# Integrated Circuit Bus-Hold Input Pins

 Bus-Hold Input Pins Pull-up resistors should not be used with an IC that has Bus-Hold. An IC with Bus-Hold prevents a floating line by providing a small amount of positive feedback current on the IC input pin. Glue logic ICs which contain Bus-Hold will have an 'H' in their part number. In this case the 'H' does not mean 'High-Speed' as it did with some older TTL and CMOS logic families. Circuit with Bus-Hold should be used in place of a resistor. In addition, a Bus-Hold does not require the input line to be tied high or low, the line may be left open [floating]. Bus-Hold circuit consume in the range of 50 to 75uA of current to hold the line in its last state. A current of 500uA [per connected pin] is required to 'Over-Drive' the circuit causing it to change state. Adding a pull-up resistor to a pin with Bus-Hold will cause higher then necessary current demand. If the pull-up is small enough [increasing the load current], the driver may not be able to switch in the time required. Thevenin or parallel terminated lines are not recommended because of their very low values.

Follow the equations below in the event a resistor is used on a Bus-Hold input with a totem-pole output driver.
Vcc = 3.3v, Vin = 0.8v (@ 75uA hold spec)
Rpull-up = (3.3v - 0.8v) / 75uA = 33K ohm resistor pull-up value

Vcc = 3.3v, Vin = 2.0v (@ 75uA hold spec)
Rpull-down = (2.0v) / 75uA = 26K ohm resistor pull-down value
Pull-up resistors used with totem-pole output drivers contend with the driver. So the resistor needs to be scaled to a higher value.

Follow these equations in the event a resistor is used on a Bus-Hold input with an Open-Collector output driver.
Rpull-up = [Vcc minimum - Vtrip] / I BHLO (@ 500uA Bus-Hold switch current)
Rpull-up = [ 3.1v - 1.5v] / 500uA
Rpull-up = 3200 ohms
While Pull-up resistors used with open collector drivers need to be scaled to supply the Bus-Hold switching current; I BHLO [500uA]. There is no contention with the driver on an open collector IC, because the driver will not pull the line high [only low]. The calculation also holds true when a resistor is pulling a switch or other device up from ground or when a resistor is pulling up capacitor from ground, as in an RC circuit timing circuit.

Terms -
VCC: The voltage applied to the power pin(s). In most cases the voltage the device needs to operate at.
VIH: [Voltage Input High] The minimum positive voltage applied to the input which will be accepted by the device as a logic high.
VIL: [Voltage Input Low] The maximum positive voltage applied to the input which will be accepted by the device as a logic low.
VOL: [Voltage Output Low] The maximum positive voltage from an output which the device considers will be accepted as the maximum positive low level.
VOH: [Voltage Output High] The maximum positive voltage from an output which the device considers will be accepted as the minimum positive high level.
VT: [Threshold Voltage] The voltage applied to a device which is "transition-Operated", which cause the device to switch. May also be listed as a '+' or '-' value.

Description of TTL, ECL and CMOS Glue Logic Families

 . Standard Logic Voltage Thresholds . . Bus Logic Thresholds . . Logic Speed x Power Chart . . Trace Termination . . Ground/Power Planes .

Back to the IC Input/Output Pull-up values, or Logic Design Page.

Modified 6/13/15