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C.6
LESSON 4 – ANALOG-TO-DIGITAL CONVERTERS AND COMPARATORS
This lesson is broken up into two programs which demonstrate how to use the
analog-to-digital converter, the comparator and the internal voltage reference. The first
program set is comp.asm (written in assembly) and comp.c (written in C). This program
shows the very basic implementation of using the comparator with the internal voltage
reference. The second program set is atod.asm (written in assembly) and atod.c
(written in C). This program uses the LED display library to show the value of the
analog-to-digital (A-D) converter.
C.6.1
Comparator Lesson
For this lesson, the comparator is configured as a dedicated comparator driving the
GP2 pin directly. With <CM2:CM0> = 101, the comparator has multiplexed inputs.
GP0/CIN+ is selected as the input to the V IN - by setting CIS = 1, thus making the analog
potentiometer the one input. The internal voltage reference is used as the other input
to the comparator on V IN +. The module compares the potentiometer voltage with the
reference, and the logic is setup (CINV = 1) such that the output is high when the input
voltage is higher than the reference. LED3 is driven directly by the comparator and is
lit when the output of the module is high. To light D3, you must configure RA4 (GPIO4)
and RA2 (GPIO2) as digital outputs via the TRIS register, RA4 is set low and RA2 is
driven directly by the comparator without additional software overhead. The remaining
I/O's are configured as high impedance inputs.
Note:
The schematics, as shown in Appendix A, display the 14-pin device pinout.
This is compatible with 8-pin devices.
The internal voltage reference is essentially a variable resistor based voltage divider
between V DD and V SS . Use the low range mode (VRR = 1) and set the internal
reference to be 0.5 V DD by selecting <VR3:VR0> = 1100, thus C VREF = V DD *(12/24).
By stepping through the different voltage reference settings, a basic-low resolution
analog-to-digital converter can be implemented.
The comparator module has seven different modes. Three of the modes can drive an
output pin directly. The software can monitor the output directly or create an interrupt
on change. Bits CMIE and PEIE must be set to enable the comparator interrupt which
can be used to wake the device from SLEEP.
Figure C-11 shows the project setup for the assembly and C projects.
FIGURE C-11:
? 2004 Microchip Technology Inc.
ASSEMBLY AND C PROJECT SETUP
DS40051D-page 57
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