understanding EEPROM Addressing

Mike W

I am using A 24C02 EEPROM with a SX28

the A2,A1,A0 pins are hard wired to VCC

I used the I2C help and examples to come up with this code. and all is fine, I can write and read to and from the EEPROM.

My example writes the value 25 to memory locarion 256. and then reades it back

( I am also using a LCD display but omitted that code to make the post shorter)

My question is?

When I write to the EEPROM I have to use an Address (Addr) of 256 or larger. WHY? Is this because I have A2,A1,A0 hard wired to VCC or have I done something else wrong and missed it

Thanks

Mike





'

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DEVICE SX28, OSC4MHZ, TURBO, STACKX, OPTIONX

FREQ 4_000_000

'

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' IO Pins

'

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Serial_Data PIN RA.0 INPUT ' SDA

Serial_Clock PIN RA.1 INPUT ' SCL

'

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' Constants

'

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SlaveID CON %10101110 ' for 24C02 A0,A1,A2 hard wired high (174 $AE)

Ack CON 0 ' Control Protocol to acknowledge receipt

Nak CON 1 ' negative acknowledgment or not acknowledged

Addr CON $100 ' Word Address in 24C02 256 or larger

'

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' Variables

'

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flags VAR Byte ' flag for read or write eight bit

ackVal VAR flags.0 ' ackVal bit.0, (0 = write 1 = read)

tmpB1 VAR BYTE ' work vars

tmpB2 VAR BYTE '

tmpB3 VAR BYTE '

tmpW1 VAR WORD '

' =========================================================================

PROGRAM Start

' =========================================================================

'

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' Subroutine Declarations

'

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DEVICE_WRITE SUB 3 ' write value to memory

DEVICE_READ FUNC 1, 2 ' read byte from memory

I2C_IN FUNC 1, 1 ' read byte from Serial_Data

I2C_START SUB 0 ' generate I2C Start

I2C_STOP SUB 0 ' generate I2C Stop

I2C_OUT SUB 1, 1 ' write byte to Serial_Data

'

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' Program Code

'

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Start:

Main:

tmpB2 = 25

Device_Write Addr, tmpB2 write 25 to address 256

(more code Here)

tmpB2 = Device_Read Addr ' read from 24C02



END

'

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' Subroutines Code

'

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'

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SUB DEVICE_WRITE

tmpW1 = __WPARAM12 ' copy Address

tmpB1 = __PARAM3 ' copy value

I2C_START

tmpW1_MSB = tmpW1_MSB & $03 ' get block value

tmpW1_MSB = tmpW1_MSB << 1

tmpW1_MSB = tmpW1_MSB | SlaveID ' create control byte

tmpW1_MSB.0 = 0 ' set RW bit for write

I2C_OUT tmpW1_MSB ' send slave ID

I2C_OUT tmpW1_LSB ' send word Address

I2C_OUT tmpB1 ' send data byte

I2C_STOP ' finish

DO ' let write cycle finish

I2C_START

I2C_OUT tmpW1_MSB ' resend slave ID

LOOP UNTIL ackVal = Ack

ENDSUB

'

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FUNC DEVICE_READ

tmpW1 = __WPARAM12 ' copy Address

I2C_START

tmpW1_MSB = tmpW1_MSB & $03 ' get block value

tmpW1_MSB = tmpW1_MSB << 1

tmpW1_MSB = tmpW1_MSB | SlaveID ' create control byte

tmpW1_MSB.0 = 0 ' set RW bit for write

I2C_OUT tmpW1_MSB ' send slave ID

I2C_OUT tmpW1_LSB ' send word Address

I2C_START ' restart for read

tmpW1_MSB.0 = 1 ' set RW bit for Read

I2C_OUT tmpW1_MSB '

tmpB1 = I2C_IN Nak '

I2C_STOP '

RETURN tmpB1

ENDFUNC

'

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SUB I2C_START '

I2CSTART Serial_Data '

ENDSUB

'

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SUB I2C_STOP '

I2CSTOP Serial_Data '

ENDSUB

'

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SUB I2C_OUT '

I2CSEND Serial_Data, __PARAM1, ackVal '

ENDSUB

'

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FUNC I2C_IN '

ackVal = __PARAM1.0 '

I2CRECV Serial_Data, tmpB3, ackVal '

RETURN tmpB3 '

ENDFUNC

'

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Bean

Mike,
You're making it too hard. You just just need to send the ID, then the address. The example used an EEPROM that had an odd addressing scheme.

Bean.

Post Edited By Moderator (Chris Savage (Parallax)) : 11/2/2007 3:41:11 PM GMT