Rainbow Electronics AT91CAP9S250A - Manual (Page 173)

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6264A–CAP–21-May-07
AT91CAP9S500A/AT91CAP9S250A
22.7.2.1 Byte Write Access
Byte write access supports one byte write signal per byte of the data bus and a single read
signal.
Note that the SMC does not allow boot in Byte Write Access mode.
• For 16-bit devices: the SMC provides NWR0 and NWR1 write signals for respectively byte0
(lower byte) and byte1 (upper byte) of a 16-bit bus. One single read signal (NRD) is provided.
Byte Write Access is used to connect 2 x 8-bit devices as a 16-bit memory.
• For 32-bit devices: NWR0, NWR1, NWR2 and NWR3, are the write signals of byte0 (lower
byte), byte1, byte2 and byte 3 (upper byte) respectively. One single read signal (NRD) is
provided.
Byte Write Access is used to connect 4 x 8-bit devices as a 32-bit memory.
Byte Write option is illustrated on Figure 22-6 .
22.7.2.2 Byte Select Access
In this mode, read/write operations can be enabled/disabled at a byte level. One byte-select line
per byte of the data bus is provided. One NRD and one NWE signal control read and write.
• For 16-bit devices: the SMC provides NBS0 and NBS1 selection signals for respectively
byte0 (lower byte) and byte1 (upper byte) of a 16-bit bus.
Byte Select Access is used to connect one 16-bit device.
• For 32-bit devices: NBS0, NBS1, NBS2 and NBS3, are the selection signals of byte0 (lower
byte), byte1, byte2 and byte 3 (upper byte) respectively. Byte Select Access is used to
connect two 16-bit devices.
Figure 22-7 shows how to connect two 16-bit devices on a 32-bit data bus in Byte Select Access
mode, on NCS3 (BAT = Byte Select Access).