ATMEGA169P-16MUR【PDF 20/274 页】IC MCU AVR 16K 16MHZ IND 64QFN

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116

8018P–AVR–08/10

ATmega169P

prevents the occurrence of odd-length, non-symmetrical PWM pulses, thereby making the out-

put glitch-free.

The OCR1x Register access may seem complex, but this is not case. When the double buffering

is enabled, the CPU has access to the OCR1x Buffer Register, and if double buffering is dis-

abled the CPU will access the OCR1x directly. The content of the OCR1x (Buffer or Compare)

automatically as the TCNT1 and ICR1 Register). Therefore OCR1x is not read via the high byte

temporary register (TEMP). However, it is a good practice to read the low byte first as when

accessing other 16-bit registers. Writing the OCR1x Registers must be done via the TEMP Reg-

ister since the compare of all 16 bits is done continuously. The high byte (OCR1xH) has to be

written first. When the high 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

For more information of how to access the 16-bit registers refer to ”Accessing 16-bit Registers”

on page 109.

15.7.1

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 COMx1:0 bits settings define whether the OC1x pin is set, cleared or

toggled).

15.7.2

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.

15.7.3

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 down counting.

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 Com-

pare (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.

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