HD64F2638F20J Datasheet, PDF (481/1512 Pages) Renesas Technology Corp – Renesas 16-Bit Single-Chip Microcomputer H8S Family/H8S/2600 Series

H8S/2639, H8S/2638, H8S/2636,
H8S/2630, H8S/2635 Group
Section 12 Watchdog Timer
Bit 7—Overflow Flag (OVF): Indicates that TCNT has overflowed from H'FF to H'00.
Bit 7
OVF
Description
0
[Clearing conditions]
(Initial value)
• Cleared when 0 is written to the TME bit (Only applies to WDT1)
• Cleared by reading TCSR^* when OVF = 1, then writing 0 to OVF
1
[Setting condition]
• When TCNT overflows (changes from H'FF to H'00)
When internal reset request generation is selected in watchdog timer mode, OVF is
cleared automatically by the internal reset.
Note: * When interval timer interrupts are disabled and OVF is polled, read the OVF = 1 state at
least twice.
In the interval timer mode, the OVF flag can be cleared in the interval timer interrupt routine by
writing 0 to OVF after reading TCSR when OVF is set to 1, in accordance with the conditions for
clearing the OVF flag.
However, when attempting to poll the OVF flag when interval timer interrupts are prohibited the
OVF value will not be recognized as 1 (even though it is set to 1) if there is a conflict between the
timing used to set the OVF flag and the timing used to read the OVF flag.
In such cases it is possible to completely satisfy the conditions for clearing the OVF flag by
reading OVF two or more times while its value is 1. In a situation such as the above, the OVF flag
should be read two or more times while its value is 1 and then cleared.
Bit 6—Timer Mode Select (WT/IT): Selects whether the WDT is used as a watchdog timer or
interval timer. This selection determines whether WDT0 issues an internal reset when TCNT
overflows while bit RSTE of the reset control/status register (RSTCSR) is set to 1. In the interval
timer mode, WDT0 sends a WOVI interrupt request to the CPU. WDT1, on the other hand,
requests a reset or an NMI interrupt from the CPU if the watchdog timer mode is chosen, whereas
it requests a WOVI interrupt from the CPU if the interval timer mode is chosen.
REJ09B0103-0800 Rev. 8.00
May 28, 2010
Page 431 of 1458