MPC555CME Freescale Semiconductor, MPC555CME Datasheet - Page 224

• Manufacturer Part Number: MPC555CME
• Description: KIT EVALUATION FOR MPC555
• Manufacturer: Freescale Semiconductor
• Type: Microcontrollerr

Specifications of MPC555CME

• Contents: Module Board, Installation Guide, Power Supply, Cable, Software and more
• Processor To Be Evaluated: MPC555
• Data Bus Width: 32 bit
• Interface Type: RS-232
• For Use With/related Products: MPC555
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

6.10 Software Watchdog Timer (SWT)
MPC555
USER’S MANUAL
pitrtclk
Clock
The timeout period is calculated as:
Solving this equation using a 4-MHz external clock and a pre-divider of 256 gives:
This gives a range from 64 microseconds, with a PITC of 0x0000, to 4.19 seconds,
with a PITC of 0xFFFF. When a 20-MHz crystal is used with a pre-divider of 256, the
range is between 12.8 microseconds to 0.84 seconds.
The software watchdog timer (SWT) prevents system lockout in case the software be-
comes trapped in loops with no controlled exit. The SWT is enabled after system reset
to cause a system reset if it times out. it. The SWT requires a special service sequence
to be executed on a periodic basis. If this periodic servicing action does not occur, the
SWT times out and issues a reset or a non-maskable interrupt (NMI), depending on
the value of the SWRI bit in the SYPCR.
The SWT can be disabled by clearing the SWE bit in the SYPCR. Once the SYPCR is
written by software, the state of the SWE bit cannot be changed.
The SWT service sequence consists of the following two steps:
The service sequence clears the watchdog timer and the timing process begins again.
If any value other than 0x556C or 0xAA39 is written to the SWSR, the entire sequence
must start over.
1. Write 0x556C to the Software Service Register (SWSR)
2. Write 0xAA39 to the SWSR
/
MPC556
PITF (PISCR)
(PISCR)
Disable
Clock
PTE
SYSTEM CONFIGURATION AND PROTECTION
PIT
Figure 6-7 PIT Block Diagram
period
Rev. 15 October 2000
=
PITperiod
(PISCR)
Modulus
Counter
PITC
----------------------- -
F
PITC
16-bit
pitrtclk
+
1
=
=
---------------------------------------------- -


ExternalClock
---------------------------- -
PITC
----------------------- -
15625
PITC
4or256
PIE (PISCR)
PS (PISCR)
+
+
1
1


MOTOROLA
Interrupt
PIT
6-16