3/24/97 2-20 Architectural Overview
Figure 2.14 Typical External Data Read.
Figure 2.15 Typical External Data Write.
2.7 Ports
Standard I/O ports on the XA have been enhanced to provide better versatility and
programmability than was previously available in the 80C51 and most of its derivatives. Access
to the I/O ports from a program is through SFR addresses assigned to those ports. Ports may be
read and written is the same manner as any other SFR.
The XA provides more flexibility in the use of I/O ports by allowing different output
configurations. See Figure 2.16. Port outputs may be programmed to be quasi-bidirectional
(80C51 style ports), open drain, push-pull, and high impedance (input only).
ALE
RD
Address bus
Data bus
address
data in to XA
data address
ALE
WRL/WRH
Address bus
Data bus
address
data out from XA
data address
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