Serial Comms and Crystals

[richard]

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

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

[Tabsoft]

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

Instead of this...

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...

 
 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.

[falingtrea]

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.

[Scampy]

Guys, thanks for the additional replies,

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