DSP56362P datasheet(12/152 Pages) FREESCALE

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External Memory Expansion Port (Port A)

DSP56362 Technical Data, Rev. 4

2-6

Freescale Semiconductor

2.5.3

External Bus Control

Table 2-7 External Bus Control Signals

Signal Name

Type

State during Reset

Signal Description

AA0–AA3/RA

S0–RAS3

Output

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 can be tri-stated outputs with programmable polarity.

CAS

Output

Tri-Stated

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.

Output

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.

Output

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, the signals are tri-stated.

Input

Ignored Input

Transfer Acknowledge—If the DSP56362 is the bus master and there is

no external bus activity, or the DSP56362 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 CLKOUT. 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.

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