Serial Comms and Crystals

[HenrikOlsson]

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.

[Scampy]

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.

[Demon]

...

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

[Scampy]

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

[richard]

dht11/dht22
http://www.picbasic.co.uk/forum/showthread.php?t=19800

[Charlie]

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

[Scampy]

dht11/dht22
http://www.picbasic.co.uk/forum/showthread.php?t=19800

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 ?

[richard]

this should be close

if ((tmr6 <75)||(tmr6>85))then goto badread

would become

if ((tmr6 <45)||(tmr6>55))then goto badread

and

if ((tmr6>20)&&(tmr6<80)) then     ' noise ?
           if (tmr6 >50 ) then

would become

 if ((tmr6>12)&&(tmr6<50 then     ' noise ?
           if (tmr6 >31 ) then