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9219A–RFID–01/11

Atmel ATA5505 [Preliminary]

The OCR1A/B Register access may seem complex, but this is not case. When the double

buffering is enabled, the CPU has access to the OCR1A/B Buffer Register, and if double buff-

ering is disabled the CPU will access the OCR1A/B directly. The content of the OCR1A/B

(Buffer or Compare) Register is only changed by a write operation (the Timer/Counter does

not update this register automatically as the TCNT1 and ICR1 Register). Therefore OCR1A/B

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 OCR1A/B Registers

must be done via the TEMP Register since the compare of all 16bits is done continuously. The

high byte (OCR1A/BH) 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

(OCR1A/BL) is written to the lower eight bits, the high byte will be copied into the upper 8-bits

of either the OCR1A/B buffer or OCR1A/B Compare Register 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 115.

12.7.1

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 (FOC1A/B) bit. Forcing compare match will not

set the OCF1A/B flag or reload/clear the timer, but the OC1A/Bi pins will be updated as if a

real compare match had occurred (the COM1A/B1:0 bits settings define whether the OC1A/Bi

pins are set, cleared or toggled - if the respective OCnxi bit is set).

12.7.2

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 OCR1A/B to be initial-

ized to the same value as TCNT1 without triggering an interrupt when the Timer/Counter clock

is enabled.

12.7.3

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 channels, independent of whether the Timer/Counter is running or not. If the value

written to TCNT1 equals the OCR1A/B 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 con-

tinue to 0xFFFF. Similarly, do not write the TCNT1 value equal to BOTTOM when the counter

is downcounting.

The setup of the OC1A/B should be performed before setting the Data Direction Register for

the port pin to output. The easiest way of setting the OC1A/B value is to use the Force Output

Compare (FOC1A/B) strobe bits in Normal mode. The OC1A/B Register keeps its value even

when changing between Waveform Generation modes.

Be aware that the COM1A/B1:0 bits are not double buffered together with the compare value.

Changing the COM1A/B1:0 bits will take effect immediately.

12.8

Compare Match Output Unit

The Compare Output mode (COM1A/B1:0) bits have two functions. The Waveform Generator

uses the COM1A/B1:0 bits for defining the Output Compare (OC1A/B) state at the next com-

pare match. Secondly the COM1A/B1:0 and OCnxi bits control the OC1A/Bi pin output source.

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