110
2467S–AVR–07/09
ATmega128
Timer/Counter Prescaler
Figure 45.
Prescaler for Timer/Counter0
The clock source for Timer/Counter0 is named clk
T0
. clk
T0
is by default connected to the
main system clock clk
I/O
. By setting the AS0 bit in ASSR, Timer/Counter0 is asynchro-
nously clocked from the TOSC1 pin. This enables use of Timer/Counter0 as a Real
Time Counter (RTC). When AS0 is set, pins TOSC1 and TOSC2 are disconnected from
Port C. A crystal can then be connected between the TOSC1 and TOSC2 pins to serve
as an independent clock source for Timer/Counter0. The Oscillator is optimized for use
with a 32.768 kHz crystal. Applying an external clock source to TOSC1 is not
recommended.
For Timer/Counter0, the possible prescaled selections are: clk
T0S
/8, clk
T0S
/32, clk
T0S
/64,
clk
T0S
/128, clk
T0S
/256, and clk
T0S
/1024. Additionally, clk
T0S
as well as 0 (stop) may be
selected. Setting the PSR0 bit in SFIOR resets the prescaler. This allows the user to
operate with a predictable prescaler.
Special Function IO Register –
SFIOR
• Bit 7 – TSM: Timer/Counter Synchronization Mode
Writing the TSM bit to one activates the Timer/Counter Synchronization mode. In this
mode, the value that is written to the PSR0 and PSR321 bits is kept, hence keeping the
corresponding prescaler reset signals asserted. This ensures that the corresponding
Timer/Counters are halted and can be configured to the same value without the risk of
one of them advancing during configuration. When the TSM bit is written to zero, the
PSR0 and PSR321 bits are cleared by hardware, and the Timer/Counters start counting
simultaneously.
10-BIT T/C PRESCALER
TIMER/COUNTER0 CLOCK SOURCE
clk
I/O
clk
T0S
TOSC1
AS0
CS00
CS01
CS02
clk
T0S
/8
clk
T0S
/64
clk
T0S
/128
clk
T0S
/1024
clk
T0S
/256
clk
T0S
/32
0
PSR0
Clear
clk
T0
Bit
7
6
5
4
3
2
1
0
TSM
–
–
–
ACME
PUD
PSR0
PSR321
SFIOR
Read/Write
R/W
R
R
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
