MMC2001
TIMER/RESET MODULE
MOTOROLA
REFERENCE MANUAL
9-13
When the counter reaches a count of zero, the PIT interrupt flag (ITIF) is set in the
ITCSR and the value in the modulus latch is loaded into the counter to be decre-
mented towards zero. If the PIT interrupt enable (ITIE) bit is set in the ITCSR, the
interrupt flag issues an interrupt to the CPU.
The counter may by directly initialized, without having to wait for the count to reach
zero, when the ITDR is written with the OVW bit in the ITCSR set.
Figure 9-14 Counter Reloading from the Modulus Latch
9.6.3 PIT as a “Free-Running” Timer
This mode of operation is selected when the RLD bit in the ITCSR is cleared to a
value of zero. In this mode, the counter rolls over from 0x0000 to 0xFFFF, without
reloading from the modulus latch, and continues to count.
When the counter reaches a count of zero, the PIT interrupt flag (ITIF) is set in the
ITCSR. If the PIT interrupt enable (ITIE) bit is set in the ITCSR, the interrupt flag can
issue an interrupt to the CPU.
The counter may by directly initialized, without having to wait for the count to reach
zero, when the ITDR is written while the OVW bit is set.
Figure 9-15 Counter in Free-Running Mode
9.6.4 Interval Timer Registers
The interval timer has three registers: the control/status register (ITCSR), the data
register (ITDR), and the alternate data register (ITADR).
0x0002
0x0001
0x0000
0x0005
0x0005
OSC
OSC/4
COUNTER
MODULUS
ITIF
0x0002
0x0001
0xFFFF
0x0005
OSC
OSC/4
COUNTER
MODULUS
ITIF
0x0000
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