Intel mcs-48 manual Multi-Chip MCS-48 Systems

EXPANDED MCS-48 SYSTEM

Interconnection to the_8048 lL very straightforward with BUS, RD, and WR con- necting directly to the corresponding pins on the 8255. The only design consideration is the way in which the internal registers of the 8255 are to be addressed. If the registers are to be addressed as external data memory using the MOVX instructions, the appropriate number of address bits (in this case, 2) must be latched on BUS using ALE as described in the section on external data memories. If only a single device is connected to BUS, the 8255 may be continuously selected by grounding CS. If multiple 8255'sare used, additional address bits can be latched and used as chip selects.

A second addressing method eliminates ex- ternal latches and chip select decoders by using output port lines as address and chip select lines directly. This method, of course, requires the setting of an output port with ad- dress information prior to executing a MOVX instruction.

3.4 Multi-Chip MCS-48 Systems

The accompanying figure shows the addition of two memory expanders to the 8048, one 8355/8755 ROM and one 8156 RAM. The main consideration in designing such a sys- tem is the addressing of the various memories and I/O ports. Note that in this configuration address lines A,o and A" have been ORed to chip select the 8355. This ensures that the chip is active for all external program memory fetches in the 1K to 3K range and is disabled for all other addresses. This gating has been added to allow the I/O port of the 8355 to be used. If the chip was left selected all the time there would be conflict between these ports and the RAM and I/O of the 8156. The NOR gate could be eliminated and~" connected directly to the CE (instead of CE) input of the 8355; however, this would create a 1K word "hole" in the program memory by causing the 8355 to be active in the 2K to 4K range in- stead of the normal 1K to 3K range.

8156/8355

8048

PORT

A

PORT

B

PORT

C

---TIMER

OUT

THE THREE COMPONENT MCS-48 SYSTEM

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