SPI need Pull ups?

Discussion in 'Embedded Systems and Microcontrollers' started by nileshkhupase, Jul 5, 2011.

  1. nileshkhupase

    Thread Starter Member

    Mar 22, 2011
    33
    0
    Hi,

    i have a SPI serial EEPROM XAT25320,32Kbit and it is interfaced with 16 bit controller C167CR.

    but every pin of EEPROM(CS,SO,SI,SCLK) is pulled up externally through a resistor by 5V supply. What would be the purpose?
     
  2. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    most devices have better sinking capacity than sourcing. By adding the pull ups you will 'assist' the devices ability to source, and help clean up the rising transition.
     
  3. nileshkhupase

    Thread Starter Member

    Mar 22, 2011
    33
    0
    i am not getting it..how will it increase sourcing current......
    please can you explain it...
     
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    all circuitry has some capacitive quality. Some problematic, some negligable. In addition, a large 'fan out', or loading, can cause slow rise times (high slew rate) with limited sourcing current. By adding the logic voltage/pullup resistive current to the circuit, rise times will improve. The problem is less noticable on the falling transistion because as mentioned, sinking capacity is typically higher for most devices.
     
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