P87C51RB2BBD NXP Semiconductors, P87C51RB2BBD Datasheet - Page 39

• Manufacturer Part Number: P87C51RB2BBD
• Description: MCU 8-Bit 87C 80C51 CISC 16KB EPROM 3.3V/5V 44-Pin LQFP Tray
• Manufacturer: NXP Semiconductors

Specifications of P87C51RB2BBD

• Package: 44LQFP
• Device Core: 80C51
• Family Name: 87C
• Maximum Speed: 33 MHz
• Ram Size: 512 Byte
• Program Memory Size: 16 KB
• Operating Supply Voltage: 3.3|5 V
• Data Bus Width: 8 Bit
• Program Memory Type: EPROM
• Number Of Programmable I/os: 32
• Interface Type: UART
• Operating Temperature: 0 to 70 °C
• Number Of Timers: 3

Philips Semiconductors
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 30 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.
In order to hold off the reset, the user has three options:
1. periodically change the compare value so it will never match the
2. periodically change the PCA timer value so it will never match
3. disable the watchdog by clearing the WDTE bit before a match
2003 Jan 24
80C51 8-bit microcontroller family
with 512B/1KB RAM, low voltage (2.7 to 5.5 V), low power, high
speed (30/33 MHz)
PCA timer,
the compare values, or
occurs and then re-enable it.
WRITE TO
CCAP4H
1
WRITE TO
CCAP4L
0
RESET
ENABLE
Figure 30. PCA Watchdog Timer mode (Module 4 only)
8KB/16KB/32KB/64KB OTP
CCAP4H
16–BIT COMPARATOR
CH
PCA TIMER/COUNTER
CIDL
––
ECOMn
WDTE
CCAP4L
CL
CAPPn
39
––
0
MODULE 4
timer is never disabled as in option #3. If the program counter ever
MATCH
The first two options are more reliable because the watchdog
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 31 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 31.
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
2
16
CAPNn
count of the PCA timer.
––
0
MATn
––
1
P87C51RA2/RB2/RC2/RD2
TOGn
CPS1
X
CPS0
PWMn
0
ECCFn
ECF
X
CMOD
(D9H)
RESET
CCAPM4
(DEH)
SU01612
Product data