MC68331CEH16 Datasheet, PDF (20/84 Pages) Freescale Semiconductor, Inc – 32-Bit Modular Microcontroller

Freescale Semiconductor, Inc.
3.2.5 Spurious Interrupt Monitor
The spurious interrupt monitor issues BERR if no interrupt arbitration occurs during an interrupt-ac-
knowledge cycle.
3.2.6 Software Watchdog
The software watchdog is controlled by SWE in the SYPCR. Once enabled, the watchdog requires that
a service sequence be written to SWSR on a periodic basis. If servicing does not take place, the watch-
dog times out and issues a reset. This register can be written at any time, but returns zeros when read.
SWSR —Software Service Register
15
RESET:
NOT USED
Register shown with read value
$YFFA27
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Perform a software watchdog service sequence as follows:
1. Write $55 to SWSR.
2. Write $AA to SWSR.
Both writes must occur before time-out in the order listed, but any number of instructions can be exe-
cuted between the two writes.
The watchdog clock rate is affected by SWP and SWT in SYPCR. When SWT[1:0] are modified, a
watchdog service sequence must be performed before the new time-out period takes effect.
The reset value of SWP is affected by the state of the MODCLK pin on the rising edge of reset, as shown
in the following table.
MODCLK
0
1
SWP
1
0
3.2.7 Periodic Interrupt Timer
The periodic interrupt timer (PIT) generates interrupts of specified priorities at specified intervals. Timing
for the PIT is provided by a programmable prescaler driven by the system clock.
PICR — Periodic Interrupt Control Register
$YFFA22
15
14
13
12
11
10
8
7
0
0
0
0
0
0
PIRQL
PIV
RESET:
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
This register contains information concerning periodic interrupt priority and vectoring. Bits [10:0] can be
read or written at any time. Bits [15:11] are unimplemented and always return zero.
PIRQL[2:0] —Periodic Interrupt Request Level
The following table shows what interrupt request level is asserted when a periodic interrupt is generat-
ed. If a PIT interrupt and an external IRQ signal of the same priority occur simultaneously, the PIT in-
terrupt is serviced first. The periodic timer continues to run when the interrupt is disabled.
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