External Data Bus
2.7
External Data Bus
Table 2-6 External Data Bus Signals
State during
Reset
Signal Name
D0–D23
Type
Signal Description
Input/Output
Tri-Stated
Data Bus—When the DSP is the bus master, D0–D23 are active-high,
bidirectional input/outputs that provide the bidirectional data bus for external
program and data memory accesses. Otherwise, D0–D23 are tri-stated.
2.8
External Bus Control
Table 2-7 External Bus Control Signals
State During
Reset
Signal Name Type
Signal Description
AA0–AA2/
RAS0–RAS2
Output
Output
Tri-Stated
Tri-Stated
Address Attribute or Row Address Strobe—When defined as AA, these signals can
be used as chip selects or additional address lines. When defined as RAS, these
signals can be used as RAS for DRAM interface. These signals are tri-statable outputs
with programmable polarity.
CAS
Column Address Strobe— When the DSP is the bus master, CAS is an active-low
output used by DRAM to strobe the column address. Otherwise, if the bus mastership
enable (BME) bit in the DRAM control register is cleared, the signal is tri-stated.
RD
WR
TA
Output
Output
Input
Tri-Stated
Tri-Stated
Read Enable—When the DSP is the bus master, RD is an active-low output that is
asserted to read external memory on the data bus (D0-D23). Otherwise, RD is tri-stated.
Write Enable—When the DSP is the bus master, WR is an active-low output that is
asserted to write external memory on the data bus (D0-D23). Otherwise, WR is tri-stated.
Ignored Input Transfer Acknowledge—If the DSP is the bus master and there is no external bus
activity, or the DSP is not the bus master, the TA input is ignored. The TA input is a data
transfer acknowledge (DTACK) function that can extend an external bus cycle
indefinitely. Any number of wait states (1, 2. . .infinity) may be added to the wait states
inserted by the BCR by keeping TA deasserted. In typical operation, TA is deasserted
at the start of a bus cycle, is asserted to enable completion of the bus cycle, and is
deasserted before the next bus cycle. The current bus cycle completes one clock
period after TA is asserted synchronous to the internal system clock. The number of
wait states is determined by the TA input or by the bus control register (BCR),
whichever is longer. The BCR can be used to set the minimum number of wait states
in external bus cycles.
In order to use the TA functionality, the BCR must be programmed to at least one wait
state. A zero wait state access cannot be extended by TA deassertion, otherwise
improper operation may result. TA can operate synchronously or asynchronously,
depending on the setting of the TAS bit in the operating mode register (OMR).
TA functionality may not be used while performing DRAM type accesses, otherwise
improper operation may result.
DSP56367 Technical Data, Rev. 2.1
Freescale Semiconductor
2-5
FREESCALE [ Freescale ]
