PIC32MX: I2C External RAM

From Mech
Jump to: navigation, search

DO NOT USE THE FOLLOWING CODE AS WRITTEN - IT DOES NOT WORK - MODIFY BEFORE USING

Overview

This page discusses work toward sending data to a PCF8570P SRAM chip. Basic I2C communication information, chip data, circuit diagram, and a partially functioning code are given. Prior to the code is a brief explanation of where the code is failing and why no data is being transmitted between the PIC 32 and the PCF8570P.

Data is sent to the PCF8570 SRAM chip by first sending a 7-bit slave address, then a 8-bit write address and then 8-bit data strings. The eighth bit in the slave address is set to either a 0 (write) or 1 (read) which tells the master and the slave what operation is happening. After each string is sent, there is a ninth bit which is the acknowledge bit. This bit is set low if the master or slave receives the information (acknowledge) or high if they do not (not acknowledge).

Circuit

Given below is the circuit diagram for communication between the PIC32 board used in class and the PCF8570P SRAM chip [1].

The PCF8570P chip has a built-in 4 bit hardware address and 3-bit user-defined hardware address allowing multiple chips to be on the same I2C data line. The diagram below shows the following 7-bit hardware address (bits shown in bold are given by the manufacturer: 1010010

I2C RAM.jpg

Code

The following code generates the proper clock and data pulses from the PIC32, however, the PCF8570P chip continuously failed to return an "Acknowledge" bit suggesting that the chip was not processing the data being sent to it by the PIC.

DO NOT USE THE FOLLOWING CODE AS WRITTEN - IT DOES NOT WORK - MODIFY BEFORE USING


// INCLUDES
#include <HardwareProfile.h>
#include <HardwareProfile_NU32.h>
#include <plib.h>
// Configuration Bit settings
// SYSCLK = 80 MHz (8MHz Crystal/ FPLLIDIV * FPLLMUL / FPLLODIV)
// PBCLK = 50 KHz
// Primary Osc w/PLL (XT+,HS+,EC+PLL)
// WDT OFF
// Other options are don't care
//
#pragma config FPLLMUL = MUL_20, FPLLIDIV = DIV_2, FPLLODIV = DIV_1, FWDTEN = OFF
#pragma config POSCMOD = HS, FNOSC = PRIPLL, FPBDIV = DIV_2 
// DEFINES
#define SYSCLK	(80000000)
#define PBCLK  (SYSCLK/2) // set PBCLK to 800 KHz
#define Fsck	50000
#define BRG_VAL 	((PBCLK/2/Fsck)-2)
#define Nop() asm( "nop" )
void i2c_wait(unsigned int cnt)
{
	while(--cnt)
	{
		Nop();
		Nop();
	}
}
// WRITE TO SLAVE
int main(void)
{
while(1)
{
	unsigned char SlaveAddress;
	char i2cData[0];
	int  DataSz;
	//Enable channel
       OpenI2C1( I2C_EN, BRG_VAL );
SlaveAddress = 0x50; (0b01010000)
i2cData[0] = ((SlaveAddress << 1) | 0 ); //slave address and write command
i2cData[1] = 0x05; //word address
i2cData[2] = 0xAA; //data bits 10101010
DataSz = 3;
 StartI2C1();	//Send the Start condition to the clock and data lines
 IdleI2C1();		//Wait to complete
MasterWriteI2C1 (i2cData[0]); //sends first string of data to PCF8570P 
IdleI2C1();
 if( I2C1STATbits.ACKSTAT ) //looks for acknowledge low bit to be returned
  break;
MasterWriteI2C1 (i2cData[1]);
IdleI2C1();
 if( I2C1STATbits.ACKSTAT )
  break;
MasterWriteI2C1 (i2cData[2]);
IdleI2C1();
 if( I2C1STATbits.ACKSTAT )
  break;
StopI2C1(); //sends the stop condition to the PCF8570P clock and data lines
IdleI2C1();
// READ DATA FROM RAM
//First sends write command to write which word the PIC wants to read from
i2cData[0] = ((SlaveAddress << 1) | 0); 
i2cData[1] = 0x05;
DataSz = 2;
 StartI2C1();	//Send the Start Bit
  IdleI2C1();	//Wait to complete
MasterWriteI2C1 (i2cData[0]); 
IdleI2C1();
 if( I2C1STATbits.ACKSTAT )
  break;
MasterWriteI2C1 (i2cData[1]);
IdleI2C1();
 if( I2C1STATbits.ACKSTAT )
  break;
RestartI2C1(); //restarts the PCF8570P
IdleI2C1();
MasterWriteI2C1( (SlaveAddress << 1) | 1 ); //transmit slave address and read command
 IdleI2C1();
MasterWriteI2C1(i2cData[1]);
 IdleI2C1();
unsigned char i2cbyte;
 i2cbyte = MasterReadI2C1(); //PIC reads data from PCF8570P
StopI2C1();
IdleI2C1();
}
}
Personal tools