Rainbow-electronics ATmega128L Uživatelský manuál stáhnout pdf (Strana 104)

104

ATmega128(L)

2467B–09/01

Timer/Counter Prescaler Figure 44. Prescaler for Timer/Counter0

The clock source for Timer/Counter0 is named clk

T0S

. clk

T0S

is by default connected to

the main system clock clk

OSC

. By setting the AS0 bit in ASSR, Timer/Counter0 is asyn-

chronously 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 TSM to one, PSR0 and PSR321 becomes registers that hold their value until

rewritten, or the TSM bit is written zero. This mode is useful for synchronizing

timer/counters. By setting both TSM and the appropriate PSR bit(s), the appropriate

timer/counters are halted, and can be configured to same value without the risk of one of

them advancing during configuration. When the TSM bit written zero, the

Timer/Counters start counting simultaneously.

10-BIT T/C PRESCALER

TIMER/COUNTER0 CLOCK SOURCE

clk

OSC

clk

T0S

TOSC1

AS0

CS00

CS01

CS02

clk

T0S

clk

T0S

/64

clk

T0S

/128

clk

T0S

/1024

clk

T0S

/256

clk

T0S

/32

PSR0

Clear

clk

Bit 7 6 5 4 3 2 1 0

TSM – – ADHSM ACME PUD PSR0 PSR321 SFIOR

Read/Write R/W R R R/W R/W R/W R/W R/W

Initial value 0 0 0 0 0 0 0 0

1 2 ... 99 100 101 102 103 104 105 106 107 108 109 ... 330 331

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