Motorola MPC823e User Manual

System Interface Unit
12-12 MPC823e REFERENCE MANUAL MOTOROLA
SYSTEM INTERFACE UNIT
12
12.5 THE POWERPC DECREMENTER
The 32-bit decrementing counter is defined by the PowerPC architecture to provide a
decrementer interrupt. This binary counter is clocked by the same frequency as the
timebase. In the MPC823e, the decrementer is clocked by the TMBCLK clock, so you must
enable the TBE bit in the TBSCR for the decrementer to start.
The state of the decrementer is not affected by HRESET
and SRESET and, therefore, must
be initialized by the software. The decrementer runs continuously after power-up. It
continues counting while HRESET
and SRESET are asserted and it is implemented with the
following requirements in mind. The decrementer interrupt is also sent to the power-down
wake-up logic, which allows the CPU to be awaken from power-down mode.
• The operation of the timebase and decrementer are coherent, which means the
counters are driven by the same fundamental timebase.
• The decrementer is unaffected when read.
• When storing to the decrementer, the value in the decrementer is replaced with the
value in the GPR.
• When Bit 0 (MSB) of the decrementer changes from 0 to 1, an interrupt request is
signaled. If multiple decrementer interrupt requests are received before the first one is
reported, only one interrupt is reported.
• If the decrementer is altered by the software and the content of Bit 0 is changed from
0 to 1, an interrupt request is signaled.
A decrementer exception causes a pending decrementer interrupt request in the core. When
the decrementer interrupt is taken, the request is automatically cleared. The following chart
shows some of the periods available for the decrementer, assuming that a 4MHz oscillator
is used.
COUNT VALUE TIMEOUT COUNT VALUE TIMEOUT
0 1 microsecond 999999 1.0 second
9 10. microseconds 9999999 10.0 seconds
99 100. microseconds 99999999 100.0 seconds
999 1.0 millisecond 999999999 1000. seconds
9999 10.0 milliseconds FFFFFFFF(hex) 4295 seconds
T
dec
32
2
F
tmbclk
()
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Motorola MPC823e Specifications

General

Brand	Motorola
Model	MPC823e
Category	Computer Hardware
Language	English