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ST62T46B/E46B

4.2 TIMER 1 & 2

The MCU features two on-chip Timer peripheral

named TIMER 1 & TIMER 2. Each of these timers

consist of an 8-bit counter with a 7-bit programma-

ble prescaler, giving a maximum count of 2

15.

The content of the 8-bit counter can be read/writ-

ten in the Timer/Counter register, TCR, while the

state of the 7-bit prescaler can be read in the PSC

the TSCR register as described in the following

paragraphs.

The 8-bit counter is decremented by the output

(rising edge) coming from the 7-bit prescaler and

can be loaded and read under program control.

When it decrements to zero then the TMZ (Timer

Zero) bit in the TSCR is set to “1”. If the ETI (Ena-

ble Timer Interrupt) bit in the TSCR is also set to

“1”, an interrupt request is generated as described

in the Interrupt Chapter. The Timer interrupt can

be used to exit the MCU from WAIT mode.

The prescaler input is the internal frequency fINT

divided by 12 (TIMER 1 & 2). The prescaler decre-

ments on the rising edge. Depending on the divi-

sion factor programmed by PS2, PS1 and PS0 bits

in the TSCR. The clock input of the timer/counter

vision factor 1, the clock input of the prescaler is

also that of timer/counter; for factor 2, bit 0 of the

prescaler register is connected to the clock input of

TCR. This bit changes its state at half the frequen-

cy of the prescaler input clock. For factor 4, bit 1 of

the PSC is connected to the clock input of TCR,

and so forth. The prescaler initialize bit, PSI, in the

TSCR register must be set to “1” to allow the pres-

caler (and hence the counter) to start. If it is

cleared to “0”, all the prescaler bits are set to “1”

and the counter is inhibited from counting. The

prescaler can be loaded with any value between 0

and 7Fh, if bit PSI is set to “1”. The prescaler tap is

selected by means of the PS2/PS1/PS0 bits in the

control register.

Figure 21 illustrates the Timer’s working principle.

Figure 21. Timer Working Principle

BIT0

BIT1

BIT2

BIT3

BIT6

BIT5

BIT4

CLOCK

7-BIT PRESCALER

8-1 MULTIPLEXER

8-BIT COUNTER

BIT0

BIT1

BIT2

BIT3

BIT4

BIT5

BIT6

BIT7

PS0

PS1

PS2

VA00186

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