MC68HC05X16 Datasheet, PDF (138/232 Pages) Freescale Semiconductor, Inc – High-density complementary metal oxide semiconductor HCMOS) microcontroller unit

Freescale Semiconductor, Inc.
Main CPU
clock
S
R
Latch
f_osc/2 _{÷ 4}
f_osc/32 prescaler
÷ 256
(Bit 7 of free
running counter)
÷ 7 watchdog
counter
Power-on
Reset
pin
WDOG bit
Control logic
Schmitt
trigger
Input
protection
Figure 10-3 Watchdog system block diagram
The watchdog system can be automatically enabled, following power-on or external reset, via a
mask option (see Section 1.2), or it can be enabled by software by writing a ‘1’ to the WDOG bit in
the miscellaneous register at $000C (see Section 10.1.2). Once enabled, the watchdog system
cannot be disabled by software (writing a ‘zero’ to the WDOG bit has no effect at any time). In
addition, the WDOG bit acts as a reset mechanism for the watchdog counter. Writing a ‘1’ to this
bit clears the counter to its initial value and prevents a watchdog timeout.
10
WDOG — Watchdog enable/disable
The WDOG bit can be used to enable the watchdog timer previously disabled by a mask option.
Following a watchdog reset the state of the WDOG bit is as defined by the mask option specified.
1 (set) – Watchdog enabled and counter cleared.
0 (clear) – The watchdog cannot be disabled by software; writing a zero to this
bit has no effect.
The divide-by-7 watchdog counter will generate a main reset of the chip when it reaches its final
state; seven clocks are necessary to bring the watchdog counter from its clear state to its final
state. This reset appears after time t_DOG since the last clear or since the enable of the watchdog
counter system. The watchdog counter, therefore, has to be cleared periodically, by software, with
a period less than t_DOG.
The reset generated by the watchdog system is apparent at the RESET pin (see Figure 10-3). The
RESET pin level is re-entered in the control logic, and when it has been maintained at level ‘zero’
for a minimum of t_DOGL, the RESET pin is released.
RESETS AND INTERRUPTS
MC68HC05X16
For More Information On This Product,
Rev. 1
Go to: www.freescale.com