Extend+PIC+Microcontroller‘s+RAM+by+without+using+EMI

Introduction
Virtually all PIC microcontrollers have some banking mechanism to extend addressing to additional memory space. But this external data memory is not directly addressable (except in some high versions of PIC18 devices, which include PIC18F8520, PIC18F6620, etc.). In this post we describe easy to implement external memory interface for PIC microcontrollers. Theoretically most of the PIC microcontrollers can use this setup to extend its RAM space. At the prototyping stage we test this system with PIC16F877, PIC16F887, PIC18F4550 and PIC18F4620 microcontrollers.

Schematic


With this given schematic user may be able to address RAM space up to 192KB. But this can be extended up to 448KB by adding more SRAM modules to the system. In this given setup we use 24512 – 64K x 8 CMOS SRAMs as a memory modules. 74HC373 latch is used as 8bit to 16bit address extender and 74HC138 demultiplexer is used as memory bank selector.

Function to write data to the external memory bank
code format="c" //============================================================================== // Function to write data to the external memory bank // Input parameters: //  bank: Memory band number. Valid range is between 1 to 7. 0 for no bank. //  address: Memory address to write. //  value: Value to write // Return: //  none //============================================================================== void WriteExMem(char bank, unsigned int address, char value) { asm { clrf TRISD clrf TRISB movlw 0xe0 andwf TRISC, 1 movlw 0xc3 andwf PORTC, 1 nop nop movf FARG_WriteExMem_address+1, 0 movwf PORTD movlw 0xfd andwf PORTC, 1 nop nop bsf PORTC, 1 movf FARG_WriteExMem_address, 0 movwf PORTD movf FARG_WriteExMem_value, 0 movwf PORTB nop nop movlw 0xfe andwf PORTC, 1 movf FARG_WriteExMem_bank, 0 movwf R0   rlcf R0, 1 bcf R0, 0 rlcf R0, 1 bcf R0, 0 movf R0, 0 iorwf PORTC, 1 nop nop bsf PORTC, 0 movlw 0xe3 andwf PORTC, 1 } } code

Function to read data from external memory bank
code format="c" //============================================================================== // Function to read data from external memory bank // Input parameters: //  bank: Memory band number. Valid range is between 1 to 7. 0 for no bank. //  address: Memory address to read. // Return: //  Data value //============================================================================== char ReadExMem(char bank, unsigned int address) { asm { clrf TRISD movlw 0xff movwf TRISB movlw 0xe0 andwf TRISC, 1 movlw 0xc3 andwf PORTC, 1 nop nop movf FARG_ReadExMem_address+1, 0 movwf PORTD movlw 0xfd andwf PORTC, 1 nop nop bsf PORTC, 1 movf FARG_ReadExMem_address, 0 movwf PORTD movf FARG_ReadExMem_bank, 0 movwf R0   rlcf R0, 1 bcf R0, 0 rlcf R0, 1 bcf R0, 0 movf R0, 0 iorwf PORTC, 1 nop nop movf PORTB, 0 movwf R1   movlw 0xe3 andwf PORTC, 1 movf R1, 0 movwf R0 } } code

Sample Code to test SRAM system
code format="c" void main { unsigned int address_; char bank = 1; char txt_buffer[20];

ADCON1 = 15; ADCON0 = 0; SSPCON1 = 0; CCP1CON = 0;

UART1_Init(4800); Delay_ms(100); UART_Write_Text("Start Testing...\n");

while(bank < 4) {   sprintf(txt_buffer, "Testing bank %c\n", (bank+48)); UART_Write_Text(txt_buffer); for(address_ = 0; address_ < 65535; address_++) WriteExMem(bank, address_, 170);          //170 is a magic number to test for(address_ = 0; address_ < 65535; address_++) if(ReadExMem(bank, address_)!=170) {       UART_Write_Text(" Test FAIL "); sprintf(txt_buffer, "address: %u\n", address_); UART_Write_Text(txt_buffer); bank = 5; break; }   if(bank < 5) UART_Write_Text(" Test PASS \n"); bank++; } UART_Write_Text("SRAM test completed\n"); while(1);

} code

The above listed source code is compatible with MikroC Pro for PIC compiler and it can use with other compilers with some slight modifications.

Project Resources
[|Project source codes and schematic files]