MPC555
/
MPC556
SYSTEM CONFIGURATION AND PROTECTION
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
6-30
6.13.4 System Timer Registers
The following sections describe registers associated with the system timers. These fa-
cilities are powered by the KAPWR and can preserve their value when the main power
supply is off. Refer to
for details on the required actions needed in
order to guarantee this data retention.
6.13.4.1 Decrementer Register
The 32-bit decrementer register is defined by the MPC555 / MPC556 architecture. The
values stored in this register are used by a down counter to cause decrementer excep-
tions. The decrementer causes an exception whenever bit zero changes from a logic
zero to a logic one. A read of this register always returns the current count value from
the down counter.
Contents of this register can be read or written to by the
mfspr
or the
mtspr
instruc-
tion. The decrementer register is reset by PORESET. HRESET and SRESET do not
affect this register. The decrementer is powered by standby power and can continue
to count when standby power is applied.
Refer to
3.9.5 Decrementer Register (DEC)
for more information on this register.
6.13.4.2 Time Base SPRs
The TB is a 64-bit register containing a 64-bit integer that is incremented periodically.
There is no automatic initialization of the TB; the system software must perform this
Table 6-15 TESR Bit Descriptions
Bit(s)
Name
Description
0:17
—
Reserved
18
IEXT
Instruction external transfer error acknowledge. This bit is set if the cycle was terminated by an
externally generated TEA signal when an instruction fetch was initiated.
19
IBMT
Instruction transfer monitor time out. This bit is set if the cycle was terminated by a bus monitor
time-out when an instruction fetch was initiated.
20:25
—
Reserved
26
DEXT
Data external transfer error acknowledge. This bit is set if the cycle was terminated by an exter-
nally generated TEA signal when a data load or store is requested by an internal master.
27
DBM
Data transfer monitor time out. This bit is set if the cycle was terminated by a bus monitor time-
out when a data load or store is requested by an internal master.
28:31
—
Reserved
DEC
— Decrementer Register
SPR 22
MSB
0
LSB
31
Decrementing Counter
PORESET
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
HRESET/SRESET: UNCHANGED
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Freescale Semiconductor, Inc.
For More Information On This Product,
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