Nuts and Volts - May 2010

Code from the main program calling the function:

WRITE AA TO X555

edata = read_eeprom(eaddr, ce_master);

Figure 2.

Read EEPROM function definition.

Function name

WRITE 55 TO XAAA

Read Function: Parameters passed from calling program – EEPROM address and chip to read from (Master or Slave)

byte read_eeprom (int address, byte ce) {

WRITE A0 TO X555

byte fdata;

// Code to address chip and request a read omitted for simplicity fdata = PINA;

Type of data returned by function. “Void” if no data returned

PERFORM BYTE OR PAGE WRITE OPERATIONS

return fdata;

}

Figure 3. Write sequence required by the X88C64.

work. I then considered reading 4K into an array and swapping the chips twice to complete the copy. Since we only had two good EEPROMs remaining, I didn’t want to take a chance of accidentally overwriting a working chip. I settled on a design that had two ZIF sockets: one for the working EEPROM (Master) and one for the blank EEPROM (Slave). I could then read and write on a byte-by-byte basis and not require RAM storage in the Mega. This also would allow me to scale up to any size EEPROM. The downside was more involved wiring on the breadboard.

Direct Port I/O

The digital Write Arduino command is actually setting

Read data from Port A into the fdata variable Value returned to the calling program

one bit in an eight-bit register. The ATmega1280 CPU used in the Mega has 11 registers designated PORTA through PORTL (PORTI is not defined) of which 54 digital bits are brought out to connectors on the Mega board. [See the ATmega 1280 datasheet section 13 (in Links) for more details.] For my programmer, I need to control all 13 address bits and read or write eight data bits, so setting bits one at a time would be quite inefficient. Also, it takes much less time to control the ports directly, even though (in my case) performance was not an issue. Each port pin actually consists of three register bits — DDRxn, PORTxn, and PINxn — where x designates the port (A through L) and n specifies the bit (0 through 7). The DDRxn bit specifies the direction of the pin. If DDRxn is set to a logic one, the pin is an output. If DDRxn is set to a logic zero, the pin is an input. PORTxn is used to set output data and