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SN74CB3T3245: PCI pull up resistors

Yossy Goldenberg
Prodigy 240 points
Part Number: SN74CB3T3245

Hello,

I intend  to use SN74cb3t3245 as buffer for a PC104-P board design (PCI bus) . 

The SN74cb3t3245 is used to translate from a 3.3V/5V PCI bus to a 3.3V FPGA IOs. The FPGA IOs are pulled up internally.

 What will be the best pull-up for  the  PCI bus side?

If I am using only 3.3V PCI bus, pull-up resistors are still required?

Thanks!

Yossy Goldenberg

  • 0
    TI__Intellectual 2694 points

    Hi Yossy,

    The SN74CB3T3245 is a passive switch that supports level translation internally on the B-side. 

    For a VCC = 3.3V, the A-side can be driven anywhere from 0V to 5V and the B-side will track to VCC i.e. 3.3V. The device by itself does not require any external pull-up resistors on either A-side or B-side as all it does is connect or disconnect A-side to B-side and helps to reduce system cost by providing internal translation on B-side thus reducing need of external voltage translator.

    If the PCI bus output is a open drain then you will need a Pull-up resistor on the A-side and the value will dependent on the strength of the pull down FET of the PCI driver. If you have a Push-pull driver, you should be good without a pull-up.

    Below is a good resource for general Pull-up resistor calculation that we recommend for I2C bus but can but used for any open drain system.

    https://www.ti.com/lit/an/slva689/slva689.pdf

    Thank you,

    Regards,

    Sandesh

  • 0 in reply to Sandesh Rawool
    Prodigy 240 points

    Hello Sandesh,

    Thank you for your detailed answer !

    I want to make sure: for non open drain signals, there is no need for pull-up resistors for both 3.3V & 5V PCI bus (A side - not the FPGA)? 

    I might misunderstood the following thread https://e2e.ti.com/support/logic/f/151/t/212146 .

    Thanks!

    regards,

    Yossy Goldenberg

  • 0 in reply to Yossy Goldenberg
    TI__Intellectual 2694 points

    Hi Yossy,

    The VCC dependency and need for pull-up resistor is mainly for the Output side voltage (FPGA in this case). Since you already have a pull-up on the FPGA side you are covered for the different scenarios and output will be pulled to FPGA level/VCC level.

    If you look at Fig. 5 from the datasheet, the Output behavior is based on Input w.r.t VCC.

    Example:

    If VCC = 3.3V, INPUT SIGNAL = 2.3V to 4.3V, VOUT will be close to VCC (~3.3V, slightly low). If full VCC is needed, external pull-up is recommended that you have on FPGA. The device itself does not require pull-up on INPUT side if the voltage conditions are met.

    If there is any concern with the pull-up strength of the input PCI, might not be a bad idea to add pull-up resistor (to VCC) footprint on INPUT with "DO NOT INSTALL" as back up. Not a requirement but having a footprint available will give you option in case the PCI driver is weak.

    Hope this helps.

    Thank you.

    Regards,
    Sandesh

  • 0 in reply to Sandesh Rawool
    Prodigy 240 points

    Hello Sandesh,

    Your help is greatly appreciated.

    Thank you for your support and detailed answers.

    Regards,

    Yossy Goldenberg