ST6387 datasheet(32/84 Pages) STMICROELECTRONICS

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ST6365, ST6375, ST6385 ST6367, ST6377, ST6387

TIMERS (Cont’d)

4.2.1 Timer Operating Modes

Since in the ST638x devices the external TIMER

pin is not connected, the only allowed operating

mode is the output mode, which is selected by set-

ting bit 4 and by clearing bit 5 in the TSCR1 regis-

ter. This procedure will enable Timer 1 and

Timer 2.

Output Mode (TSCR1 D4 = 1, TSCR1 D5 = 0). On

this mode the timer prescaler is clocked by the

prescaler clock input (OSC/12). The user can se-

lect the desired prescaler division ratio through the

PS2/PS1/PS0 bits. When TCR count reaches 0, it

sets the TMZ bit in the TSCR.

The TMZ bit can be tested under program control

to perform timer functions whenever it goes high.

Bits D4 and D5 on TSCR2 (Timer 2) register are

not implemented.

Timer Interrupt

When the counter register decrements to zero and

the software controlled ETI (enable timer interrupt)

bit is set to one then an interrupt request associat-

ed to interrupt vector #3 (for Timer 1), to interrupt

vector #1 (for Timer 2) is generated. When the

counter decrements to zero also the TMZ bit in the

TSCR register is set to one.

Notes:

TMZ is set when the counter reaches 00h; howev-

er, it may be set by writing 00h in the TCR register

or setting the bit 7 of the TSCR register. TMZ bit

must be cleared by user software when servicing

the timer interrupt to avoid undesired interrupts

when leaving the interrupt service routine. After re-

set, the 8-bit counter register is loaded to FFh

while the 7-bit prescaler is loaded to 7Fh, and the

TSCR register is cleared which means that timer is

stopped (PSI=0) and timer interrupt disabled.

A write to the TCR register will predominate over

the 8-bit counter decrement to 00h function, i.e. if a

write and a TCR register decrement to 00h occur

simultaneously, the write will take precedence,

and the TMZ bit is not set until the 8-bit counter

reaches 00h again. The values of the TCR and the

PSC registers can be read accurately at any time.

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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