Datasheet

 2018 Microchip Technology Inc. Data Sheet Complete DS40002060A-page 70

ATtiny4/5/9/10

12.6.3 Using the Output Compare Unit

Since writing TCNT0 in any mode of operation will block all compare matches for one timer clock cycle, there are risks

involved when changing TCNT0 when using any of the Output Compare channels, independent of whether the Timer/

Counter is running or not. If the value written to TCNT0 equals the OCR0x value, the compare match will be missed, result-

ing in incorrect waveform generation. Do not write the TCNT0 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 TCNT0

value equal to BOTTOM when the counter is downcounting.

The setup of the OC0x should be performed before setting the Data Direction Register for the port pin to output. The easi-

est way of setting the OC0x value is to use the Force Output Compare (0x) strobe bits in Normal mode. The OC0x Register

keeps its value even when changing between Waveform Generation modes.

Be aware that the COM0x1:0 bits are not double buffered together with the compare value. Changing the COM0x1:0 bits

will take effect immediately.

12.7 Compare Match Output Unit

The Compare Output Mode (COM0x1:0) bits have two functions. The Waveform Generator uses the COM0x1:0 bits for

defining the Output Compare (OC0x) state at the next compare match. Secondly the COM0x1:0 bits control the OC0x pin

output source. Figure 12-7 on page 70 shows a simplified schematic of the logic affected by the COM0x1:0 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 COM0x1:0 bits are shown. When referring to the OC0x state, the reference is for

the internal OC0x Register, not the OC0x pin. If a system reset occur, the OC0x Register is reset to “0”.

Figure 12-7. Compare Match Output Unit, Schematic (non-PWM Mode)

The general I/O port function is overridden by the Output Compare (OC0x) from the Waveform Generator if either of the

COM0x1:0 bits are set. However, the OC0x pin direction (input or output) is still controlled by the Data Direction Register

(DDR) for the port pin. The Data Direction Register bit for the OC0x pin (DDR_OC0x) must be set as output before the

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

DATA BU

FOCnx

clk

I/O