ATmega16 datasheet(93/315 Pages) ATMEL

93 / 315 page

ATmega16(L)

2466E–AVR–10/02

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 86.

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 COM11:0 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 channels, 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 out-

put 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”.

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