"DiffOsc" Experiment - Programming and Customizing PICmicro® Microcontrollers







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"DiffOsc" Experiment
Most PICmicro® MCU part numbers can take advantage of several
different types of oscillators. In this experiment, the PIC16C505 is used to demonstrate
the built in, external crystal and external ceramic resonator 4 MHz oscillators. The
experiment is designed to run for sixteen minutes (960 or B'001111000000' Seconds), at
which time, the difference between it and a calibrated clock is checked.
The experiment is run three times, with each of the three different
circuits shown below:



The parts needed for this experiment are listed in the table:


Part
Description


PICmicro® MCU
PIC16C505-JW


Vdd/Vss Decoupling Capacitor
0.1 uF (Any Type)


_MCLR Pull Up Resistor
10K, 1/4 Watt


4 MHz Crystal
"Parallel Cut"


27-33 pF Capacitorys
2x Any Type


4 MHz Ceramic Resonator
Three Leaded Ceramic Resonator with Built in 27-33pF Capacitors


LED Current Limiting Resistors
6x 220W, 1/4 Watt


LED Display
10 LED "Bargraph" Recommended


Breadboard
Any Type


+5 Volt "Vcc" Power Supply
Any Type


Using a breadboard, the experiment is wired using the guide:

This circuit can be built on a EMU-II or YAP-II, but note that
the oscillator will have to be connected as shown in the breadboard diagram.
The source code listed below can be accessed from the
CD-ROM by clicking Here. Note that the
first instruction (movwf OSCCAL) is always present; even when the external
crystal or external ceramic resonator is being used for the experiment.

title "DiffOsc - Increment PORTC Once a Minute"
;
; This Application Tests the time keeping Ability of the 16C505
; With Different Clocks by updating the PORTC Value once every
; 1,000,000 Cycles (or One Second).
;
;
; Hardware Notes:
; PIC16C505 Running at 4 MHz with Different Clocks
; _MCLR is Pulled Up
; All 6 bits of PORTC are Pulled up and Connected to LEDs
; PORTC.5 ("TOCK1") I/O set to I/O
;
; Myke Predko
; 99.12.29
;
LIST P=16C505, R=DEC
INCLUDE "p16c505.inc"

; Register Usage
CBLOCK 0x010 ; Start Registers at End of the Values
Dlay:3 ; Counter for 1,000,000 Cycles
ENDC


PAGE
; __CONFIG _MCLRE_ON & _CP_OFF & _WDT_OFF & _XT_OSC
__CONFIG _MCLRE_ON & _CP_OFF & _WDT_OFF & _IntRC_OSC_RB4EN

; Mainline of DiffOsc
org 0

movwf OSCCAL ; Save the Oscillator Calibration Value

movlw 0x0FF ^ (1