AT90CAN128-16AE ATMEL Corporation, AT90CAN128-16AE Datasheet - Page 143

• Manufacturer Part Number: AT90CAN128-16AE
• Description: 8-bit Avr Microcontroller With 128K Bytes of Isp Flash And CAN Controller.flash (Kbytes) 128 Vcc (V) 2.7-5.5 EEPROM (Kbytes) 4 SRAM (bytes) 4K CAN (mess. Obj.) 15
• Manufacturer: ATMEL Corporation

Timer/Counter Clock
Sources
Counter Unit
4250C–CAN–03/04
Table 65. Definitions
The Timer/Counter can be clocked by an internal synchronous or an external asynchro-
nous clock source. The clock source is selected by the clock select logic which is
controlled by the clock select (CS22:0) bits located in the Timer/Counter control register
(TCCR2).The clock source clk
AS2 bit in the ASSR Register is written to logic one, the clock source is taken from the
Timer/Counter Oscillator connected to TOSC1 and TOSC2 or directly from TOSC1. For
details on asynchronous operation, see “Asynchronous Status Register – ASSR” on
page 155. For details on clock sources and prescaler, see “Timer/Counter2 Prescaler”
on page 159.
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
Figure 61 shows a block diagram of the counter and its surrounding environment.
Figure 61. Counter Unit Block Diagram
Figure 62.
Signal description (internal signals):
Depending on the mode of operation used, the counter is cleared, incremented, or dec-
remented at each timer clock (clk
clock source, selected by the Clock Select bits (CS22:0). When no clock source is
selected (CS22:0 = 0) the timer is stopped. However, the TCNT2 value can be accessed
BOTTOM
MAX
TOP
count
direction
clear
clk
top
bottom
T2
DATA BUS
TCNTn
The counter reaches the BOTTOM when it becomes zero (0x00).
The counter reaches its MAXimum when it becomes 0xFF (decimal 255).
The counter reaches the TOP when it becomes equal to the highest
value in the count sequence. The TOP value can be assigned to be the
fixed value 0xFF (MAX) or the value stored in the OCR2A Register. The
assignment is dependent on the mode of operation.
Increment or decrement TCNT2 by 1.
Selects between increment and decrement.
Clear TCNT2 (set all bits to zero).
Timer/Counter clock.
Signalizes that TCNT2 has reached maximum value.
Signalizes that TCNT2 has reached minimum value (zero).
direction
count
clear
T2
bottom
T2
is by default equal to the MCU clock, clk
Control Logic
). clk
T2
top
can be generated from an external or internal
TOVn
(Int.Req.)
clk
Tn
Prescaler
clk
TnS
AT90CAN128
Oscillator
I/O
T/C
clk
. When the
I/O
TOSC2
TOSC1
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