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4501

SINGLE-CHIP 4-BIT CMOS MICROCOMPUTER

MITSUBISHI ELECTRIC  

MITSUBISHI MICROCOMPUTERS

4501 Group

SINGLE-CHIP 4-BIT CMOS MICROCOMPUTER

TIMERS

The 4501 Group has the following timers.

• Programmable timer

The programmable timer has a reload register and enables the

frequency dividing ratio to be set. It is decremented from a set-

ting value n. When it underflows (count to n + 1), a timer interrupt

request flag is set to “1,” new data is loaded from the reload reg-

ister, and count continues (auto-reload function).

• Fixed dividing frequency timer

The fixed dividing frequency timer has the fixed frequency divid-

ing ratio (n). An interrupt request flag is set to “1” after every n

count of a count pulse.

F F1 6

n : Counter initial value

Count starts

Reload

n

1st underflow

Reload

2nd underflow

0016

Timer interrupt “1”

request flag “0”

n+1 count

n+1 count

Time

An interrupt occurs or

a skip instruction is executed.

Fig. 21 Auto-reload function

The 4501 Group timer consists of the following circuits.

• Prescaler : frequency divider

• Timer 1 : 8-bit programmable timer

• Timer 2 : 8-bit programmable timer

(Timers 1 and 2 have the interrupt function, respectively)

• 16-bit timer

Prescaler and timers 1 and 2 can be controlled with the timer con-

trol registers W1, W2 and W6. The 16-bit timer is a free counter

which is not controlled with the control register.

Each function is described below.

Table 9 Function related timers

Circuit

Structure

Count source

Prescaler

Timer 1

Timer 2

16-bit timer

Frequency divider

• Instruction clock

8-bit programmable • Prescaler output (ORCLK)

binary down counter

(link to INT input)

8-bit programmable • Timer 1 underflow

binary down counter • Prescaler output (ORCLK)

• CNTR input

• System clock

16-bit fixed dividing • Instruction clock

frequency binary down

counter

Frequency

dividing ratio

4, 16

1 to 256

1 to 256

65536

Use of output signal

• Timer 1 and 2 count sources

• Timer 2 count source

• CNTR output

• Timer 1 interrupt

• CNTR output

• Timer 2 interrupt

• Watchdog timer

(The 16th bit is counted twice)

Control

register

W1

W1

W2

W2

24

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