
196
Table 7.2 lists the external signals related to bus-state controller function.
Table 7.2
Bus State Controller (BSC) External Signals
Signal
I/O
State with
Bus
Released
Function
A23–A0
O
Hi-Z
Address bus. The 24 bits specify a total 16 Mbytes in the
selected memory space.
BACK
O
Bus usage grant acknowledge to an external bus master
BREQ
I
Bus request input from an external bus master
BS
O
Hi-Z
Indicates the start of a bus cycle for an interface to ordinary
space. The signal is asserted for a single clock cycle
simultaneously with the address output. The start of the bus
cycle can be determined by this signal.
CAS3
, CAS2
O
Hi-Z*
1
Column address strobe (CAS ) signals for byte lane in a
DRAM interface. CAS3 refers to bits 31–24, and CAS2 refers
to bits 23–16.
CAS1
/RFSH
O
Hi-Z*
1
Column address strobe (CAS ) signal for the byte lane
composed of bits 15–8 in a DRAM interface
For an interface to pseudo-SRAM that requires separate OE
and RFSH signals, this pin carries the RFSH signal
CAS0
/OE /
RFSH
O
Hi-Z*
1
Column address strobe (CAS ) signal for the byte lane
composed of bits 7–0 in a DRAM interface
For an interface to pseudo-SRAM that requires a multiplexed
OE
/RFSH signal, this pin carries the OE /RFSH signal
CS1
, CS0
O
Hi-Z
Chip selects for CS0 and CS1 16-Mbytes external memory
regions
CS2
/RAS2
O
Hi-Z*
1
Chip select for the CS2 16-Mbytes external memory region
This pin also carries the row address strobe (RAS2 ) signal
when the CS2 region is configured for DRAM
CS3
/RAS3 /
CE
O
Hi-Z*
1
Chip select for the CS3 16-Mbytes external memory region.
When the CS3 region is configured for DRAM, this pin also
carries the row address strobe (RAS3 ) signal; when the CS3
region is configured for pseudo-SRAM, this pin also carries
the chip enable (CE ) signal.
D31–D0
I/O
Hi-Z
32-bit data bus. When reading or writing a 16-bit width area,
use D15–D0; when reading or writing a 8-bit width area, use
D7–D0. With 8-bit accesses that read or write a 32-bit width
area, input and output the data via the byte position
determined by the lower address bits of the 32-bit bus.
DACK1, DACK0 O
O
DMA acknowledge for channels 0 and 1