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SECTION 12
STANDBY RAM WITH TPU EMULATION
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The standby RAM module with TPU emulation capability (TPURAM) consists of a
control register block and a 3.5-Kbyte array of fast (two system clock) static RAM,
which is especially useful for system stacks and variable storage. The TPURAM
responds to both program and data space accesses. The TPURAM can also be used
to emulate TPU microcode ROM.
12.1 General
The TPURAM can be mapped to the lower 3.5 Kbytes of any 4-Kbyte boundary in the
address map, but must not overlap the module control registers as overlap makes the
registers inaccessible. Data can be read or written in bytes, words or long words. The
TPURAM is powered by VDD in normal operation. During power-down, TPURAM contents can be maintained by power from the VSTBY input. Power switching between
sources is automatic.
12.2 TPURAM Register Block
There are three TPURAM control registers: the TPURAM module configuration register (TRAMMCR), the TPURAM test register (TRAMTST), and the TPURAM base
address and status register (TRAMBAR). To protect these registers from accidental
modification, they are always mapped to supervisor data space.
The TPURAM control register block begins at address $7FFB00 or $FFFB00, depending on the value of the module mapping (MM) bit in the SIM configuration register
(SIMCR). Refer to 5.2.1 Module Mapping for more information on how the state of
MM affects the system.
The TPURAM control register block occupies eight bytes of address space. Unimplemented register addresses are read as zeros, and writes have no effect. Refer to D.9
Standby RAM Module with TPU Emulation Capability (TPURAM) for register block
address map and register bit/field definitions.
12.3 TPURAM Array Address Mapping
The base address and status register TRAMBAR specifies the TPURAM array base
address in the MCU memory map. TRAMBAR[15:4] specify the 12 high-order bits of
the base address. The TPU bus interface unit compares these bits to address lines
ADDR[23:12]. If the two match, then the low order address lines and the SIZ[1:0] signals are used to access the RAM location in the array.
The RAM disable (RAMDS) bit, the LSB of TRAMBAR, indicates whether the
TPURAM array is active (RAMDS = 0) or disabled (RAMDS = 1). The array is disabled
coming out of reset and remains disabled if the base address field is programmed with
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an address that overlaps the address of the module control register block. Writing a
valid base address to TRAMBAR[15:4] clears RAMDS and enables the array.
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TRAMBAR can be written only once after a reset. This prevents runaway software
from accidentally re-mapping the array. Because the locking mechanism is activated
by the first write after a reset, the base address field should be written in a single word
operation. Writing only one-half of the register prevents the other half from being
written.
12.4 TPURAM Privilege Level
The RASP field in TRAMMCR specifies whether access to the TPURAM can be made
from supervisor mode only, or from either user or supervisor mode. If supervisor-only
access is specified, an access from user mode is ignored by the TPURAM control logic
and can be decoded externally. Refer to 4.7 Privilege Levels and 5.5.1.7 Function
Codes for more information concerning privilege levels.
12.5 Normal Operation
During normal operation, the TPURAM control registers and array can be accessed by
the CPU32, by byte, word, or long word. A byte or aligned word access takes one bus
cycle (two system clock cycles). A long word access requires two bus cycles.
Misaligned accesses are not permitted by the CPU32 and will result in an address
error exception. Refer to 5.6 Bus Operation for more information concerning access
times. The TPU cannot access the array and has no effect on the operation of the
TPURAM during normal operation.
12.6 Standby Operation
Standby mode maintains the RAM array when the MCU main power supply is turned
off.
Relative voltage levels of the VDD and VSTBY pins determine whether the TPURAM is
in standby mode. TPURAM circuitry switches to the standby power source when specified limits are exceeded. The TPURAM is essentially powered by the power supply
pin with the greatest voltage (for example, V DD or VSTBY). If specified standby supply
voltage levels are maintained during the transition, there is no loss of memory when
switching occurs. The RAM array cannot be accessed while the TPURAM is powered
from VSTBY. If standby operation is not desired, connect the VSTBY pin to the VSS pin.
ISB (SRAM standby current) may exceed specified maximum standby current during
the time VDD makes the transition from normal operating level to the level specified for
standby operation. This occurs within the voltage range VSB – 0.5 V Š VDD Š VSS + 0.5
V. Typically, I SB peaks when VDD ≈ VSB – 1.5 V, and averages 1.0 mA over the transition period.
Refer to APPENDIX A ELECTRICAL CHARACTERISTICS for standby switching and
power consumption specifications.
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STANDBY RAM WITH TPU EMULATION
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12.7 Low-Power Stop Operation
Setting the STOP bit in TRAMMCR places the TPURAM in low-power stop mode. In
low-power stop mode, the array retains its contents, but cannot be read or written by
the CPU32. STOP can be written only when the processor is operating in supervisor
mode. STOP is set during resets. Low-power stop mode is exited by clearing STOP.
The TPURAM module will switch to standby mode while it is in low-power mode, provided the operating constraints discussed above are met.
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12.8 Reset
Reset places the TPURAM in low-power stop mode, enables supervisor mode access
only, clears the base address register, 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.
Refer to 5.7 Reset for more information concerning resets.
12.9 TPU Microcode Emulation
The TPURAM array can emulate the microcode ROM in the TPU module. This provides a means for 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 timing of the TPU microcode 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 11 TIME
PROCESSOR UNIT and to the TPU Reference Manual (TPURM/AD) for more
information.
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USER’S MANUAL
STANDBY RAM WITH TPU EMULATION
Rev. 15 Oct 2000
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MC68336/376
USER’S MANUAL
STANDBY RAM WITH TPU EMULATION
Rev. 15 Oct 2000
For More Information
On This Product,
Go to: www.freescale.com
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