DM163045 Microchip Technology, DM163045 Datasheet - Page 73

• Manufacturer Part Number: DM163045
• Description: PICDEM Lab Development Kit (with PICkit 3) General Purpose
• Manufacturer: Microchip Technology
• Series: PICDEM™r
• Type: MCUr

Specifications of DM163045

• Contents: Board, PIC kit 3
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• For Use With/related Products: PIC Micro® MCU

© 2009 Microchip Technology Inc.
reference, the cycle repeats and the system oscillates. The frequency of this oscillation
is dependant on the RC time constant (
capacitor to 37% of its initial voltage. As either the resistance or capacitance
decreases, so will
is replaced with a Negative Temperature Coefficient (NTC) thermistor where resistance
decreases as temperature increases, any temperature change would cause a shift in
resistance with a subsequent shift in the frequency of the oscillator.
This oscillator can be created quite easily by simply initializing the comparator and
nothing more. However, with the addition of some intelligence and some additional
peripherals, a high resolution sensor measurement application can be achieved.
The PIC16F690 features a 16-bit timer/counter peripheral Timer1. This timer can either
use the internal instruction clock (F
on the Timer1 Clock Input (T1CKI) pin to increment two 8-bit registers, TMR1H and
TMR1L, to obtain a combined 16-bit result. In this application, the oscillator described
will be used as the Timer1 clock source. Therefore, the TMR1H:TMR1L will increment
with each low-to-high transition effectively counting the number of pulses. The Timer0
peripheral features an interrupt-on-overflow (255-0) that will be used to provide a fixed
time frame in which the TMR1H:TMR1L registers will count. On a Timer0 overflow inter-
rupt, the Timer1 peripheral stops counting and the current value in the upper 4-bits of
TMR1H will be output to four LEDs connected to PORTC pins RC3, RC2, RC1 and
RC0. In order to obtain a usable result, it is important that Timer0 triggers an interrupt
before the TMR1H:TMR1L result overflows. If the temperature to the thermistor
changes, the oscillator frequency will shift resulting in a change in the number of counts
the Timer1 peripheral was able to implement before the fixed Timer0 interrupt with a
different result displayed on the LEDs.
The schematic for this lab is shown in Figure 4-5.
FIGURE 4-5:
LED1
J8
V
R4
470Ω
10
SS
1
2
3
4
5
6
7
8
9
T1CKI
RC3
LED2
τ
effectively increasing the frequency of the oscillator. If the resistor
SCHEMATIC FOR COMPARATOR LAB 3
U2
V
R5
470Ω
C12IN0-
SS
C1OUT
RC0
RC1
RC2
LED3
Comparator Peripheral Labs
20
19
18
17
16
15
14
13
12
11
OSC
V
R6
470Ω
SS
J9
/4) as its time base or an external clock source
τ
LED4
= R x C), or the time it takes to discharge the
V
R7
470Ω
SS
D1
1N4148
V
SS
C
1μF
1
V
SS
DS41369A-page 69
R3
10KΩ
NTC thermistor
R1
100KΩ
V
R2
10KΩ
SS