Rainbow Electronics AT75C310 - Manual (Page 117)

AT75C310
117
WD: Watchdog Timer
The AT75C310 has an internal watchdog timer which can
be used to prevent system lock-up if the software becomes
trapped in a deadlock. In normal operation, the user
reloads the watchdog at regular intervals before the timer
overflow occurs. If an overflow does occur, the watchdog
timer generates one or a combination of the following sig-
nals depending on the parameters in WD_OMR (Overflow
Mode Register):
• If RSTEN is set, an internal reset is generated
(WD_RESET as shown in Figure 28).
• If IRQEN is set, a pulse is generated on the signal
WDIRQ which is connected to the Advanced Interrupt
Controller.
• If EXTEN is set, a low level is driven on the NWDOVF
signal for a duration of eight ACLK cycles.
The watchdog timer has a 16-bit down counter. Bits 12 to
15 of the value loaded when the watchdog is restarted are
programmable using the HPCV parameter in WD_CMR
(Clock Mode). Four clock sources are available to the
watchdog counter: ACLK/8, ACLK/32, ACLK/128 or
ACLK/1024. The selection is made using the WDCLKS
parameter in WD_CMR. This provides a programmable
time-out period of 1.3 ms to 2.6 seconds with a 24 MHz
system clock.
All write accesses are protected by control access keys to
help prevent corruption of the watchdog should an error
condition occur. To update the contents of the mode and
control registers it is necessary to write the correct bit pat-
tern to the control access key bits at the same time as the
control bits are written (the same write access).
Figure 28. Watchdog Timer Block Diagram
WD User Interface
WD Base Address: 0xFF028000
Offset Register Description Register Name Access Reset State
0x00 Overflow Mode Register WD_OMR Read/write 0
0x04 Clock Mode Register WD_CMR Read/write 0
0x08 Control Register WD_CR Write-only –
0x0C Status Register WD_SR Read-only 0
Advanced
Peripheral
Bus (APB)
WD_RESET
WDIRQ
ACLK/8
ACLK/32
ACLK/128
ACLK/1024
Control Logic
Clock Select
16-Bit
Programmable
Down Counter
CLK_CNT
Clear
Overflow
NWDOVF