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8XC196MC, MD, MH USER’S MANUAL
15-22
15.5.1 Standard Bus-control Mode
In the standard bus-control mode, the microcontroller generates the standard bus-control signals:
ALE, RD#, WR#, and BHE# (see Figure 15-10). ALE is asserted while the address is driven, and
it can be used to latch the address externally. RD# is asserted for every external memory read, and
WR# is asserted for every external memory write. When asserted, BHE# selects the bank of mem-
ory that is addressed by the high byte of the data bus.
Figure 15-10. Standard Bus Control
When the microcontroller is configured to use a 16-bit bus, separate low- and high-byte write sig-
nals must be generated for single-byte writes. Figure 15-11 shows a sample circuit that combines
BHE# and AD0 to produce these signals (WRL# and WRH#). A similar pair of signals for read
is unnecessary. For a single-byte read with the 16-bit bus, both bytes are placed on the data bus
and the processor discards the unwanted byte.
Figure 15-11. Decoding WRL# and WRH#
ALE
WR# or RD#
BHE#
AD15:0
Valid
Addr Data Out
16-bit Bus Cycle
ALE
WR# or RD#
AD7:0
AD15:8 Address High
Addr Low Data Out
8-bit Bus Cycle
A3077-01
BHE#
WR#
AD0
WRH#
WRL#
A3109-03
A0
74AC
373