MC68332
STANDBY RAM WITH TPU EMULATION
USER’S MANUAL
8-3
itance, VDD supply ramp time, available standby voltage, and available standby
current must be known. Assuming that the rate of change is constant as VDD changes
from 0.0 V to 5.5 V (nominal VSS to nominal VDD) and that VSB also drops during this
period, capacitance is calculated using the following expression:
Where:
C = Desired capacitance
I = ISB differential (Transient ISB – Available supply current)
t = time of maximum ISB (Typically in the range VSB – 1.5 V ± 0.5 V)
V = VSB differential (Available supply voltage – Specified minimum VSB)
8.7 Low-Power Stop Operation
Setting the STOP bit in the TRAMMCR switches the TPURAM module to low-power
mode. In low-power mode, the array retains its contents, but cannot be read or written
by the CPU. STOP can be written only when the processor is operating at the super-
visor privilege level. STOP is set during reset. Stop mode is exited by clearing STOP.
The TPURAM module will switch to standby mode while it is in low-power mode, pro-
vided the operating constraints discussed above are met.
8.8 Reset
Reset places the TPURAM in low-power mode, enables supervisor-level access only,
clears the base address, and disables the array. These actions make it possible to
write a new base address into the base address register.
When a synchronous reset occurs while a byte or word TPURAM access is in
progress, the access is completed. If reset occurs during the first word access of a
long-word operation, only the first word access is completed. If reset occurs during the
second word access of a long-word operation, the entire access is completed. Data
being read from or written to the TPURAM may be corrupted by asynchronous reset.
cerning resets.
8.9 TPU Microcode Emulation
The TPURAM array can emulate the microcode ROM in the TPU module. This pro-
vides a means of developing custom TPU code. The TPU selects TPU emulation
mode.
The TPU is connected to the TPURAM via a dedicated bus. While the TPURAM array
is in TPU emulation mode, the access timing of the TPURAM module matches the tim-
ing of the TPU microinstruction ROM to ensure accurate emulation. Normal accesses
through the IMB are inhibited and the control registers have no effect, allowing external
RAM to emulate the TPURAM at the same addresses. Refer to SECTION 7 TIME and to the
TPU Reference Manual (TPURM/AD) for more infor-
mation.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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