7549 datasheet(44/86 Pages) RENESAS

44 / 86 page

Rev.2.02

Mar 31, 2009

Page 42 of 81

REJ03B0202-0202

7549 Group

Watchdog Timer

The watchdog timer gives a means for returning to a reset status

when the program fails to run on its normal loop due to a runaway.

The watchdog timer consists of an 8-bit watchdog timer H and an

8-bit watchdog timer L, being a 16-bit counter. The operation of

the watchdog timer is controlled by bits 2 to “0” in function set

ROM data 2 and the watchdog timer control register.

• Watchdog timer disable bit

When the watchdog timer disable bit (bit 1 in function set ROM

data 2(FSROM2)) is set to “0”, the watchdog timer is enabled

and starts counting after reset.

Setting this bit to “1” does not operate the watchdog timer.

This bit cannot be rewritten by executing the instruction.

To use the watchdog timer, always set this bit to “0”.

After reset, the watchdog timer cannot start counting by a program.

• Watchdog timer source clock selection bit

The count source of the watchdog timer is selected by the

watchdog timer source clock selection bit (bit 0 in FSROM2).

This bit cannot be rewritten by executing the instruction.

When this bit is set to “0”, the count source is always set to the

low-speed on-chip oscillator output/16.

When this bit is set to “1”, the count source is set to

φSOURCE/

16.

φSOURCE is changed by setting the clock selection bits (bits

5 and 4 in the clock mode register (CLKM: address 003716)).

• Watchdog timer H count source selection bit

The count source of watchdog timer H is selected by the

watchdog timer control register (WDTCON: address 003916).

When the watchdog timer H count source selection bit (bit 7 in

WDTCON) is set to “0”, the count source is set to an underflow

signal from watch dog timer L. When this bit is set to “1”, the

clock selected as the count source of watchdog timer L is input to

watchdog timer H.

The initial value of this bit after releasing reset can be set by the

bit 2 in FSROM2.

• Watchdog Timer Operation

Resetting or writing any data to WDTCON sets watchdog timer H

to “FF16” and watchdog timer L to “FF16”. When the watchdog

timer starts, the selected clock is counted and internal reset occurs

by the watchdog timer H underflow. Writing to WDTCON is

usually programmed to be performed before underflow.

Reading WDTCON reads the values of the high-order 6 bits in

the watchdog timer H counter and the watch dog timer count

source selection bit.

The following shows the time to watchdog timer underflow after

writing to the watchdog timer control register.

The example applies when the XIN input clock is selected as

φSOURCE and f(XIN) = 8 MHz.

• Watchdog timer H count source selection bit = 0: 131.072 ms

• Watchdog timer H count source selection bit = 1: 512

µs

Fig 53. Structure of Function set ROM data 2

Fig 54. Structure of watchdog timer control register

Watchdog timer source clock selection bit

0 : Low-speed on-chip oscillator/16

1 :

φSOURCE/16

Watchdog timer disable bit

0 : Watchdog timer enabled

1 : Watchdog timer disabled

Watchdog timer H count source initial value

selection bit

0 : Initial value of bit 7 of WDTCON

after reset release is “0”

1 : Initial value of bit 7 of WDTCON

after reset release is “1”

STP instruction function selection bit

0 : System enters into the stop mode

at the STP instruction execution

1 : Internal reset occurs at the STP

instruction execution

Low-speed on-chip oscillator control bit

(Note 1)

0 : Stop of low-speed on-chip oscillator

disabled

1 : Stop of low-speed on-chip oscillator

enabled

Set “0” to these bits certainly.

Function set ROM data 2

(FSROM2: address FFDA16)

Note 1: If “0” is set to this bit, it is not possible to write “1” to bit 0 in the clock

mode register. Also, the low-speed on-chip oscillator does not stop

even if in the stop mode.

Watchdog timer H (read only for

high-order 6-bit)

Not used (returns “0” when read)

Watchdog timer H count source

selection bit

0 : Watchdog timer L underflow

1 : Low-speed on-chip oscillator/16

φSOURCE/16

Watchdog timer control register (Note)

(WDTCON: address 003916,

initial value: X01111112)

Note: The initial value of this register is changes by setting of

function set ROM data 2.

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