7. Microcontroller Bus and Memory Expansion
Data Memory Expansion (SRAM)
6264 is the most commonly used. A0-A12 address lines, D0-D7 data lines. CE chip select OE read enable WE write enable
Reading external data RAM timing, MOVX A, @Ri or MOVX A, @DPTR starts reading
When low, the eight-bit address data enters the latch, P0 is in a high-impedance state. Waiting for external data to come in
Two methods, both utilize byte overflow, one is to define R7 for overflow, the other is to use the DPL feature address for overflow.
External memory can only be accessed indirectly
I/O Interface Expansion 8255
Three operating modes of 8255
Peripherals sending to 8255 are called inputs
Only PC6 and PC7 can serve as I/O ports
The microcontroller sending to 8255 is called output
Example:
