Extended memory circuit design

　　The STC89C516RD microcontroller contains 1280 bytes of RAM data storage, which is sufficient for general applications. However, since the system uses a real-time multi-tasking operating system RTOS, which requires a portion of RAM, and a large amount of data needs to be stored, the generation of the LCD display interface also requires RAM space, so we choose to expand the RAM data storage by 32K bytes. The expanded data storage is completed by CY62256.

　   CY62256 is a 32KX8-bit static random access memory chip. A0~A14 are 15-bit address lines on the chip, I/0 port is 8-bit bidirectional data line, CE is chip select signal (low level is valid), 0E is read enable signal line (low level is valid), WE is write signal line (low level is valid). CY62256 chip select signal CE is connected to A15 of microcontroller. CY62256 is a 32K (215) capacity RAM occupying 15 address lines. According to this connection, the decoding address of CY62256 is 0000H, EFFFH, a total of 32K. 74HC373 in Figure 3.6 is an address latch. Its function is to separate and latch the address signal in the time-sharing multiplexing port of the address/data bus of the microcontroller for use by the extended data memory or program memory (this system does not have an extended program memory). The ALE of the microcontroller is the low 8-bit address latch control signal, which is connected to the LE pin of 74HC373 so that 373 can latch the address signal.