BUS ARBITRATION AND SHARED MEMORY
MOTOROLA
PORT A
4 - 21
When DSP#1 wants control of the memory the following steps are performed (see Figure 4-16):
1. DSP#1 makes an external access, thereby asserting BS, which asserts WT
(causing DSP#1 to execute wait states in the current cycle) and asserts
DSP#2 BR (requesting that DSP#2 release the bus).
2. When DSP#2 finishes its present bus cycle, it three-states its bus drivers and
asserts BG. Asserting BG enables the three-state buffers, placing the DSP#1
signals on the memory bus. Asserting BG also deasserts WT, which allows
DSP#1 to finish its bus cycle.
3. When DSP#1’s memory cycle is complete, it releases BS, which deasserts
BR. DSP#2 then deasserts BG, three-stating the buffers and allowing DSP#2
to access the memory bus.
MEMORY
D
A
C
DSP #1
D0 - D23
A0 - A15
RD
,
WR
,
DS
,
PS
, X/
Y
BS
WT
THREE-STATE
BUFFER
DIR
ENABLE
DSP #2
D0 - 23
A0 - A15
RD, WR,
DS, PS, X/Y
BG
BR
Figure 4-15 Bus Arbitration Using BR and BG,
and WT and BS with No Overhead
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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