Datasheet
163
2549O–AVR–05/12
ATmega640/1280/1281/2560/2561
17.11.15 TCNT4H and TCNT4L –Timer/Counter 4
17.11.16 TCNT5H and TCNT5L –Timer/Counter 5
The two Timer/Counter I/O locations (TCNTnH and TCNTnL, combined TCNTn) give direct
access, both for read and for write operations, to the Timer/Counter unit 16-bit counter. To
ensure that both the high and low bytes are read and written simultaneously when the CPU
accesses these registers, the access is performed using an 8-bit temporary High Byte Register
(TEMP). This temporary register is shared by all the other 16-bit registers. See “Accessing 16-bit
Registers” on page 138.
Modifying the counter (TCNTn) while the counter is running introduces a risk of missing a com-
pare match between TCNTn and one of the OCRnx Registers.
Writing to the TCNTn Register blocks (removes) the compare match on the following timer clock
for all compare units.
17.11.17 OCR1AH and OCR1AL – Output Compare Register 1 A
17.11.18 OCR1BH and OCR1BL – Output Compare Register 1 B
17.11.19 OCR1CH and OCR1CL – Output Compare Register 1 C
Bit 76543210
(0xA5) TCNT4[15:8] TCNT4H
(0xA4) TCNT4[7:0] TCNT4L
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Bit 76543210
(0x125) TCNT5[15:8] TCNT5H
(0x124) TCNT5[7:0] TCNT5L
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Bit 76543210
(0x89) OCR1A[15:8] OCR1AH
(0x88) OCR1A[7:0] OCR1AL
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Bit 76543210
(0x8B) OCR1B[15:8] OCR1BH
(0x8A) OCR1B[7:0] OCR1BL
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Bit 76543210
(0x8D) OCR1C[15:8] OCR1CH
(0x8C) OCR1C[7:0] OCR1CL
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0