at43usb320a ATMEL Corporation, at43usb320a Datasheet - Page 46

• Manufacturer Part Number: at43usb320a
• Description: Full-speed Usb Microcontroller With An Embedded Hub
• Manufacturer: ATMEL Corporation

46
AT43USB320A
Timer/Counter1 – TCNT1H and TCNT1L
This 16-bit register contains the prescaled value of the 16-bit Timer/Counter1. 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 register (TEMP). This tem-
porary register is also used when accessing OCR1A, OCR1B and ICR1. If the main program
and also interrupt routines perform access to registers using TEMP, interrupts must be dis-
abled during access from the main program and from interrupt routines if interrupts are
allowed from within interrupt routines.
• TCNT1 Timer/Counter1 Write:
When the CPU writes to the high byte TCNT1H, the written data is placed in the TEMP regis-
ter. Next, when the CPU writes the low byte TCNT1L, this byte of data is combined with the
byte data in the TEMP register, and all 16 bits are written to the TCNT1 Timer/Counter1 regis-
ter simultaneously. Consequently, the high byte TCNT1H must be accessed first for a full 16-
bit register write operation.
• TCNT1 Timer/Counter1 Read:
When the CPU reads the low byte TCNT1L, the data of the low byte TCNT1L is sent to the
CPU and the data of the high byte TCNT1H is placed in the TEMP register. When the CPU
reads the data in the high byte TCNT1H, the CPU receives the data in the TEMP register.
Consequently, the low byte TCNT1L must be accessed first for a full 16-bit register read
operation.
The Timer/Counter1 is realized as an up or up/down (in PWM mode) counter with read and
w rite acce ss. If Time r/Cou nte r1 is written to an d a clo ck so urce is selecte d, the
Timer/Counter1 continues counting in the timer clock cycle after it is preset with the written
value.
Read/Write
Initial Value
$2D ($4D)
$2C ($4C)
Bit
MSB
R/W
R/W
15
–
7
0
0
R/W
R/W
14
–
–
6
0
0
R/W
R/W
13
–
–
5
0
0
R/W
R/W
12
–
–
4
0
0
R/W
R/W
11
–
–
3
0
0
R/W
R/W
10
–
–
2
0
0
R/W
R/W
9
–
–
1
0
0
LSB
R/W
R/W
8
–
0
0
0
1443E–USB–4/04
TCNT1H
TCNT1L