MC68332 SYSTEM INTEGRATION MODULE
USER’S MANUAL 4-17
4.4 External Bus Interface
The external bus interface (EBI) transfers information between the internal MCU bus
and external devices. Figure 4-7 shows a basic system with external memory and pe-
ripherals.
Figure 4-7 MCU Basic System
The external bus has 24 address lines and 16 data lines. The EBI provides dynamic
sizing between 8-bit and 16-bit data accesses. It supports byte, word, and long-word
transfers. Ports are accessed through the use of asynchronous cycles controlled by
the data transfer (SIZ1 and SIZ0) and data size acknowledge pins (DSACK1
and
DSACK0
). Multiple bus cycles may be required for a transfer to or from an 8-bit port.
The maximum number of bits transferred during an access is referred to as port width.
Widths of eight and sixteen bits can be accessed by asynchronous bus cycles con-
trolled by the data size (SIZ[1:0]) and the data and size acknowledge (DSACK[1:0]
)
signals. Multiple bus cycles may be required for a dynamically-sized transfer.
To add flexibility and minimize the necessity for external logic, MCU chip-select logic
can be synchronized with EBI transfers. Refer to 4.8 Chip Selects for more informa-
tion.
32 EXAMPLE SYS BLOCK
ADDR[23:0]
SIZ
CLKOUT
AS
FC
DSACK
DS
R/W
DATA[15:0]
CS3
CS5
IRQ
CSBOOT
ADDR[15:0]
SIZ
CLK
DSACK
AS
DS
CS
DATA[15:0]
IACK
IRQ
ADDR[23:0]
DATA[15:8]
CS
R/W
MEMORY
ADDR[23:0]
DATA[7:0]
CS
R/W
MEMORY
MCU
ASYNC BUS
PERIPHERAL
1
2
2
2
1. Can be decoded to provide additional address space.
2. Varies depending upon peripheral memory size.
Fr
ees
cale S
em
iconduct
or
, I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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