AT90S/LS4434 and AT90S/LS8535
36
The Stop condition provides a Timer Enable/Disable function. The CK down divided modes are scaled directly from the CK
oscillator clock. If the external pin modes are used, the corresponding setup must be performed in the actual Direction Con-
trol Register (cleared to zero gives an input pin).
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 tem-
porary register (TEMP). This temporary 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 disabled 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 register. 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 register 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 write access. If Timer/Counter1
is written to and a clock source is selected, the Timer/Counter1 continues counting in the timer clock cycle after it is preset
with the written value.
Table 14.
Clock 1 Prescale Select
CS12
CS11
CS10
Description
0
0
0
Stop, the Timer/Counter1 is stopped.
0
0
1
CK
0
1
0
CK/8
0
1
1
CK/64
1
0
0
CK/256
1
0
1
CK/1024
1
1
0
External Pin T1, falling edge
1
1
1
External Pin T1, rising edge
Bit
15
14
13
12
11
10
9
8
$2D ($4D)
MSB
TCNT1H
$2C ($4C)
LSB
TCNT1L
7
6
5
4
3
2
1
0
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
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
0
0
0
0
0
0
0
0
