ATtiny44 datasheet(93/236 Pages) ATMEL

93 / 236 page

8006H–AVR–10/09

ATtiny24/44/84

(WGM13:0) bits and Compare Output mode (COM1x1:0) bits. The TOP and BOTTOM signals

are used by the Waveform Generator for handling the special cases of the extreme values in

some modes of operation (“Modes of Operation” on page 96).

A special feature of Output Compare unit A allows it to define the Timer/Counter TOP value (i.e.,

counter resolution). In addition to the counter resolution, the TOP value defines the period time

for waveforms generated by the Waveform Generator.

Figure 12-4 on page 93 shows a block diagram of the Output Compare unit. The small “n” in the

cates Output Compare unit (A/B). The elements of the block diagram that are not directly a part

of the Output Compare unit are gray shaded.

Figure 12-4. Output Compare Unit, Block Diagram

The OCR1x Register is double buffered when using any of the twelve Pulse Width Modulation

(PWM) modes. For the Normal and Clear Timer on Compare (CTC) modes of operation, the

double buffering is disabled. The double buffering synchronizes the update of the OCR1x Com-

pare Register to either TOP or BOTTOM of the counting sequence. The synchronization

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-

OCFnx (Int.Req.)

= (16-bit Comparator )

OCRnx Buffer (16-bit Register)

OCRnxH Buf. (8-bit)

OCnx

TEMP (8-bit)

DATA BUS (8-bit)

OCRnxL Buf. (8-bit)

TCNTn (16-bit Counter)

TCNTnH (8-bit)

TCNTnL (8-bit)

COMnx1:0

WGMn3:0

OCRnx (16-bit Register)

OCRnxH (8-bit)

OCRnxL (8-bit)

Waveform Generator

TOP

BOTTOM

ATtiny44 Datasheet(PDF) 93 Page - ATMEL Corporation

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