TC74A0 Temperature sensor giving me constant 255 ???

[Izzy]

Hello Folk,

I have been having this problem with TC74 temperature sensor. No matter what I do its always giving me constant 255?? Why is it?

I had this problem long ago and I couldnt fix it. So I thought the TC74 I had was bad. Today I bought another one, and it does the same thing.

I have TC74A0 and according to the data sheet I should use 0x90 and 0x91 for read and write address. But it is still giving me constant 255.


Yes I have searched and read a lot in this forum but I couldnt fix it. So desperately seeking for help.

Here is my sample code:

[PCM programmer]

Typically, you will read 0xFF if the i2c slave is not responding.
This could be caused by leaving off the pull-up resistors, or
using the wrong slave address, or swapping the sda and scl lines,
or something else.

[Izzy]

Hello PCM Programmer,
Thanks for your reply.

I have checked the connections and they are not swapped. I have 3.3k pullup resistor on both I2C lines. And I have checked the addresses too. But it is still giving me constant 255?? Why?

According to data sheet I am suppsed to use 0x90 and 0x91 for read and write addresses right? I have also tried 0x9A and 0x9B.

On page 13:
http://ww1.microchip.com/downloads/en/devicedoc/21462c.pdf


Any clue?

[Izzy]

Infact I have checked all the possible addresses now. And still no luck.

Voltage measurement at PINs:

VDD = 4.95V
SCLK = 4.85V
GND = 0V
SDA = 4.85V

Incase if it helps..here is the picture of the breadboarded circuit:
http://img363.imageshack.us/img363/2861/dsc00684zi4.jpg




Thanks

[Ttelmah]

Some small comments.
Add decoupling close to the PIC. You should get into the 'habit' of this, especially if working on a setup like a breadboard. 0.1uF, right close to the supply pins. Do the same close to the temperature sensor, and the LCD.
Try setting an I2C clock rate. The TC74, supports 100K _max_. I don't know what the default I2C rate will be from the compiler, but it might well be selecting a faster rate than this.
90, and 91, should be correct for the address.
Try adding an I2C stop, rather then using the restart. So:

[Izzy]

Thanks for your reply.

I tried adding an I2C stop, rather then using the restart. But still its giving me 255.
I have also tried adding SLOW :

#use i2c(Master,SDA=PIN_C4, SCL=PIN_C3, Force_HW, Slow)

Still same problem.


How do I set I2c clock rate? I am using 40Mhz oscillator.

Thanks

[Izzy]

OK now this is very WEIRD!

I am getting the reading, but only If I touch pin C4 (i.e. SDI/SDA pin) with a metal or my skin. I touched it with the probe of my DMM. Just 1 probe. It started giving me

25
25
(I warmed up the sensor)
26
26
27
28..

So its working, BUT only if I keep touching that C4 pin with a my skin or metal. If I leave, it goes back to 255??


WHY is it ?

[ckielstra]

You have found the problem to be related to the voltage levels on the I2C lines. Touching the lines with your hand or a probe will add extra capacitance.

Do you have by any chance a series resistor on the I2C lines? If yes, than this resistor in combination with the pull-up resistor would create a voltage divider and the voltage wouldn't go low enough.

[Izzy]

Hmm..As far as I know, I dont have a voltage divider on I2c lines.


Please take a look at the circuit, incase if I am not seeing one.

http://img363.imageshack.us/img363/2861/dsc00684zi4.jpg

[PCM programmer]

What is the MCLR pin connected to ? This is pin 1 on the PIC.
It looks like there is a yellow wire that goes up to the LCD.
If you are using pin1 as an input (RE3), then you need to add
the NOMCLR fuse to your #fuses statement. If you are using
it as a MCLR pin, then you should have a 10K pull-up on it, and
don't connect it to anything else.

Also, add 100 nF (0.1uF) ceramic capacitors between the Vdd and Vss
pins on each side of the PIC. Your photo doesn't show any caps.

Also, the way you did this breadboard is not the best. I would have
put the PIC in the middle of one of the breadboard sections. You have
it straddling the power and GND bus. If you had put it in the middle
of a breadboard section, then there would be only a short distance
between the Vdd and Vss pins on each side of the PIC, and the busses
on the breadboard. That would be a much cleaner layout, and much
better electrically. With your existing layout, you have snaked the
+5v and Gnd wires around the bottom of the PIC, to connect to the
Vdd and Vss pins on the top side. That's not the best way.

Also, I would have used one color (Red) for +5v and black for GND.
Then use another color for signal wires (white or something).
Then it's very clear where the power and Gnd wires are going.

[Ttelmah]

For the I2C 'speed', you can set what 'slow', and 'fast', actually give (in compilers for the last couple of years), using (for example):

SLOW=60000

I'd rewire the whole of this section of the layout. Add the capacitors both PCM, and myself mentioned, replace the actual wires used, and improve the ground run to both the PIC, and the chip. You may have something as 'simple', as a resistive connection on the C4 pin, or the connection at the TC that goes to this.

Best Wishes

[Izzy]

Thanks a lot guys for your advices! I really appreciate them.

[Izzy]

Phew!

Finally a 10K resistor instead of 3.3K in the SDA line solved the problem.

Thanks a lot guys!