SIGNAL DESCRIPTIONS
MOTOROLA
DSP56002 PIN DESCRIPTIONS
2 - 5
2.2.2.2
Data Memory Select (DS)
This three-state output is asserted only when external data memory is referenced (see Table 2-1).
2.2.2.3
X/Y Select (X/Y)
This three-state output selects which external data memory space (X or Y) is referenced
by DS (see Table 2-1).
2.2.2.4
Read Enable (RD)
This three-state output is asserted to read external memory on the data bus (D0–D23).
2.2.2.5
Write Enable (WR)
This three-state output is asserted to write external memory on the data bus (D0–D23).
2.2.2.6
Bus Needed (BN)
The BN output pin is asserted whenever the chip requires the external memory expansion
port (Port A). During instruction cycles where the external bus is not required, BN is deas-
serted.
If an external device has requested the bus by asserting the BR input and the DSP
has granted the bus (by asserting BG), the DSP will continue processing as long as no
external accesses are required. If an external access is required and the chip is not the
bus master, it will stop processing and remain in wait states until bus ownership is re-
turned. If the BN pin is asserted when the chip is not the bus master, this indicates that
processing has stopped and the DSP is waiting to acquire bus ownership. An external ar-
biter may use this pin to help decide when to return bus ownership to the DSP.
Note:
The BN pin cannot be used as an early indication of imminent external bus access
because it is valid later than the other bus control signal BS.
During hardware reset, BN is deasserted.
2.2.2.7
Bus Request (BR)
When the bus request input (BR) is asserted, the DSP56002 will always relinquish the bus
to an external device such as a processor or DMA controller. The external device will be-
come the new master of the external address and data buses while the DSP continues
internal operations using internal memory spaces. When BR is deasserted, the
DSP56002 will again assume bus mastership.
When BR is asserted, the DSP56002 will always release Port A, including A0–A15, D0–
D23, and the bus control pins (PS, DS, X/Y, RD, WR, and BS) by placing them in the high-
impedance state, after the execution of the current instruction has been completed.
Note:
To prevent erroneous operation, the BR pin should be pulled up when it is not in use.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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