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duncangray
Joined: 27 Apr 2007
Posts: 14
Location: UK
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| Sorted: DAC not working on PIC16F753 |
 Posted: Fri Nov 04, 2016 5:13 am |
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Good morning CCS Forum,
Yet again I am trying something new and failing. Please can someone steer me in the right direction.
I want to make an approximation of a sine wave generator using a PIC DAC and my starter is supposed to be a ramp generator using the 16F753. Compiler is 5.062 and my code is | Code: | #include <16F753.h>
#use delay(internal=8000000)
void main()
{
int8 value;
setup_dac(DAC_VDD | DAC_OUTPUT);
setup_opamp1(OPAMP_ENABLED | OPAMP_NI_TO_DAC);
while(TRUE)
{
dac_write(value++);
delay_ms(1);
output_toggle(PIN_C2);
}
} |
The opamp may not be necessary as I am looking at A0 using a high impedance oscilloscope - it doesn't work with that line commented out either!
C2 is toggling so I know the code is executing but A0 is 0V (with the ICD disconnected).
Please can someone help as the CCS documentation seems slightly thin in the DAC department and I haven't found a DAC example in the examples directory.
Thanks Duncan
Last edited by duncangray on Fri Nov 04, 2016 1:14 pm; edited 1 time in total |
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temtronic
Joined: 01 Jul 2010
Posts: 9205
Location: Greensville,Ontario
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| Posted: Fri Nov 04, 2016 5:48 am |
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OK, I've never used that PIC but looked at the datasheet..
things to consider..
1) other peripherals use that pin must be disabled
2) the DAC has an 'enable' pin, confirm that IS set...
3) add a 10K to gnd 'load' resistor on DAC out pin
4) dump the listing and compare code to the datasheet for proper register selection and setting of bits. Perhaps the compiler has a 'bug'.
5) can you just 'toggle' the pin ? Confirming it's not shorted or 'busted'?
Having zero output I suspect the DAC output is not being enabled.
Hopefully someone else who has either used this PIC or one similar will respond...
Jay |
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duncangray
Joined: 27 Apr 2007
Posts: 14
Location: UK
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| Posted: Fri Nov 04, 2016 6:51 am |
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Jay,
Thanks for the rapid response - you're an early bird - responding before 7am!
Your points in the wrong order...
5) A0 toggles so it's not bust - it's also one of the programming pins.
3) 10k to gnd does nothing (not sure why you suggested this one as A0 was already at zero volts but I'm sure there was a reason).
1) Ignoring A0's use as an input - it is used for ICSPDAT and GPIO as well as DAC. As it toggles it is working as GPIO but I had assumed that setting DAC_OUTPUT would reassign it exclusively for the DAC. I also tried to output to the opamp DAC_OPA1OUT and setup_opamp1 to OPAMP_ENABLED | OPAMP_NI_TO_DAC in the hope that the DAC output would appear on C2 (having removed my toggle). Still nothing.
I have dumped the lst file but I'm afraid I don't know how to do 2 or 4. Please can you guide me.
Duncan |
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temtronic
Joined: 01 Jul 2010
Posts: 9205
Location: Greensville,Ontario
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| Posted: Fri Nov 04, 2016 8:06 am |
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hmm..good news the pin ain't 'busted'...
Since it's used for programming, be sure to have the fuse NODEBUG.
Come to think of it, you don't have any #fuses, so it could be one of them has disabled the DAC...
Also if using MPLAB, it has to have the build option set to release...though if a pin toggles at the correct speed, that may not be the problem.
Yyou can send the listing to me at sparky.miller [at] cogeco.ca, it's only 3*C here and need an 'excuse' to stay inside !
You could send the 'value' variable to a PC to confirm it is incrementing(0,1,254,255,0).
However ZERO ouput on the DAC pin does seem to suggest it's not enabled.
Must be after lunch on your side of pond... Jay |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19452
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| Posted: Fri Nov 04, 2016 8:39 am |
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I'll go 'backwards'. 
To setup the op-amp to give a voltage follower, it needs the extra define 'OPAMP_IN_UNITY_GAIN_MODE'. Currently the inverting input on the op-amp is not being connected. You have the DAC going to the non inverting input, but the other input undefined.
However you are not testing on the op-amps output pin anyway.
I'd remove the op-amp stuff, since some of the setting here do affect the DAC settings (DACOPA1OUT, is saying to use the op-amp and output on A1). Might be causing a problem.
With that done:
| Code: |
#include <16F753.h>
#use delay(internal=8000000)
void main()
{
int16 value;
setup_dac(DAC_VDD | DAC_OUTPUT | DAC_RIGHT_JUSTIFIED);
while(TRUE)
{
dac_write(value++);
delay_ms(1);
output_toggle(PIN_C2);
if (value==512)
value=0;
}
}
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Your chip requires a 16bit value for the DAC value (512 levels). In left justified mode (default), the top 9 bits of the 16bit value are used. Feeding it with an int8, in left justified mode, is only going to give an output of a few mV.... |
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duncangray
Joined: 27 Apr 2007
Posts: 14
Location: UK
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| Posted: Fri Nov 04, 2016 10:23 am |
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Thanks Jay (it's getting dark again - time to start work) and big thanks too to Ttelmah.
I had tried left justified without knowing what I was doing but I grasped the logic of your code/comments and realised just how much I still have to learn!
My 16F753 now works as a 0 to 5V ramp generator and it's time to get started on a sine wave.
You haven't got time to write a book have you?
Thanks again, Duncan |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19452
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| Posted: Fri Nov 04, 2016 12:35 pm |
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Good.
Left justified is what you get by default. Now, if you had tried right justified it'd have started working (but 0 to 2.5v).
Have fun. Flag this thread as 'solved' (you just edit the title). |
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