MPC555
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MPC556
SYSTEM CONFIGURATION AND PROTECTION
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
6-15
Figure 6-6 RTC Block Diagram
6.9 Periodic Interrupt Timer (PIT)
The periodic interrupt timer consists of a 16-bit counter clocked by the PITRCLK clock
supplied by the clock module.
The 16-bit counter counts down to zero when loaded with a value from the PITC. After
the timer reaches zero, the PS bit is set and an interrupt is generated if the PIE bit is
is a logic one. The software service routine should read the PS bit and then write it to
zero to terminate the interrupt request. At the next input clock edge, the value in the
PITC is loaded into the counter, and the process starts over again.
When a new value is loaded into the PITC, the periodic timer is updated, the divider is
reset, and the counter begins counting. If the PS bit is not cleared, an interrupt request
is generated. The request remains pending until PS is cleared. If the PS bit is set again
prior to being cleared, the interrupt remains pending until PS is cleared.
Any write to the PITC stops the current countdown, and the count resumes with the
new value in PITC. If the PTE bit is not set, the PIT is unable to count and retains the
old count value. Reads of the PIT have no effect on the counter value.
pitrtclk
FREEZE
Divide
32-bit Counter (RTC)
32-bit Register (RTCAL)
Sec
Alarm
=
Clock
Disable
Divide
MUX
4-MHz / 20-MHz crystal
Interrupt
Interrupt
By 78125
By 15625
Clock
RTSEC
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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