HT46R01A データシートの表示（PDF） - Holtek Semiconductor

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HT46R01A

Small Package A/D Type 8-Bit OTP MCU

Holtek Semiconductor 

HT46R01A

turns to its original low level. As before, the T0ON bit will

be automatically reset to zero and the timer will stop

counting. It is important to note that in the Pulse Width

Measurement Mode, the T0ON bit is automatically reset

to zero when the external control signal on the external

timer pin returns to its original level, whereas in the other

two modes the T0ON bit can only be reset to zero under

program control. The residual value in the timer, which

can now be read by the program, therefore represents

the length of the pulse received on the external timer

pin. As the T0ON bit has now been reset, any further

transitions on the external timer pin, will be ignored. Not

until the T0ON bit is again set high by the program can

the timer begin further pulse width measurements. In

this way, single shot pulse measurements can be easily

made. It should be noted that in this mode the counter is

controlled by logical transitions on the external timer pin

and not by the logic level. As in the case of the other two

modes, when the counter is full, the timer will overflow

and generate an internal interrupt signal. The counter

will also be reset to the value already loaded into the

preload register. If the external timer pin is pin-shared

with other I/O pins, to ensure that the pin is configured to

operate as a pulse width measuring input pin, two things

have to happen. The first is to ensure that the

T0M1/T0M0 bits place the Timer/Event Counter in the

pulse width measuring mode, the second is to ensure

that the port control register configures the pin as an in-

put. It should be noted that a timer overflow and corre-

sponding timer interrupt is one of the wake-up sources.

Note that the timer interrupts can be disabled by ensur-

ing that the ET0I bits in the INTC0 register are reset to

zero.

Programmable Frequency Divider - PFD

The PFD output is pin-shared with the I/O pin PA1. The

PFD on/off function and its timer source are selected via

bits in the CTRL0 register, however, if not selected, the

pin can operate as a normal I/O pin. The timer overflow

signal is the clock source for the PFD circuit. The output

frequency is controlled by loading the required values

into the timer register and if available the timer prescaler

registers to give the required frequency. The timer/event

counter, driven by the system clock and if applicable, di-

vided by the prescaler value, will begin to count-up from

this preloaded register value until full, at which point an

overflow signal will be generated, causing the PFD out-

put to change state. The counter will then be automati-

cally reloaded with the preload register value and once

again continue counting-up.

For the PFD output to function, it is essential that the

corresponding bit of the Port A control register PAC bit 1

is setup as an output. If setup as an input the PFD output

will not function, however, the pin can still be used as a

normal input pin. The PFD output will only be activated if

bit PA1 is set to ²1². This output data bit is used as the

on/off control bit for the PFD output. Note that the PFD

output will be low if the PA1 output data bit is cleared to

²0².

Using this method of frequency generation, and if a

crystal oscillator is used for the system clock, very pre-

cise values of frequency can be generated.

Prescaler

Bits T0PSC0~T0PSC2 of the TMR0C register can be

used to define a division ratio for the internal clock

source of the Timer/Event Counter enabling longer time

out periods to be setup.

I/O Interfacing

The Timer/Event Counter, when configured to run in the

event counter or pulse width measurement mode, re-

quires the use of the external timer pin for its operation.

As this pin is a shared pin it must be configured correctly

to ensure that it is setup for use as a Timer/Event Coun-

ter input pin. This is achieved by ensuring that the mode

select bits in the Timer/Event Counter control register,

select either the event counter or pulse width measure-

ment mode. Additionally the corresponding PAC Port

Control Register bit must be set high to ensure that the

pin is setup as an input. Any pull-high resistor connected

to this pin will remain valid even if the pin is used as a

Timer/Event Counter input.

T im e r O v e r flo w

P F D C lo c k

P A 1 D a ta

P F D O u tp u t a t P A 1

PFD Output Control

Rev. 1.10

20

August 13, 2008

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