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.
V_cc = 3.3v, V_in = 0.8v (@ 75uA hold spec)
R_pull-up = (3.3v - 0.8v) / 75uA = 33K ohm resistor pull-up value

V_cc = 3.3v, V_in = 2.0v (@ 75uA hold spec)
R_pull-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.
R_pull-up = [V_{cc minimum} - V_trip] / I _BHLO (@ 500uA Bus-Hold switch current)
R_pull-up = [ 3.1v - 1.5v] / 500uA
R_pull-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 -
V_CC: The voltage applied to the power pin(s). In most cases the voltage the device needs to operate at.
V_IH: [Voltage Input High] The minimum positive voltage applied to the input which will be accepted by the device as a logic high.
V_IL: [Voltage Input Low] The maximum positive voltage applied to the input which will be accepted by the device as a logic low.
V_OL: [Voltage Output Low] The maximum positive voltage from an output which the device considers will be accepted as the maximum positive low level.
V_OH: [Voltage Output High] The maximum positive voltage from an output which the device considers will be accepted as the minimum positive high level.
V_T: [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

Device Families: TTL (74xx) True TTL 74L Low power 74S Schottky 74H High speed 74LS Low power - Schottky 74AS Advanced - Schottky 74ALS Advanced - Low power - Schottky 74F(AST) Fast - (Advanced - Schottky) 74C CMOS...................check Vcc levels 74HC (U) High speed - CMOS (Unbuffered output) 74HCT High speed - CMOS - TTL inputs 74AHC Advanced - High speed - CMOS 74AHCT Advanced - High speed - CMOS - TTL inputs 74FCT (-A) Fast - CMOS - TTL inputs (speed variations) 74FCT (-T, -AT) Fast - CMOS - TTL inputs (speed variations) 74AC Advanced - CMOS 74ACT Advanced - CMOS - TTL inputs 74FACT AC, ACT (Q) series 74ACQ Advanced - CMOS - Quiet outputs 74ACTQ Advanced - CMOS - TTL inputs - Quiet outputs Bus Driver Families 74ABT Advanced - BiCMOS - Technology 74ABTE ABT - Enhanced Transceiver Logic 74ABTH Advanced - BiCMOS - Technology - bus Hold 74BCT BiCMOS - TTL inputs 74BTL Backplane - Transceiver - Logic 74GTL Gunning - Transceiver - Logic 74GTLP GTL (Motorola) Low Voltage Families 74ALB Advanced - Low Voltage - BiCMOS 74LV (U) Low - Voltage (Unbuffered output) 74LVC (R) (U) LV - CMOS (damping Resistor)(Unbuffered output) 74LVCH Low - Voltage - CMOS - bus Hold 74ALVC Advanced - Low - Voltage - CMOS 74LVT (R) (U) LV - TTL (damping Resistor(Unbuffered output) 74LVTZ Low - Voltage - TTL - High Impedance power-up 74ALVC (R) ALV - CMOS (bus Hold) (damping Resistor) 74ALVCH Advanced - Low - Voltage - CMOS - bus Hold 74LCX LV - CMOS (operates with 3v & 5v supplies) 74VCX LV - CMOS (operates with 1.8v & 3.6v supplies 4000 True CMOS (non-TTL levels) ECL Device Families: MEC I 8nS* MEC II 2nS* MEC III (16XX) 1nS* .......* = Rise & Fall Times 101xx 100 series 10K ECL, 3.5nS* 102xx 200 series 10K ECL, 2.5nS* 108xx 800 series 10K ECL, voltage compensated, 3.5nS* 10Hxxx 10K - High speed, voltage compensated, 1.8nS* 10Exxx 10K - ECLinPS, voltage compensated, 800pS* 100xxx 100K, temperature compensated 100Hxxx 100K - High speed, temperature compensated 100Exxx 100K - ECLinPS, temp, voltage comp., 800pS*

**Related pages on this site:**
. 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.


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