Interfacing Stamp with External Memory

Trang · 2009-06-30 14:43

Hello experts,

I'm working on a project in which I collect data from an accelerometer for around 10 minutes to capture motion of my board. Therefore, I needed external EEPROM to store that data. I chose the 24LC256 memory chip and now I'm trying to interface it with the Stamp. (I'm using BS2e)

So I was wondering if anyone here has tried that kind of combination -- Stamp + an external memory chip -- and could possibly tell me if it's possible. I'm thinking about using the shiftin and shiftout commands as I did with a 12-bit AD converter. But what I was concerned about is that Stamp generates its own clock implicitly in those commands and whether it's gonna cause any incompatibility with the memory chip...

Has anyone done this before? Any feedback will be very much appreciated!

achilles03 · 2009-06-30 15:14

It'll work fine with that chip. Interfacing the stamp with eeproms is common practice. Eeproms require an external clock source just like ADCs, so there's no "incompatibility" there.

However, with that chip, you'll probably need to use the I2C commands instead of the shiftin/out commands. The 2 main protocols to communicate with eeproms are SPI (shiftin and shiftout, like the ADCs you're familiar with) and I2C. It looks like the chip you chose uses I2C. You can make it easier on yourself and get an SPI eeprom, or stick with the 24LC256 and use the I2C commands. Your pick.

Hope that helps...
Dave

Mike Green · 2009-06-30 15:46

There's a Nuts and Volts Column on interfacing with I2C EEPROMs (and other I2C devices) with Stamps that don't have the built-in I2C commands. The Column includes the subroutines you need. I don't remember which one, but browse the index ... it's something like "I2C for Everyone".

Trang · 2009-06-30 16:02

Thank you so much Dave! That was really helpful. I think I'll go with SPI memory chips, since my BS2e doesn't have the I2Cin/out commands.
One more question since this was brought up: I'm really a newbie in this field, so could you also explain to me which between SPI and I2C is better to use? And if it were you, which direction would you go with? I know I picked SPI because of my Stamp chip, but if I can choose without constraints, what should I go with? --- just curious.

Thanks a lot for your help!

Trang · 2009-06-30 16:02

Oh I just read your post after I replied. Thank you so much Mike!!

Mike Green · 2009-06-30 16:07

I2C is a standardized protocol that requires only two I/O lines and allows multiple addressed devices on the same two lines. It's relatively slow with supported clock speeds of 100KHz, 400KHz, and 1MHz.

SPI is a not very standardized at all protocol that requires three or four I/O lines. Some of these lines can be shared among devices, but you always need a select line that's unique to each device. There are all sorts of variations on this protocol with minimal commonality among devices. It's very fast though, often with devices that can accept up to a 20MHz clock (although the Stamps are much slower than that).

Trang · 2009-06-30 16:12

Thank you very much! I just found the Nuts and Volts column too, very helpful...

Ron Czapala · 2009-07-02 00:26

Here is a little program I wrote to dump memory contents in ascii and hex. You might find it useful. It will work with basic stamps that support I2C and those that don't...

' {$STAMP BS2}
' {$PBASIC 2.5}
SDA            PIN      0              ' I2C Serial Data Line
SCL            PIN      1              ' I2C Serial Clock Line
ChipAddr       CON      $A0            ' 24LC128 chip address
EpromSize      CON      16             ' e.g. 16K or 32K
Ack            CON      0              ' Acknowledge Bit
temp           VAR      Byte           ' Variable Used For Work
i2cWork        VAR      Word           ' Work Byte For I2C I/O
i2cAck         VAR      Bit            ' Ack Bit From Device
start          VAR      Word
leng           VAR      Word           '# bytes to dump
incr           VAR      Nib
MemAddr        VAR      Word           ' EEPROM memory address 0 to 16383 (16k)

'  MemAddr = 16383
'  temp = 254
'  GOSUB Write_byte
' GOSUB Pippin
leng =256
DO
  DEBUG CR, "Enter ChipAddr to dump, 0 to ", DEC (1024 * EpromSize) - leng + 15, ": "
  DEBUGIN DEC5 start
  start = (start/16)*16
  MemAddr = start
  DEBUG CR
  DO
    DEBUG DEC5 MemAddr, " "
    FOR incr = 0 TO 15
  ' Reading Section
      GOSUB Read_Byte
      DEBUG CRSRX, (incr * 3) + 7, HEX2 temp
      SELECT temp
        CASE < 32
          DEBUG CRSRX, (incr * 2) + 60, "."
        CASE > 128
          DEBUG CRSRX, (incr * 2) + 60, "."
        CASE 32 TO 127
          DEBUG CRSRX, (incr * 2) + 60, temp
      ENDSELECT
      MemAddr = MemAddr + 1
    NEXT
    DEBUG CR
  LOOP UNTIL (MemAddr >= (start + leng))
LOOP
END
Pippin:
'  start=01023
  start=02048
  MemAddr = start
  temp = "P"
  GOSUB Write_byte
  MemAddr = MemAddr + 1
  temp = "I"
  GOSUB Write_byte
  MemAddr = MemAddr + 1
  temp = "P"
  GOSUB Write_byte
  MemAddr = MemAddr + 1
  temp = "P"
  GOSUB Write_byte
  MemAddr = MemAddr + 1
  temp = "I"
  GOSUB Write_byte
  MemAddr = MemAddr + 1
  temp = "N"
  GOSUB Write_byte
  MemAddr = MemAddr + 1
  MemAddr = start
RETURN
Clear:
  MemAddr = start
  temp = 0
Init:
  FOR incr = 0 TO 15
    GOSUB Write_byte
    MemAddr = MemAddr + 1
  NEXT
  IF MemAddr < start + 128 THEN
    GOTO Init
  ENDIF
RETURN
'------- I/O routines based on microcontroller type -----------
#SELECT $STAMP
' -----[noparse][[/noparse] BS2 Subroutines ]-------------------------------------
  #CASE BS2, BS2E, BS2SX
Write_Byte:
'write byte in temp to MemAddr
    GOSUB I2C_Start
    GOSUB Control_Byte_Write
    GOSUB Addrs
    GOSUB Write_Data
    GOSUB I2C_Stop
    PAUSE 10
RETURN
Read_Byte:
'read byte from MemAddr into temp
    GOSUB I2C_Start
'    PAUSE 10
    GOSUB Control_Byte_Write
    GOSUB Addrs
    GOSUB I2C_Start
    GOSUB Control_Byte_Read
'    SHIFTIN SDA, SCL, LSBFIRST, [noparse][[/noparse]i2cwork\8] ' Send Byte To Device
    SHIFTIN SDA, SCL, LSBFIRST, [noparse][[/noparse]temp\8] ' Send Byte To Device
    GOSUB I2C_Stop
RETURN
I2C_Start:                             ' I2C Start Bit Sequence
  INPUT SDA
  INPUT SCL
  LOW SDA                              ' SDA --> Low While SCL High
RETURN
I2C_Stop:                              ' I2C Stop Bit Sequence
  LOW SDA
  INPUT SCL
  INPUT SDA                            ' SDA --> High While SCL High
RETURN
Addrs:                                 ' I2C ChipAddr Byte Sequence
  i2cWork = MemAddr
  SHIFTOUT SDA, SCL, MSBFIRST, [noparse][[/noparse]i2cWork.HIGHBYTE\8] ' Send Byte To Device
  SHIFTIN SDA, SCL, MSBPRE, [noparse][[/noparse]i2cAck\1] ' Get Acknowledge Bit
  SHIFTOUT SDA, SCL, MSBFIRST, [noparse][[/noparse]i2cWork.LOWBYTE\8] ' Send Byte To Device
  SHIFTIN SDA, SCL, MSBPRE, [noparse][[/noparse]i2cAck\1] ' Get Acknowledge Bit
RETURN
Control_Byte_Write:                    ' I2C Control Write Byte Sequence
  i2cWork = ChipAddr
  i2cWork.BIT0 = 0                     ' Sets Unit To Write
  SHIFTOUT SDA, SCL, MSBFIRST, [noparse][[/noparse]i2cWork\8] ' Send Byte To Device
  SHIFTIN SDA, SCL, MSBPRE, [noparse][[/noparse]i2cAck\1] ' Get Acknowledge Bit
RETURN
Control_Byte_Read:                     ' I2C Control Read Byte Sequence
  i2cWork = ChipAddr
  i2cWork.BIT0 = 1                     ' Sets Device To Read
  SHIFTOUT SDA, SCL, MSBFIRST, [noparse][[/noparse]i2cWork\8] ' Send Byte To Device
  SHIFTIN SDA, SCL, MSBPRE, [noparse][[/noparse]i2cAck\1] ' Get Acknowledge Bit
RETURN
Write_Data:                            ' I2C Write Byte Sequence
  i2cWork = temp
  SHIFTOUT SDA, SCL, MSBFIRST, [noparse][[/noparse]i2cWork\8] 'Send Byte To Device
  SHIFTIN SDA, SCL, MSBPRE, [noparse][[/noparse]i2cAck\1] ' Get Acknowledge Bit
RETURN
' -----[noparse][[/noparse] BS2P Subroutines ]------------------------------------
  #CASE BS2P, BS2PE, BS2PX
Write_Byte:
    I2COUT SDA, ChipAddr, MemAddr.HIGHBYTE\MemAddr.LOWBYTE, [noparse][[/noparse]temp]
RETURN
Read_Byte:
    I2CIN SDA, ChipAddr+1,MemAddr.HIGHBYTE\MemAddr.LOWBYTE, [noparse][[/noparse]temp]
RETURN
#ENDSELECT

Interfacing Stamp with External Memory

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