51 MCU external expansion RAM

1. Overview

Ordinary 51 single-chip microcomputers can expand ROM and RAM with 64K bytes of space each. In actual applications, external ROM is rarely expanded, and RAM is generally expanded because the internal RAM of ordinary 51 single-chip microcomputers is too small, only 128-256 bytes, which is often not enough when processing large amounts of data. The external expansion of RAM requires the use of P0, P2, P3.6, and P3.7, consuming 18 IO ports, resulting in insufficient IO ports. In order to solve the above contradictions, large-capacity 51 single-chip microcomputers (enhanced 51 single-chip microcomputers) came into being. This type of single-chip microcomputer generally has 1K-16K RAM and 16K-64K ROM built-in, and the price is relatively expensive. In some cases, using ordinary 51 single-chip microcomputers to expand RAM externally can save costs more than directly using enhanced 51 single-chip microcomputers. Therefore, it is necessary to learn how to expand RAM externally on 51 single-chip microcomputers.

2. Circuit Design

On the WSF-51DB development board, 32K RAM (HM62256B) is expanded, and the address latch chip uses 74HC573. If the RAM is expanded off-chip, the P0 port is used as the multiplexed port for the low byte of the data and address, and there is no need to add a pull-up resistor. Of course, adding a pull-up resistor has no effect. It should be noted that when using the off-chip expansion RAM, it is best not to connect other resources to the P0 port and the P2 port. Only 15 address lines are used to access the externally expanded 32K RAM, so P2.7 is used as the enable pin of the address latch 74HC573 and the chip select of the HM62256B. As shown in the figure below, in order to reduce cross-wiring, the 74HC573 is flipped down, which looks a bit awkward.

3. Software Design

/****************************************************** **

*Program name: 51 MCU external expansion RAM test

*Program function: write data to the external RAM, read data from the external RAM, and light up the 8 LEDs of port P1

* Development tools: WSF-51DB development board

* MCU model: AT89S52-24PU

*Clock frequency: 12MHZ

*Program author: Master Wu

*Copyright statement: All rights reserved by Master Wu. Please indicate the source address and author when reprinting.

*************************************************** /

#include

#include//Contains the absolute address access function of the off-chip storage space: XBYTE[]

sbit oe=P2^7; //External RAM chip select and address latch 74HC573 enable

unsigned char code ramdata[100]= //define 100 data [page]

{

0,1,2,3,4,5,6,7,8,9,

10,11,12,13,14,15,16,17,18,19,

20,21,22,23,24,25,26,27,28,29,

30,31,32,33,34,35,36,37,38,39,

40,41,42,43,44,45,46,47,48,49,

50,51,52,53,54,55,56,57,58,59,

60,61,62,63,64,65,66,67,68,69,

70,71,72,73,74,75,76,77,78,79,

80,81,82,83,84,85,86,87,88,89,

90,91,92,93,94,95,96,97,98,99

};

//Delay ms function:

void Delayms(unsigned int t)

{

unsigned int i,j;

for(i=t;i>0;i--)

for(j=120;j>0;j--);

}

//Main function:

int main(void)

{

unsigned char i;

oe=0; //Enable RAM chip select and address latch

for(i=0;i<100;i++)

XBYTE[300+i]=ramdata[i];

//Write 100 data into the off-chip RAM, starting at address: 300

while(1)

{

for(i=0;i<100;i++)

{

P1=XBYTE[300+i]; //Read data from the off-chip RAM and light up the 8 LEDs of port P1

Delayms(500);

}

}

return 0;

}