In Digital System Board Design, It is suggested to Tie high or low all the CMOS floating and unused inputs. Leaving inputs floating causes gate puncture (gates goes to intermediate logic creating a low resistance path to the ground thereby damaging the device). It is recommended to tie the TTL unused inputs to logic low or high. The value of the pull-up/pull-down resistor should be chosen such that the ICT’s sink/source requirements are met. Below recommendations are for design of pull-up and pull-down terminations for tristate buffers, Open-collector / Open-drain pins, Open-emitter/Open-source pins, bus hold buffers, LVDS differential pins, etc.

Few Recommendations on Pull-Up & Pull-Down Resistor Terminations

  • For configurable pins, option of pull-up as well as pull-down resistor should be provided.
  • If pull-up/pull-down resistor is provided on an actively driven signal, verify the value of the resistor selected.
  • If internal pull-up/pull-down exists, external pull-up/pull-down resistor need not be used unless the internal resistor does not give the desired logic level. If external pull-up/pull-down resistor is to be used at an input pin to oppose the logic state due to the internal pull-up/pull-down resistor, the external resistor should be atleast fifteen times stronger than the internal resistor. For instance, if the internal pull down resistor is x ohms for an input pin, and a logic high is desired at this input, the external pull-up resistor should be x/15 ohms.
  • Consider using bus hold buffers on data buses for dense boards, to avoid the usage of too many pull-ups and pulldowns wherever applicable. Bus hold buffers save board space. But if there are other components in the bus with internal pull-up/pulldown resistor, which could cause a bus level conflict/possible device damage or the board can afford the resistor package (less dense) or has a cost advantage compared to the bus hold buffer, use a normal buffer without bus hold and separate resistor pack.
  • Provide pullup resistors for all open-collector/open drain pins, even if it is not used. Open-collector / Open-drain pins require pullup resistors for their proper operation. Even on unused OC/OD pins, it is recommended to have pullup, to monitor those signals which aids in debugging. Provide separate pull-up resistors for the unused outputs if individual monitoring is required.
  • Provide separate pulldown resistors for all used open-emitter/open source pins. Open-emitter/Open-source pins require a pull down resistor for proper operation. The pull down resistor provides a dc path for sinking current.
  • Provide pull-up/ pull-down resistors for all tri-statable nets. Tri-state nets should have pull-up or pull-down to have a predictable state if no device is driving that net. The pull-up/pull-down should be chosen depending upon the logic required during the undriven case.
  • If a chip has multiple unused “input only pins”, pull-up or pull-down resistors can be provided for a group of pins, rather than providing individual resistors for each pin. If a group of unused pins are provided with a single pullup or pull down resistor, then lot of board space can be saved. However caution need to be taken to make sure that it is ok to connect them together. Refer vendor datasheet and recommendations.