HD64F2149YV datasheet(446/1075 Pages) RENESAS

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Section 14 Watchdog Timer (WDT)

Rev. 3.00 Jan 18, 2006 page 418 of 1044

REJ09B0280-0300

14.3

Operation

14.3.1

Watchdog Timer Operation

To use the WDT as a watchdog timer, set the WT/

IT and TME bits in TCSR to 1. Software must

prevent TCNT overflows by rewriting the TCNT value (normally by writing H'00) before

overflow occurs. This ensures that TCNT does not overflow while the system is operating

normally. If TCNT overflows without being rewritten because of a system crash or other error, an

internal reset or NMI interrupt request is generated.

When the RST/

NMI bit is set to 1, the chip is reset for 518 system clock periods (518 φ) by a

counter overflow, and at the same time a low-level signal is output from the

RESO pin for 132

states. This is illustrated in figure 14.3. The system can be reset using this

RESO signal.

When the RST/

NMI bit cleared to 0, an NMI interrupt request is generated by a counter overflow.

In this case, the

RESO output signal remains high.

An internal reset request from the watchdog timer and reset input from the

RES pin are handled

via the same vector. The reset source can be identified from the value of the XRST bit in SYSCR.

If a reset caused by an input signal from the

RES pin and a reset caused by WDT overflow occur

simultaneously, the

RES pin reset has priority, and the XRST bit in SYSCR is set to 1.

An NMI interrupt request from the watchdog timer and an interrupt request from the NMI pin are

handled via the same vector. Simultaneous handling of a watchdog timer NMI interrupt request

and an NMI pin interrupt request must therefore be avoided.

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