms Low
pause 20' send 20ms low

high dht_data   '40us High
pauseus 40

PulsIn PORTA.1, 1, dht
if dht < 9 then goto badresponse 
for x = 31 to 0 step-1
    PulsIn PORTA.1, 1, dht[x] ' 1
next x

hum = 0 'Clear hum variable
'Stuff values into hum word variable based on pulsewidth value in dht byte arrary
j = 15
for x = 31 to 16 step - 1
   for x = 31 to 16 step - 1
    if dht(x)>=35 then hum.0[j] = 1
    j = j - 1    
next x
return

and then to display try this.

LCDOut $FE,$80+9,"RH ",dec hum/10, "." dec hum //10