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ML66525 Family User’s Manual
Chapter 16
Bus Port Functions
16 - 9
16.5 Notes Regarding Usage of Bus Port Function
16.5.1 Dummy Read Strobe Output
The instruction code of the nX-8/500S uses 8 bits as its basic unit and consists of 1 to 6 bytes.
Instructions are classified as NATIVE instructions for commonly performed operations or as
COMPOSIT instructions to realize a wide range of addressing.
NATIVE instructions consist
of 1 to 4 bytes and are used to achieve high coding and processing efficiency.
COMPOSIT instructions consist of a 1 to 3 byte address field (PREFIX) and a 1 to 3 byte
operation field (SUFFIX).
The PREFIX and SUFFIX are combined to realize a wide range of
addressing.
If instructions accompanying a write to external data memory are to be executed, an
unnecessary RDn signal (dummy RD) will be output before the actual access (WRn signal)
if
some of those instructions are COMPOSIT instructions (this is limited to cases where the
PREFIX specifies an external data memory area).
For byte and bit accesses, a dummy RD
signal is output once.
For word accesses, a dummy RD signal is output twice.
Some considerations must be exercised in cases where the above-mentioned read strobe affects
the internal operation of peripheral devices.
Using the bus port function, the specific example of connecting and accessing the 8251 serial
interface LSI chip as a peripheral device will be described.
[Example]
When the microcomputer writes to the transmit buffer of the 8251, if COMPOSIT instructions
are used with the above conditions, output of the dummy read strobe will cause data in the
receive buffer to be read.
If receive data exists in the receive buffer, the 8251 will determine
that the CPU has finished reading data, and the receive ready output signal will be reset.
Some considerations must be exercised in cases where peripheral devices operate differently
when read and write operations are performed at the same address.
These types of problems can be avoided by using NATIVE instructions.
For example, a dummy strobe is not output if load and store instructions (L, LB, ST, STB) are
used to read from and write to the accumulator (ACC).
(If programming in C language, these
sections can be written as assembler functions.)
If general-purpose memory (RAM or ROM) is connected to a bus port, the problems described
above should not occur.
Problems only occur if a connected peripheral device (functional
device) is accessed using COMPOSIT instructions as described above and the read strobe
affects the internal operation of the peripheral device.
Connect peripheral devices to bus ports based on an understanding of the operation described
herein and the function and operation of peripheral devices.
Tables 16-3 and 16-4 list PREFIX and SUFFIX combinations (instructions) that output a
dummy RD when an external data memory area is accessed.
In the tables, PREFIX addressing is inserted at the “*” in the SUFFIX column.