ST7FLCD1 datasheet(80/95 Pages) STMICROELECTRONICS

80 / 95 page

8-bit Timer with External Trigger (TIMB)

ST7FLCD1

80/95

8-bit Timer with External Trigger (TIMB)

14.1

Introduction

Timer B is an 8 bit-programmable free-running downcounter, driven by a programmable prescaler.

An external signal can also trigger the countdown. The Timer B block diagram is shown in

Figure 42.

14.2

Main Features

●

Programmable Prescaler: fCPU divided by 1, 8 or 16

●

Overflow status flag and maskable interrupt

●

Auto reload capability

●

An external signal with programmable polarity can trigger the count-down

14.3

Functional Description

The 8 bit-downcounter timer counts from a start value down to 00h. The start value is preloaded

from the associated 8-bit TIMCPRB register every time it is written, or when the counter has

reached the 00h value (Auto Reload feature) if the TAR bit is set. The OVF flag is set when the

downcounter reaches 00h. An interrupt is generated if the OVFE bit is set.

When the EXT bit is set, an external signal edge triggers the countdown start. The EDG bit controls

the rising or falling signal edge. Once detected, the selected edge sets the EEF flag, preloads the

downcounter with the start value and starts the countdown as usual.

During the countdown, the downcounter cannot be retriggered and subsequent pulses occurring

after the countdown has started are ignored until the counter reaches 00h.

Figure 42: External Timer Block Diagram

8-bit downcounter

Prescaler

OVF

OVFE

OVF interrupt request

TIMCSRB

TAR

1 / 8 / 16

TB1

TB0

EXT

EDG

EEF

Preload register

TIMCPRB

Auto reload

EXTRIG

Interrupt

EDG

EEF

EXT

Control Logic

TIMER

fCPU

Fixed

Prescaler

% 128

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