p89cv51rb2 NXP Semiconductors, p89cv51rb2 Datasheet - Page 49

• Manufacturer Part Number: p89cv51rb2
• Description: P89cv51rb2/rc2/rd2 8-bit 80c51 5 V Low Power 64 Kb Flash Microcontroller With 1 Kb Ram, Spi, 6-clock Cpu With 6/12-clock Peripherals
• Manufacturer: NXP Semiconductors

NXP Semiconductors
P89CV51RB2_RC2_RD2_1
Product data sheet
6.9.5 PCA watchdog timer
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.
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.
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:
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.
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 2
counts.
•
•
•
CLR
MOV
MOV
SETB
RET
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.
EA
CCAP4L,#00
CCAP4H,CH
EA
Rev. 01 — 5 October 2007
;Hold off interrupts
;Next compare value is within 255 counts of
;Re-enable interrupts
current PCA timer value
P89CV51RB2/RC2/RD2
Figure 24
shows a diagram of how the
80C51 with 1 kB RAM, SPI
© NXP B.V. 2007. All rights reserved.
16
PCA timer
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