9202 datasheet(54/175 Pages) STMICROELECTRONICS

54 / 175 page

UPSD3254A, UPSD3254BV, UPSD3253B, UPSD3253BV

54/175

TIMER/COUNTERS (TIMER 0, TIMER 1 AND TIMER 2)

The uPSD325X devices has three 16-bit Timer/

Counter registers: Timer 0, Timer 1 and Timer 2.

All of them can be configured to operate either as

timers or event counters and are compatible with

standard 8032 architecture.

In the “Timer” function, the register is incremented

every machine cycle. Thus, one can think of it as

counting machine cycles. Since a machine cycle

consists of 6 CPU clock periods, the count rate is

1/6 of the CPU clock frequency or 1/12 of the os-

cillator frequency (fOSC).

In the “Counter” function, the register is increment-

ed in response to a 1-to-0 transition at its corre-

sponding external input pin, T0 or T1. In this

function, the external input is sampled during

S5P2 of every machine cycle. When the samples

show a high in one cycle and a low in the next cy-

cle, the count is incremented. The new count value

appears in the register during S3P1 of the cycle

following the one in which the transition was de-

tected. Since it takes 2 machine cycles (24 fOSC

clock periods) to recognize a 1-to-0 transition, the

maximum count rate is 1/24 of the fOSC. There are

no restrictions on the duty cycle of the external in-

put signal, but to ensure that a given level is sam-

pled at least once before it changes, it should be

held for at least one full cycle. In addition to the

“Timer” or “Counter” selection, Timer 0 and Timer

1 have four operating modes from which to select.

Timer 0 and Timer 1

The “Timer” or “Counter” function is selected by

control bits C/ T in the Special Function Register

TMOD. These Timer/Counters have four operat-

ing modes, which are selected by bit-pairs (M1,

M0) in TMOD. Modes 0, 1, and 2 are the same for

Timers/ Counters. Mode 3 is different. The four op-

erating modes are de-scribed in the following text.

Table 36. Control Register (TCON)

Table 37. Description of the TCON Bits

543210

TF1

TR1

TF0

TR0

IE1

IT1

IE0

IT0

Bit

Symbol

Function

7TF1

Timer 1 overflow flag. Set by hardware on Timer/Counter overflow. Cleared by hardware

when processor vectors to interrupt routine

TR1

Timer 1 run control bit. Set/cleared by software to turn Timer/Counter on or off

5TF0

Timer 0 overflow flag. Set by hardier on Timer/Counter overflow. Cleared by hardware

when processor vectors to interrupt routine

TR0

Timer 0 run control bit. Set/cleared by software to turn Timer/Counter on or off

3IE1

Interrupt 1 Edge Flag. Set by hardware when external interrupt edge detected. Cleared

when interrupt processed

2IT1

Interrupt 1 Type Control Bit. Set/cleared by software to specify falling-edge/low-level

triggered external interrupt

1IE0

Interrupt 0 Edge Flag. Set by hardware when external interrupt edge detected. Cleared

when interrupt processed

0IT0

Interrupt 0 Type Control Bit. Set/cleared by software to specify falling-edge/low-level

triggered external interrupt

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