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MC68F375
SINGLE-CHIP INTEGRATION MODULE 2 (SCIM2E)
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
4-29
The periodic interrupt timer modulus counter is clocked by one of two signals. When
the PLL is enabled, fref is used in slow reference mode and fref ÷ 128 is used in fast
reference mode. When the PLL is disabled, fref is used. The value of the periodic timer
prescaler (PTP) bit in the periodic interrupt timer register (PITR) determines system
clock prescaling for the periodic interrupt timer. One of two options, either no prescal-
ing, or prescaling by a factor of 512, can be selected. The value of PTP is affected by
the state of the VDDSYN/MODCLK pin during reset, as shown in Table 4-13. System software can change PTP value.
Either clock signal selected by PTP is divided by four before driving the modulus
counter. The modulus counter is initialized by writing a value to the periodic interrupt
timer modulus (PITM[7:0]) field in PITR. A zero value turns off the PIT. When the mod-
ulus counter reaches zero, an interrupt is generated. The modulus counter is then
reloaded with the value in PITM[7:0] and counting repeats. If a new value is written to
PITR, it is loaded into the modulus counter when the current count is completed.
The following equation calculates the PIT period in slow reference mode:
The following equation calculates the PIT period in fast reference mode:
The following equation calculates the PIT period in external clock mode:
4.4.8 Interrupt Priority and Vectoring for the Periodic Interrupt Timer
Interrupt priority and vectoring for the PIT are determined by the values of the periodic
interrupt request level (PIRQL[2:0]) and periodic interrupt vector (PIV[7:0]) fields in the
periodic interrupt control register (PICR).
The PIRQL field is compared to the CPU32 interrupt priority mask to determine
whether the interrupt is recognized. Table 4-14 shows PIRQL[2:0] priority values.
Table 4-13 PTP Reset States
VDDSYN/MODCLK
PTP
0 (External Clock)
1 (
÷ 512)
1 (Synthesized Clock)
0 (
÷ 1)
PIT Period
PITM[7:0]
() 1 if PTP = 0, 512 if PTP = 1
() 4
()
f
ref
---------------------------------------------------------------------------------------------------------------------
=
PIT Period
128
() PITM[7:0]
() 1 if PTP = 0, 512 if PTP = 1
() 4
()
f
ref
------------------------------------------------------------------------------------------------------------------------------------
=
PIT Period
PITM[7:0]
() 1 if PTP = 0, 512 if PTP = 1
() 4
()
f
ref
---------------------------------------------------------------------------------------------------------------------
=
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Freescale Semiconductor, Inc.
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