1. More fun with Timer/Counters
Timer Input Capture
CS-280
Dr. Mark L. Hornick

2. 4 main functions built around the Timer/Counter subsystem
Pulse accumulator (T/C connected to external clock)
Allows counting of external events
Output Compare
Set a register flag when timer reaches a certain value
optionally cause an interrupt
Optionally set/clear/toggle output bits to generate variable freq.
PWM Generator
Periodic waveform generator of variable widths (duty cycle)
Input Capture
Set a flag when input bit rises/falls
optionally cause an interrupt
Can record the actual Timer/Counter value
CS-280
Dr. Mark L. Hornick

3. Timer Input Capture (TIC) records the times of external Events
Only possible with Timer/Counter 1
the16-bit T/C
“Events” are (externally-generated) signals on the Input Capture Pin (ICP)
PortD pin 6 (PD6) is the ICP
DDRD must configure PD6 for input
CS-280
Dr. Mark L. Hornick

4. Current time of T/C1 is latched when an “Event” occurs
Timer/Counter1 ticks use two 8-bit TCNT1 registers to count timer increments
TCNT1H and TCNT1L
Event time is captured in two 8-bit ICR1 registers
ICR1H and ICR1L
CS-280
Dr. Mark L. Hornick

5. Controlling Timer/Counter 1
Like T/C0, the operation of T/C1 is controlled via various I/O registers
TICIE1 – T/C1 Input Capture interrupt enable bit
OCIE1A – T/C1 Output Compare A interrupt enable
OCIE1B – T/C1 Output Compare B interrupt enable
TOIE1 – T/C1 Overflow interrupt enable
CS-280
Dr. Mark L. Hornick

6. T/C1 Flag Register
Like T/C0, the status of T/C1 overflow and output compare is readable via TIFR
ICF1 – Set on T/C1 input event
OCF1A – Set on Output Compare A match
OCF1B – Set on Output Compare B match
TOV1 – Set on Overflow
CS-280
Dr. Mark L. Hornick

7. Details: Specifying the mode of operation of T/C1 for IC
Unlike T/C0, there are two Control Registers for T/C1
TCCR1A is used when using T/C1 in Output Compare & Waveform Generation modes
Set all bits to 0 when only using Input Capture
WGMxx bits are used to set Normal/CTC/PWM modes
CS-280
Dr. Mark L. Hornick

8. Controlling the Event that gets captured
ICNC1 – noise canceller
If 1, sampling is extended over 4 clock cycles
ICES1 – Input Capture Edge Select
Specifies which “Event” triggers Input Capture
1: rising edge
0: falling edge
WGM13:12
leave 0 for Input Capture
CS12:CS10 control the counter frequency
Similar to settings for T/C0
CS-280
Dr. Mark L. Hornick

9. Servicing an Input Capture Interrupt
Example: Measuring the period T of a square wave:
Setup T/C1 for Input Capture
Enable Input Capture interrupt
Set T/C1 frequency
Set to trigger ICR1 interrupt on rising edge
Within Input Capture ISR
Read ICR1L and ICR1H
As with ADC, order is important
Save ICR1L and ICR1H
Reset ICR1 to trigger next time on falling edge
On second interrupt (falling edge)
Compute counter delta
Compute time period T from delta and frequency
Reset ICR1 to trigger on rising edge T
CS-280
Dr. Mark L. Hornick

10. An Application Example
Incremental Encoder
Used to monitor the position of rotational devices (motors, machine shafts, etc)
Often known as quadrature encoders
CS-280
Dr. Mark L. Hornick