Microprocessor Engineering

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Timer/Counters

In many microprocessor systems we need to:
 count
logic pulses
 measure the frequency of signals
 measure the period of pulses
 generate pulses of specific periods
 time events
 generate delays accurately

We can use Timer/Counters for all the above
5-1
Binary Counters
Overflow
bit
Clock signal
n-bit Counter
Counter increments by
one for every clock pulse
This bit is set to one
when the binary counter
rolls over from all 1's
back to 0's (Can be reset
to zero by software)
Counter can be pre-loaded
with a starting value
5-2
Timer/Counters

The clock source can be
a
fixed known frequency

usually the cpu clock or some fraction of it
external signal – connected to an external pin. This
allows the counting of pulses
 An


Some systems connect the overflow bit via a flip-flop
to an output pin which is toggled every time an
overflow occurs. Used for waveform generation
(PWM).
The overflow flag can be polled or used to generate
an interrupt to the CPU.
5-3
PIC 18F452 Timer0 – 8 bit mode
5-4
PIC 18F452 Timer0 – 16 bit mode
Note: Upon RESET, Timer0 is enabled in 8-bit mode with clock input from T0CKI max. prescale.
5-5
T0CON – Timer 0 Control register
5-6
To generate a fixed delay

Configure T0CON
 timer
mode
 8 or 16 bit
 prescaler

Load TMR0 with vale to give correct time delay
 NOTE:



- High byte first then low byte
Clear TMR0IF to zero (Bit 2 of INTCON register)
Start timer by setting TMR0ON (bit 7 of T0CON)
Poll TMR0IF – wait until bit is set. Alternatively use
interrupts (a future lecture topic).
5-7
To measure a time period
 Configure



T0CON
timer mode
8 or 16 bit
prescaler
 Clear
TMR0 - High byte first then low byte
 Clear TMR0IF to zero (may not be required)
 When the event starts

Start timer by setting TMR0ON (bit 7 of T0CON)
 When

the event is over
Stop the timer by clearing TMR0ON
 Read
the value from TMR0 (Read low byte first!)
5-8
A 100ms delay
CPU Clock frequency fosc= 4MHz , Timer input clock
frequency = 4MHz /4 = 1MHz so period = 1us
 Max count for 16-bit TMR0 = 2^16 = 65536 counts
 So max timer period = 65536 x 1us =65.536ms
 Need to use prescaler
Prescaler Resolution
Max period

2
4
etc.
256
2us
4us
65536x2us = 131.072ms
262.144ms
256uS
16.777216 seconds
5-9
Cont.









So need prescaler set to 2.
Timer input period = 2x1us = 2us
Number of counts = time period/timer input period =
100ms/2us = 100000/2 = 50000
Value to load into timer = 65536 – 50000 = 15536
movlw high(15536)
movwf TMR0H
movwf low(15536)
movwf TMR0L
bcf
INTCON,TMR0IF
5-10
Measure a time period
;Initialise timer 0
movlw 0x03
;Prescaler = 16, so increment timer every 16us
movwf T0CON
; with 4MHz Osc
;reset timer registers to zero
clrf TMR0H
;notice order:high then low byte
clrf TMR0L
; The event start is detected so start timer
bsf T0CON,TMR0ON
; Wait for the event to end
; The event is over so stop timer
bcf T0CON,TMR0ON
;Calc time taken
movff TMR0L,TIML
;Note order of reading - Low then high byte
movff TMR0H,TIMH
5-11
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