Picaxe 28X1 and readtemp... What am I missing ??

[Hydroid]

Hi,

I cannot seem to get the readtemp command to work with my 28X1. I didn't bother attaching a photo of my setup because I'm using an AXE091 development board - and there is only one wire connected: From the TMP pin to the 28X1's ADC0 channel (Pin2). I figure even I can't mess up one wire.....

This is my very simple code:

main:

	readtemp 0, b0
	
	debug
	
	goto main

The debug screen b0 value never changed from 0...

Just for experimentation, I installed an 08M onto the AXE091 and connected the jumper from the TMP pin to the 08M's Pin6 (ADC1 channel) and ran this code:

main:

	readtemp 1, b0
	
	debug
	
	goto main

As expected, the debug screen b0 value read 21 and increased as I put my finger on the DS18B20 showing that it is responding...

I then tried a 18M2 chip, jumper from TMP to Pin18 (C.1 ADC) and ran this code:

main:

	readtemp c.1, b0
	
	debug
	
	goto main

As with the 08M, the b0 value showed 21 and responded to my finger touch

I also tried a 20M chip - and it worked too...

I then retried my 28X1 code, using a different 28X1 chip - to rule out a bad chip.. Nothing, b0 remained at 0...

Manual 2 page 184's READTEMP description shows that this command works on a 28X1, so I assume I'm missing something really simple, but I can't figure out what.

What am I not seeing / understanding ??

Thanks, John.

 

[KeithRB]

ReadTemp is not an ADC command. It uses a special 1 wire communication with the DS18B20. You need to specify a pin number for an input/output pin.

 

[Technical]

28X1's ADC0 channel cannot be used for readtemp

You need to use an input pin, 0 through 7.

So readadc 0 is checking input 0, leg 10

 

[Hydroid]

Thank You

I figured it was something simple that I wasn't seeing. Leave it to me to just by chance pick three other chips where the ADC channel I chose also happens to be an input pin.. Talk about leading yourself down the garden path!

For my own future reference, I have annotated the manual changing the syntax statement from "Pin is the input pin" to "Pin must be an input pin"

Thanks for the clarification,

Regards, John.

 

[westaust55]

Remember also as per Manual 2"

This command cannot be used on the following pins due to silicon restrictions:
08, 08M, 08M2 3 = fixed input
14M, 14M2 C.3 = fixed input
18M2 C.4, C.5 = fixed input
20M,20M2, 20X2 C.6 = fixed input

The reason is that the pin must be capable of being bi-directional as the PICAXE must send a control signal or setting data out to the DS18B20, and also must be able to act as an input to receive the data.