HT37B90 datasheet(24/80 Pages) HOLTEK

24 / 80 page

HT37B90/HT37B70/HT37B50/HT37B30

Rev.1.00

June22,2017

ConfiguringtheTimer/EventCounterInputClock

Source

The internal timer

¢s clock can originate from various

sources, depending upon timer is chosen. The system

clock input timer source is used when the timer is in the

timermodeorinthepulsewidthmeasurementmode.

For Timer/Event Counter 0, these system clock timer

sourceisselectedbyTMR0C.5.

For Timer/Event Counter 1, 2 this system clock timer

source is first divided by a prescaler, the division ratio of

which is conditioned by the Timer Control Register bits

T1PSC0~T1PSC2.

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

event counting mode, the clock source being provided

on the external timer pin, known as TMR0 or TMR1.

These external timer pins are pin-shared with other I/O

pins. Depending upon the condition of PA share pin op-

tion, each high to low, or low to high transition on the ex-

ternaltimerinputpinwillincrementthecounterbyone.

TimerRegisters - TMR0H/TMR0L,TMR1,TMR2

The timer registers are special function registers located

in the special purpose Data Memory and is the place

where the actual timer value is stored. For the 8-bit timer,

this register is known as Timer/Event Counter 1/2. In the

case of the 16-bit timer, a pair of 8-bit registers are re-

quired to store the 16-bit timer values. These are known

as TMR1L/TMR1H. 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 for the 8-bit

timer or FFFFH for the 16-bit timers, at which point the

timer overflows and a timer internal interrupt signal is

generated. The timer value will then be reset with the ini-

tial preload register value and continue counting.

Note that to achieve a maximum full range count of FFH

for the 8-bit timer or FFFFH for the 16-bit timers, the

preload registers 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 Counters are in an OFF condition and data

is written to their preload registers, this data will be im-

mediately 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.

Note also that when the timer registers are read, the

timer clock will be blocked to avoid errors, however, as

this may result in certain timing errors, programmers

musttakethisintoaccount.

The 16-bit Timer/Event Counter have contained both

low byte and high byte timer registers, accessing these

registers is carried out in a specific way. It must be noted

that when using instructions to preload data into the low

byte register, namely TMR0L, the data will only be

placed in a low byte buffer and not directly into the low

byte register. The actual transfer of the data into the low

byte register is only carried out when a write to its asso-

ciated high byte register, namely TMR0H, is executed.

On the other hand, using instructions to preload data

into the high byte timer register will result in the data be-

ing directly written to the high byte register. At the same

time the data in the low byte buffer will be transferred

into its associated low byte register. For this reason,

when preloading data into the 16-bit timer registers, the

low byte should be written first. It must also be noted that

to read the contents of the low byte register, a read to the

b 7

T 0 E

T 0 O N

T 0 M 0

T 0 M 1

b 0

E v e n t C o u n t e r A c t i v e E d g e S e l e c t

1 : c o u n t o n f a l l i n g e d g e

0 : c o u n t o n r i s i n g e d g e

T i m e r / E v e n t C o u n t e r C o u n t i n g E n a b l e

1 : e n a b l e

0 : d i s a b l e

O p e r a t i n g M o d e S e l e c t

T 0 M 1

T 0 M 0

n o m o d e a v a i l a b l e

e v e n t c o u n t e r m o d e

t i m e r m o d e

p u l s e w i d t h m e a s u r e m e n t m o d e

P u l s e W i d t h M e a s u r e m e n t A c t i v e E d g e S e l e c t

1 : s t a r t c o u n t i n g o n r i s i n g e d g e , s t o p o n f a l l i n g e d g e

0 : s t a r t c o u n t i n g o n f a l l i n g e d g e , s t o p o n r i s i n g e d g e

T M R 0 C R e g i s t e r

N o t i m p l e m e n t e d , r e a d a s " 0 "

T 0 S

T i m e r C l o c k S o u r c e

1 : R C 1 2 K

0 : f O S C / 8

Timer/EventCounter0ControlRegister

HT37B90 Datasheet(PDF) 24 Page - Holtek Semiconductor Inc

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