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SAM9X25 [DATASHEET]
11054E–ATARM–10-Mar-2014
29.8.2.1 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.
29.8.2.2 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 29-7 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).
Figure 29-6. Connection of 2 x 8-bit Devices on a 16-bit Bus: Byte Write Option
SMC
A1
NWR0
NRD
NCS[3]
Write Enable
Read Enable
Memory Enable
NWR1
Write Enable
Read Enable
Memory Enable
D[7:0]
D[15:8]
A[24:2]
A[23:1]
A[0]