HT48R005 datasheet(31/62 Pages) HOLTEK

31 / 62 page

HT48R005/HT46R005

Rev. 1.00

November 29, 2011

Timer/Event Counters

The provision of timers form an important part of any

microcontroller, giving the designer a means of carrying

out time related functions. The devices contain from one

count-up timer of 8-bit capacity. As the timer has three

different operating modes, it can be configured to oper-

ate as a general timer, an external event counter or as a

pulse width capture device. The provision of an internal

prescaler to the clock circuitry on gives added range to

the timer.

There are two types of registers related to the

Timer/Event Counters. The first is the register that con-

tains the actual value of the timer and into which an ini-

tial value can be preloaded. Reading from this register

retrieves the contents of the Timer/Event Counter. The

second type of associated register is the Timer Control

mines how the timer is to be used. The device can have

the timer clock configured to come from the internal

clock source. In addition, the timer clock source can also

be configured to come from an external timer pin.

Configuring the Timer/Event Counter Input Clock

Source

The Timer/Event Counter clock source can originate

from various sources, an internal clock or an external

pin. The internal clock source source is used when the

timer is in the timer mode or in the pulse width capture

mode. For some Timer/Event Counters, this internal

clock source is first divided by a prescaler, the division

ratio of which is conditioned by the Timer Control Regis-

ter bits T0PSC0~T0PSC2. For Timer/Event Counter,

the internal clock source is fro the fSYS.

An external clock source is used when the timer is in the

event counting mode, the clock source being provided

on an external timer pin TMR. Depending upon the con-

dition of the T0EG bit, each high to low, or low to high

transition on the external timer pin will increment the

counter by one.

Timer Registers

- TMR

The timer register is special function register located in

the Special Purpose Data Memory and is the place

where the actual timer value is stored. This register is

known as TMR0. The value in the timer registers in-

creases by one each time an internal clock pulse is re-

ceived or an external transition occurs on the external

timer pin. The timer will count from the initial value

loaded by the preload register to the full count of FFH at

which point the timer overflows and an internal interrupt

signal is generated. The timer value will then be reset

with the initial preload register value and continue count-

ing.

Note that to achieve a maximum full range count of FFH,

the preload register must first be cleared to all zeros. It

should be noted that after power-on, the preload regis-

ters will be in an unknown condition. Note that if the

Timer/Event Counter is in an OFF condition and data is

written to its preload register, this data will be immedi-

ately written into the actual counter. However, if the

counter is enabled and counting, any new data written

into the preload data register during this period will re-

main in the preload register and will only be written into

the actual counter the next time an overflow occurs.

Timer Control Registers

- TMR0C

The flexible features of the Holtek microcontroller

Timer/Event Counters enable them to operate in three

different modes, the options of which are determined by

the contents of their respective control register.

The Timer Control Register is known as TMR0C. It is the

Timer Control Register together with its corresponding

timer register that control the full operation of the

Timer/Event Counter. Before the timer can be used, it is

essential that the Timer Control Register is fully pro-

grammed with the right data to ensure its correct opera-

tion, a process that is normally carried out during

program initialisation.

To choose which of the three modes the timer is to oper-

ate in, either in the timer mode, the event counting mode

or the pulse width capture mode, bits 7 and 6 of the

Timer Control Register, which are known as the bit pair

T0M1/T0M0, must be set to the required logic levels.

The timer-on bit, which is bit 4 of the Timer Control Reg-

ister and known as T0ON, provides the basic on/off con-

T M R

T i m e r / E v e n t C o u n t e r

M o d e C o n t r o l

T 0 O N

P r e l o a d R e g i s t e r

T i m e r / E v e n t

C o u n t e r

D a t a B u s

R e l o a d

O v e r f l o w

t o I n t e r r u p t

P r e s c a l e r

T 0 P S C 2 ~ T 0 P S C 0

T 0 M 1 T 0 M 0

f S Y S

B u z z e r

T 0 E

8-bit Timer/Event Counter Structure

HT48R005 Datasheet(PDF) 31 Page - Holtek Semiconductor Inc

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