Timer/Counters

1
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

2
Binary Counters
Overflow bit
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)
Clock signal
n-bit Counter
Counter increments by one for every clock pulse
Counter can be pre-loaded with a starting value
3
Timer/Counters

• The clock source can be
• a fixed known frequency
• usually the cpu clock or some fraction of it
• An external signal connected to an external
  pin. This allows the counting of pulses
• 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.

4
PIC 18F452 Timer0 8 bit mode
5
PIC 18F452 Timer0 16 bit mode
Note Upon RESET, Timer0 is enabled in 8-bit mode
with clock input from T0CKI max. prescale.
6
T0CON Timer 0 Control register
7
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).

8
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!)

9
A 100ms delay

• CPU Clock frequency fosc 4MHz , Timer input
  clock frequency 4MHz /4 1MHz so period 1us
• Max count for 16-bit TMR0 216 65536 counts
• So max timer period 65536 x 1us 65.536ms
• Need to use prescaler
• Prescaler Resolution Max period
• 2 2us 65536x2us 131.072ms
• 4 4us 262.144ms
• etc.
• 256 256uS 16.777216 seconds

10
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

11
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 orderhigh
  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