Serial randomness!

[skimask]

Hi everyone,
Hoping you might be able to help me with a little project I’ve been working on….
I’ve been trying to create a code that reads in a value (b0) from an
a/d converter, this then goes to a timer routine which loops ‘b0’ amount of times and then sends PORTB.3 high. I’ve programmed the following onto a pic16f628a and then added it to the circuit…. from first impressions it all looks great! However…. By watching the LED’s attached to pins 5,6,7 and 8 I can see that it’s adding up a different amount each time, even though the value from the A/D converter should be the same. Can anyone spot where I’m going wrong here as it’s left me pretty confused!
Many thanks, Alex

Where's the ADC code?
The output from the ADC should be the same...key word is 'should'.
Is your ADC input varying at all?
Are you sampling it slow enough as the datasheet suggests?
Are your parameters for the ADC (sample time, etc.) correct?
What is the input impedance to your ADC pin?
For all I know, there's a nuclear fission reactor power generating station connected to AN0 on the PIC, or it could be a small butterfly. I don't know.

[mister_e]

seems to be an external ADC on PORTB.0. Setting sampling time, clock source etc... on a 16F628 that will be hard

However i don't think it's working like that. If we knew the ADC model, we could elaborate a bit much. Almost sure it's an SPI type... SHIFTIN/SHIFTOUT would help.. but.. i only guess

[london alex]

Yeah sorry guys, should have been a bit clearer! I'm using an external TLC549CP A/D converter which has:

ADC 'data out' pin attached to pin 0 on the pic.
ADC i/o clock pin attached to pin 1 on the pic and..
ADC CS(active) pin attached to pin 2...

Attached to the ADC is a potentiometer which is supplying the variable voltage (0-5V). Throughout my testing I haven't been adjusting it and so the serial output should remain the same if it was working correctly.

Haven't tried SHIFTIN/SHIFTOUT but will have a look into it... cheers!!

[Darrel Taylor]

Alex,

When you do something like PORTB = 8, it sets all the pins in PORTB.

Since PORTB.2 is the A/D's Chip Select pin, every time you turn on an LED or Relay, you are also Enabling the A/D converter. Probably getting it confused.

And that chip can be clocked at up to 1 Mhz.
6 seconds is an awful long time to read 8 bits.
<br>

[london alex]

Hey Darrel,

Thank you loads for your response! I'm currently away from home and so unable to test your theory, but i'm convinced that will be the cause of the issue! I'll keep you updated as to when I can test it and thanks to everyone who helped.

Cheers,
Alex