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"Configuring the PIC pin for DAC output"
DAC output pin RA2/AN2/CVREF/VREF(Pin Block Diagram).
The block diagram below shows pin 1 of the 16F88 - it's useful to see how the hardware is connected so you can set up the pin correctly for voltage reference output.
RA2/AN2/CVREF/VREF pin (pin1
16F88)
It shows that the voltage reference is directly connected to the pin so all you need to do to output a voltage is to set the TRISA bit (to 1) - to make the P & N fets go into high impedance i.e. drawing no current - and set CVROE (1) obviously CVREN has to be 1 to enable the voltage reference. As well as this set ANSEL (1) (see below).
Note: The TRIS control circuit not shown on this diagram.
- ANSEL for pin RA2 to 1
- TRISA for pin RA2 to 1
- CVROE to 1
- CVREN to 1
Controlling ANSEL
The RA2 pin is the only pin that is analogue I/O (because of the voltage reference output) all other analogue pins are analogue input only.This register (ANSEL) defaults to all ones so they are all analogue I/(O) at power up but it's best to control the values so you know what they should be.
Making it go.
Measured values
Vsupply = 4.93V(i) CVRR = 1 (Low range)
Outputs 16 voltage steps from 0.01V to 3.05V
(ii) CVRR = 0 (high range)
Outputs 16 voltage steps from 1.22V to 3.53V
Data sheet information
Low volt range(i) for CVRR=1
0.00 CVRSRC to 0.625 CVRSRC with CVRSRC/24 step size
CVREF = (VR<3:0>/24) • (CVRSRC)
and 0.00 CVRSRC to 0.625 CVRSRC with CVRSRC/24 step size
CVREF = (VR<3:0>/24) • (CVRSRC)
High volt range
(ii) for CVRR=0
0.25 CVRSRC to 0.72 CVRSRC with CVRSRC/32 step size
CVREF = 1/4 • (CVRSRC) + (VR3:VR0/32) • (CVRSRC)
0.25 CVRSRC to 0.72 CVRSRC with CVRSRC/32 step size
CVREF = 1/4 • (CVRSRC) + (VR3:VR0/32) • (CVRSRC)
So for the second range (higher voltage output range) you can have finer step sizes and a slightly higher maximum voltage output (but the minimum is not zero).
CVRSRC = 4.93V
| Measured (i) | 0.01V to | 3.05V | ||
| For (i) Range is | 0V to | 4.93*0.625 = | 3.08V | |
| Measured (ii) | 1.22V to | 3.53V | ||
| For (ii) Range is | 0.25*4.93 = | 1.23V to | 4.93*0.72 = | 3.55V |
This shows the practical measurements match the datasheet predictions very well.
Speed
Running at 8MHz with the following code produces a pulse width for an output step of 11us (90kHz) - obviously this will change depending on the code used but you could probably create a simple signal generator.Note: You can increase the output speed by using a 20MHz crystal - for the current project it is using the internal oscillator at 8MHz so there's room for improvement.
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