MPC555
/
MPC556
DUAL-PORT TPU RAM (DPTRAM)
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
18-7
In order to guarantee valid DPTRAM data during power-down, external low voltage in-
hibit circuitry (external to the MCU) must be designed to force the RESET pin of the
MCU into the active state before VDDL drops below its normal limit. This is necessary
to inhibit spurious writes to the DPTRAM during power-down.
18.4.3 Reset Operation
When a synchronous reset occurs, a bus master is allowed to complete the current
access. Thus a write bus cycle (byte or half word) that is in progress when a synchro-
nous reset occurs will be completed without error. Once a write already in progress
has been completed, further writes to the RAM array are inhibited.
NOTE
A word (32-bit) write will be completed coherently only if the reset oc-
curs during the second (16-bit) write bus cycle. If reset occurs during
the first write bus cycle, only the first half word will be written to the
RAM array and the second write will not be allowed to occur. In this
case, the word data contained in the DPTRAM will not be coherent.
The first half word will contain the most significant half of the new
word information and the second half word will contain the least sig-
nificant half of the old word information.
If a reset is generated by an asynchronous reset such as the loss of clocks or software
watchdog time-out, the contents of the RAM array are not guaranteed. (Refer to
for a description of MPC555 / MPC556 reset sources, operation, con-
trol, and status.)
Reset will also reconfigure some of the fields and bits in the DPTRAM control registers
to their default reset state. See the description of the control registers to determine the
effect of reset on these registers.
18.4.4 Stop Operation
Setting the STOP control bit in the DPTMCR causes the module to enter its lowest
power-consuming state. The DPTMCR can still be written to allow the STOP control
bit to be cleared.
In stop mode, the DPTRAM array cannot be read or written. All data in the array is re-
tained. The BIU continues to operate to allow the CPU to access the STOP bit in the
DPTMCR. The system clock remains stopped until the STOP bit is cleared or the DP-
TRAM module is reset.
The STOP bit is initialized to logical zero during reset. Only the STOP bit in the DPT-
MCR can be accessed while the STOP bit is asserted. Accesses to other DPTRAM
registers may result in unpredictable behavior. Note also that the STOP bit should be
set and cleared independently of the other control bits in this register to guarantee
proper operation. Changing the state of other bits while changing the state of the
STOP bit may result in unpredictable behavior.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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