99
AT94KAL Series FPSLIC
Rev. 1138G–FPSLI–11/03
• Bits 2,1,0 - CS12, CS11, CS10: Clock Select1, Bits 2, 1 and 0
The Clock Select1 bits 2,1 and 0 define the prescaling source of Timer/Counter1.
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 for
Timer/Counter1, transitions on PE4/(T1) will clock the counter even if the pin is configured as
an output. This feature can give the user SW control of the counting.
Timer/Counter1 Register – 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 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.
Table 29.
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 PE4 (T1), falling edge
1
1
1
External pin PE4 (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