ST6255CB3 datasheet(44/86 Pages) STMICROELECTRONICS

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ST62T55C ST62T65C/E65C

TIMER (Cont’d)

4.2.1 Timer Operating Modes

There are three operating modes, which are se-

lected by the TOUT and DOUT bits (see TSCR

two clocks which can be connected to the 7-bit

prescaler (fINT ÷ 12 or TIMER pin signal), and to

the output mode.

4.2.1.1 Gated Mode

(TOUT = “0”, DOUT = “1”)

In this mode the prescaler is decremented by the

Timer clock input (fINT ÷ 12), but ONLY when the

signal on the TIMER pin is held high (allowing

pulse width measurement). This mode is selected

by clearing the TOUT bit in the TSCR register to

“0” (i.e. as input) and setting the DOUT bit to “1”.

PC1 must be configured in input mode

4.2.1.2 Event Counter Mode

(TOUT = “0”, DOUT = “0”)

In this mode, the TIMER pin is the input clock of

the prescaler which is decremented on the rising

edge.

4.2.1.3 Output Mode

(TOUT = “1”, DOUT = data out)

The TIMER pin is connected to the DOUT latch,

hence the Timer prescaler is clocked by the pres-

caler clock input (fINT ÷ 12).

The user can select the desired prescaler division

ratio through the PS2, PS1, PS0 bits. When the

TCR count reaches 0, it sets the TMZ bit in the

TSCR. The TMZ bit can be tested under program

control to perform a timer function whenever it

goes high. The low-to-high TMZ bit transition is

used to latch the DOUT bit of the TSCR and trans-

fer it to the TIMER pin. This operating mode allows

external signal generation on the TIMER pin.

Table 12. Timer Operating Modes

4.2.2 Timer Interrupt

When the counter register decrements to zero with

the ETI (Enable Timer Interrupt) bit set to one, an

interrupt request is generated as described in the

Interrupt Chapter. When the counter decrements

to zero, the TMZ bit in the TSCR register is set to

one.

Figure 26. Timer Working Principle

TOUT

DOUT

Timer Pin

Timer Functi on

Input

Event Counter

Input

Gated Input

Output

Output “0”

Output

Output “1”

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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