ST10R272L - PARALLEL PORTS
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7.1.1
Alternate functions of PORT0
When an external bus is enabled, PORT0 is used as data bus or address/data bus.
Note that an external 8-bit demultiplexed bus only uses P0L, while P0H is free for I/O
(provided that no other bus mode is enabled).
PORT0 is also used to select the system start-up configuration. During reset, PORT0 is
configured to input, and each line is held high through an internal pullup device. Each line
can then be individually pulled to a low level (see DC-level specifications in the respective
Data Sheets) through an external pulldown device. A default configuration is selected when
the respective PORT0 lines are at a high level. Through pulling individual lines to a low level,
this default can be changed according to the needs of the applications.
The internal pullup devices are designed such that an external pulldown resistors (see Data
Sheet specification) can be used to apply a correct low level. These external pulldown
resistors can remain connected to the PORT0 pins also during normal operation, however,
care has to be taken such that they do not disturb the normal function of PORT0 (this might
be the case, for example, if the external resistor is too strong).
With the end of reset, the selected bus configuration will be written to the BUSCON0
register. The configuration of the high byte of PORT0, will be copied into the special register
RP0H. This read-only register holds the selection for the number of chip selects and
segment addresses. Software can read this register in order to react according to the
selected configuration, if required. When the reset is terminated, the internal pullup devices
are switched off, and PORT0 will be switched to the appropriate operating mode.
During external accesses in multiplexed bus modes PORT0 first outputs the 16-bit
intra-segment address as an alternate output function. PORT0 is then switched to
high-impedance input mode to read the incoming instruction or data. In 8-bit data bus mode,
two memory cycles are required for word accesses, the first for the low byte and the second
for the high byte of the word. During write cycles PORT0 outputs the data byte or word after
