mc68331cpv20b1 Freescale Semiconductor, Inc, mc68331cpv20b1 Datasheet - Page 49

• Manufacturer Part Number: mc68331cpv20b1
• Description: Mc68331 32 Bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

4.2.11 Periodic Interrupt Timer
MC68331
USER’S MANUAL
LPSTOP
FREEZE
EXTAL
SWT0
SWT1
SWP
SWE
PTP
The periodic interrupt timer allows the generation of interrupts of specific priority at pre-
determined intervals. This capability is often used to schedule control system tasks
that must be performed within time constraints. The timer consists of a prescaler, a
modulus counter, and registers that determine interrupt timing, priority and vector as-
signment. Refer to SECTION 5 CENTRAL PROCESSING UNIT for further information
about interrupt exception processing.
The periodic interrupt modulus counter is clocked by a signal derived from the buffered
crystal oscillator (EXTAL) input pin unless an external frequency source is used. The
value of the periodic timer prescaler (PTP) bit in the periodic interrupt timer register
(PITR) determines system clock prescaling for the watchdog timer. One of two op-
tions, either no prescaling, or prescaling by a factor of 512, can be selected. The value
of PTP is affected by the state of the MODCLK pin during reset, as shown in Table 4-
5. System software can change PTP value.
Either clock signal (EXTAL or EXTAL
ulus counter (PITCLK). The modulus counter is initialized by writing a value to the pe-
riodic timer modulus timer modulus (PITM) field in the PITR. A zero value turns off the
periodic timer. When the modulus counter value reaches zero, an interrupt is generat-
ed. The modulus counter is then reloaded with the value in PITM and counting repeats.
If a new value is written to PITR, it is loaded into the modulus counter when the current
count is completed.
Figure 4-3 Periodic Interrupt Timer and Software Watchdog Timer
DISABLE
CLOCK
Table 4-5 MODCLK Pin and PTP Bit at Reset
Freescale Semiconductor, Inc.
PRESCALER (2
For More Information On This Product,
0 (External Clock)
1 (Internal Clock)
SYSTEM INTEGRATION MODULE
MODCLK
Go to: www.freescale.com
9
)
PRECLK
512) is divided by four before driving the mod-
CLOCK
MUX
1 ( 512)
0 ( 1)
PTP
SWCLK
4
PITCLK
2
9
DIVIDER CHAIN (2
8-BIT MODULUS
COUNTER
15 STAGE
2
11
PITR
2
13
15
)
2
15
PIT
INTERRUPT
RESET
PIT BLOCK
4-7
4