byte I/O location is written by the CPU, the TEMP Register will be updated by the value
written. Then when the low byte (OCR1xL) is written to the lower eight bits, the high byte
will be copied into the upper 8-bits of either the OCR1x buffer or OCR1x Compare Reg-
ister in the same system clock cycle.
For more information of how to access the 16-bit registers refer to “Accessing 16-bit
Registers” on page 89.
Force Output Compare
In non-PWM Waveform Generation modes, the match output of the comparator can be
forced by writing a one to the Force Output Compare (FOC1x) bit. Forcing compare
match will not set the OCF1x Flag or reload/clear the timer, but the OC1x pin will be
updated as if a real compare match had occurred (the COM1x1:0 bits settings define
whether the OC1x pin is set, cleared or toggled).
Compare Match Blocking by
TCNT1 Write
All CPU writes to the TCNT1 Register will block any compare match that occurs in the
next timer clock cycle, even when the timer is stopped. This feature allows OCR1x to be
initialized to the same value as TCNT1 without triggering an interrupt when the
Timer/Counter clock is enabled.
Using the Output Compare
Unit
Since writing TCNT1 in any mode of operation will block all compare matches for one
timer clock cycle, there are risks involved when changing TCNT1 when using any of the
output compare units, independent of whether the Timer/Counter is running or not. If the
value written to TCNT1 equals the OCR1x value, the compare match will be missed,
resulting in incorrect waveform generation. Do not write the TCNT1 equal to TOP in
PWM modes with variable TOP values. The compare match for the TOP will be ignored
and the counter will continue to 0xFFFF. Similarly, do not write the TCNT1 value equal
to BOTTOM when the counter is downcounting.
The setup of the OC1x should be performed before setting the Data Direction Register
for the port pin to output. The easiest way of setting the OC1x value is to use the force
output compare (FOC1x) strobe bits in Normal mode. The OC1x Register keeps its
value even when changing between waveform generation modes.
Be aware that the COM1x1:0 bits are not double buffered together with the compare
value. Changing the COM1x1:0 bits will take effect immediately.
Compare Match Output
Unit
The Compare Output mode (COM1x1:0) bits have two functions. The Waveform Gener-
ator uses the COM1x1:0 bits for defining the Output Compare (OC1x) state at the next
compare match. Secondly the COM1x1:0 bits control the OC1x pin output source. Fig-
ure 44 shows a simplified schematic of the logic affected by the COM1x1:0 bit setting.
The I/O Registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of
the general I/O Port Control Registers (DDR and PORT) that are affected by the
COM1x1:0 bits are shown. When referring to the OC1x state, the reference is for the
internal OC1x Register, not the OC1x pin. If a System Reset occur, the OC1x Register is
reset to “0”.
96
ATmega32(L)
2503J–AVR–10/06
ATMEL [ ATMEL ]
