PXAS30 datasheet(26/52 Pages) PHILIPS

26 / 52 page

Philips Semiconductors

Preliminary specification

XA-S3

XA 16-bit microcontroller

32 K/1 K OTP/ROM/ROMless, 8-channel 8-bit A/D, low voltage (2.7 V–5.5 V),

I2C, 2 UARTs, 16 MB address range

2000 Dec 01

––

ECOMn

CAPPn

CAPNn

MATn

TOGn

PWMn

ECCFn

CCAPM4

(495H)

CCAP4H

CCAP4L

RESET

PCA TIMER/COUNTER

16–BIT COMPARATOR

MATCH

ENABLE

WRITE TO

CCAP4H

RESET

WRITE TO

CCAP4L

CMOD

(490H)

CIDL

WDTE

––

CPS1

CPS0

ECF

SU01313

MODULE 4

Figure 18. PCA Watchdog Timer m(Module 4 only)

PCA Watchdog Timer

An on-board watchdog timer is available with the PCA to improve

the reliability of the system without increasing chip count. Watchdog

timers are useful for systems that are susceptible to noise, power

glitches, or electrostatic discharge. Module 4 is the only PCA

module that can be programmed as a watchdog. However, this

module can still be used for other modes if the watchdog is not

needed.

Figure 18 shows a diagram of how the watchdog works. The user

pre-loads a 16-bit value in the compare registers. Just like the other

compare modes, this 16-bit value is compared to the PCA timer

value. If a match is allowed to occur, an internal reset will be

generated. This will not cause the RST pin to be driven low.

In order to hold off the reset, the user has three options:

1. periodically change the compare value so it will never match the

PCA timer,

2. periodically change the PCA timer value so it will never match

the compare values, or

3. disable the watchdog by clearing the WDTE bit before a match

occurs and then re-enable it.

The first two options are more reliable because the watchdog timer

is never disabled as in option #3. If the program counter ever goes

astray, a match will eventually occur and cause an internal reset.

The second option is also not recommended if other PCA modules

are being used. Remember, the PCA timer is the time base for all

modules; changing the time base for other modules would not be a

good idea. Thus, in most applications the first solution is the best

option.

Figure 19 shows the code for initializing the watchdog timer. Module

4 can be configured in either compare mode, and the WDTE bit in

CMOD must also be set. The user’s software then must periodically

change (CCAP4H,CCAP4L) to keep a match from occurring with the

PCA timer (CH,CL). This code is given in the WATCHDOG routine in

Figure 19.

This routine should not be part of an interrupt service routine,

because if the program counter goes astray and gets stuck in an

infinite loop, interrupts will still be serviced and the watchdog will

keep getting reset. Thus, the purpose of the watchdog would be

defeated. Instead, call this subroutine from the main program within

216 count of the PCA timer.

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