P89CV51RB2FBC datasheet(49/73 Pages) NXP

49 / 73 page

P89CV51RB2_RC2_RD2_1

Product data sheet

Rev. 01 — 5 October 2007

49 of 73

NXP Semiconductors

P89CV51RB2/RC2/RD2

80C51 with 1 kB RAM, SPI

module’s CCAPnL SFR, the output will be LOW; when it is equal to, or greater, the output

will be HIGH. When CL overﬂows from FFH to 00H, CCAPnL is reloaded with the value in

CCAPnH. This allows the PWM to be updated without glitches. The PWM and ECOM bits

in the module’s CCAPMn register must be set to enable PWM mode.

6.9.5 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 24 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 HIGH.

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 subroutine WATCHDOG

shown below.

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

• Periodically change the compare value so it will never match the PCA timer.

• Periodically change the PCA timer value so it will never match the compare values.

• 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

the third option. If the program counter ever reaches an undesired value, 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 that the PCA timer is the time base for

all modules; changing the time base for other modules is not recommended. Thus, in

most applications the ﬁrst option is best.

;CALL the following WATCHDOG subroutine periodically.

CLR

;Hold off interrupts

MOV

CCAP4L,#00

;Next compare value is within 255 counts of

current PCA timer value

MOV

CCAP4H,CH

SETB

;Re-enable interrupts

RET

Do not use this routine as part of an interrupt service routine, because if the program

counter would enter an inﬁnite loop, still interrupts will be serviced and the watchdog will

continually keep getting reset. Because this would defeat the purpose of the watchdog, it

is recommended that this subroutine is called from the main program within 216 PCA timer

counts.

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