DSP56321 View Datasheet(PDF) - Freescale Semiconductor

Part Name

Description

Manufacturer

DSP56321

24-Bit Digital Signal Processor

Freescale Semiconductor 

External Memory Expansion Port (Port A)

Table 1-7. External Bus Control Signals (Continued)

Signal Name

Type

TA

Input

BR

Output

BG

Input

BB

Input/ Output

State During

Reset, Stop, or

Wait

Signal Description

Ignored Input

Transfer Acknowledge—If the DSP56321 is the bus master and there is no

external bus activity, or the DSP56321 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) can be added to the wait states inserted by the bus control register

(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 BCR, whichever is longer. The

BCR can be used to set the minimum number of wait states in external bus

cycles.

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.

Reset: Output

(deasserted)

State during

Stop/Wait

depends on BRH

bit setting:

• BRH = 0: Output

(deasserted)

• BRH = 1:

Maintains last

state (that is, if

asserted, remains

asserted)

Bus Request—Asserted when the DSP requests bus mastership. BR is

deasserted when the DSP no longer needs the bus. BR may be asserted or

deasserted independently of whether the DSP56321 is a bus master or a bus

slave. Bus “parking” allows BR to be deasserted even though the DSP56321 is

the bus master. (See the description of bus “parking” in the BB signal

description.) The bus request hold (BRH) bit in the BCR allows BR to be

asserted under software control even though the DSP does not need the bus.

BR is typically sent to an external bus arbitrator that controls the priority,

parking, and tenure of each master on the same external bus. BR is affected

only by DSP requests for the external bus, never for the internal bus. During

hardware reset, BR is deasserted and the arbitration is reset to the bus slave

state.

Ignored Input

Bus Grant—Asserted by an external bus arbitration circuit when the DSP56321

becomes the next bus master. When BG is asserted, the DSP56321 must wait

until BB is deasserted before taking bus mastership. When BG is deasserted,

bus mastership is typically given up at the end of the current bus cycle. This may

occur in the middle of an instruction that requires more than one external bus

cycle for execution.

Ignored Input

To ensure proper operation, the user must set the asynchronous bus arbitration

enable (ABE) bit (Bit 13) in the Operating Mode Register. When this bit is set,

BG and BB are synchronized internally. This adds a required delay between the

deassertion of an initial BG input and the assertion of a subsequent BG input.

Bus Busy—Indicates that the bus is active. Only after BB is deasserted can the

pending bus master become the bus master (and then assert the signal again).

The bus master may keep BB asserted after ceasing bus activity regardless of

whether BR is asserted or deasserted. Called “bus parking,” this allows the

current bus master to reuse the bus without rearbitration until another device

requires the bus. BB is deasserted by an “active pull-up” method (that is, BB is

driven high and then released and held high by an external pull-up resistor).

Notes: 1. See BG for additional information.

2. BB requires an external pull-up resistor.

DSP56321 Technical Data, Rev. 11

Freescale Semiconductor

1-5

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