Motorola MPC823e - Software Service Register

System Interface Unit
MOTOROLA MPC823e REFERENCE MANUAL 12-27
SYSTEM INTERFACE UNIT
12
The decrementer begins counting when it is loaded with a value from the SWTC field. After
the timer reaches 0x0, a software watchdog expiration request is issued to the reset or NMI
control logic. At reset, the value in the SWTC register is set to the maximum value and is
loaded into the software watchdog register (SWR) again, thus starting the process over.
When a new value is loaded into the SWTC register, the software watchdog timer will not be
updated until the servicing sequence is written to the SWSR register. If the SWE bit is loaded
with the value 0, the modulus counter will not count.
12.9.1 Software Service Register
The software service register (SWSR) is the location that the software watchdog timer
servicing sequence writes to. To prevent a SWT timeout, a write of 0x556C followed by
0xAA39 must be written to this register. The SWSR can be written at any time, but returns
all zeros when read.
SEQ—Sequence
This field is the pattern that is used to control the state of the software watchdog timer.
Figure 12-7. Software Watchdog Timer Block Diagram
SWSR
BIT 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
FIELD SEQ
RESET 0
R/W W
ADDR (IMMR & 0xFFFF0000) + 0x00E
DISABLE
CLOCK
FRZ
SWR / DECREMENTER
TIMEOUT
16-BIT
SWTC
SWE
SERVICE
LOGIC
RELOAD
ROLLOVER = 0
RESET
SWSR
MUX
2,048
CORE
SWP
CLOCK
DIVIDE BY
OR NMI