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kongfu1
Joined: 26 Apr 2010
Posts: 56
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| Using Comparator with 16f1938 |
 Posted: Wed Jul 06, 2011 4:22 pm |
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Hi,
I have a project to use 16F1938 to monitor an analog signal.
Currently analog signal is connected to RA2 and plan to use DAC as reference voltage of comparator.
I am confused by those options of comparator defined in 16f1938.h:
| Code: |
#define CP2_A0_A2 0x80000000
#define CP2_A1_A2 0x80010000
#define CP2_B3_A2 0x80020000
#define CP2_B1_A2 0x80030000
#define CP2_A0_DAC 0x80100000
#define CP2_A1_DAC 0x80110000
#define CP2_B3_DAC 0x80120000
#define CP2_B1_DAC 0x80130000
#define CP2_A0_FVR 0x80200000
#define CP2_A1_FVR 0x80210000
#define CP2_B3_FVR 0x80220000
#define CP2_B1_FVR 0x80230000
#define CP2_A0_VSS 0x80300000
#define CP2_A1_VSS 0x80310000
#define CP2_B3_VSS 0x80320000
#define CP2_B1_VSS 0x80330000
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My question is that which option should be used with setup_comparator knowing RA2 is connected to analog input and DAC used as reference voltage? Any example code?
Thanks,
Howard |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Wed Jul 06, 2011 5:20 pm |
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| Quote: |
analog signal is connected to RA2 and plan to use DAC as reference
voltage of comparator.
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I don't think this combination is possible.
Look at the pin diagram for the 16F1938 on page 5 of the data sheet:
| Quote: |
Pin Diagram – 28-Pin SPDIP/SOIC/SSOP (PIC16F1938, PIC16LF1938) |
For pin A2, it shows that it's also the C2IN+ comparator input:
| Quote: | COM2/DACOUT/VREF-/C2IN+/AN2/RA2
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Now look at this diagram in the 16F1938 data sheet (on page 174):
| Quote: |
FIGURE 18-2: COMPARATOR MODULE SIMPLIFIED BLOCK DIAGRAM |
It shows that CxIN+ and the DAC and the FVR buffer 2, and Vss
are all connected to the same 4:1 multiplexer. You select from one
of those 4 inputs to drive the + input of the comparator. This means
your proposed combination of inputs is not possible.
The other input of the comparator (the - input) is driven by another
4:1 multiplexor with these 4 choices as inputs:
| Quote: |
C12IN0- AN0 RA0
C12IN1- AN1 RA1
C12IN2- AN9 RB3
C12IN3- AN10 RB1
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You need to choose inputs from the available options.
16F1938 data sheet:
http://ww1.microchip.com/downloads/en/DeviceDoc/41574A.pdf |
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kongfu1
Joined: 26 Apr 2010
Posts: 56
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| Posted: Wed Jul 06, 2011 7:12 pm |
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PCM,
Thanks for the quick response and helps.
I have revised the board based on your suggestion to connect analog signal to RA0. With two options available on setup_comparator,
| Code: |
#define CP1_A0_DAC 0x8010
#define CP2_A0_DAC 0x80100000
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If CP1 is used, comparator output will be on RA4 and CP2 goes with RA5. Correct?
If CP1_A0_DAC is selected, and DAC is set as
| Code: |
setup_dac(DAC_VSS_VDD);
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then dac_write(x) where x could be anything between 0-31 could be used to set the Vref for comparator. Correct?
Is there any example code on this?
Thanks,
Howard |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Thu Jul 07, 2011 2:08 pm |
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To give you a 100% accurate answer, I would have to test it in hardware
with your compiler version. It's better if try to make it work first. Then
if doesn't work, post your test program and test method, and I'll look at it. |
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