
MOTOROLA
SYSTEM INTEGRATION MODULE
MC68336/376
5-18
USER’S MANUAL
Either clock signal selected by the 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 periodic timer. When
the modulus counter value 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.
When a fast reference frequency is used, the PIT period can be calculated as follows:
When an externally input clock frequency is used, the PIT period can be calculated as
follows:
5.4.7 Interrupt Priority and Vectoring
Interrupt priority and vectoring are determined by the values of the periodic interrupt
request level (PIRQL[2:0]) and periodic interrupt vector (PIV) fields in the periodic in-
terrupt control register (PICR).
The PIRQL field is compared to the CPU32 interrupt priority mask to determine wheth-
er the interrupt is recognized. Table 5-8 shows PIRQL[2:0] priority values. Because of
SIM hardware prioritization, a PIT interrupt is serviced before an external interrupt re-
quest of the same priority. The periodic timer continues to run when the interrupt is dis-
abled.
The PIV field contains the periodic interrupt vector. The vector is placed on the IMB
when an interrupt request is made. The vector number is used to calculate the address
of the appropriate exception vector in the exception vector table. The reset value of
the PIV field is $0F, which corresponds to the uninitialized interrupt exception vector.
Table 5-8 Periodic Interrupt Priority
PIRQL[2:0]
Priority Level
000
Periodic interrupt disabled
001
Interrupt priority level 1
010
Interrupt priority level 2
011
Interrupt priority level 3
100
Interrupt priority level 4
101
Interrupt priority level 5
110
Interrupt priority level 6
111
Interrupt priority level 7
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
---------------------------------------------------------------------------------------------------------------------
=