Logic Family Slew Rate

The slew rate of a device is the rate of change of it's output [from high to low, or from low to high]. Or the amount of time it takes an IC to switch from 10% to 90% of it's final value in a given time. This graphic provides an equation for determining IC slew rate. The formula requires the signal rise time, and the output low and high voltages. Some devices may have a different rise time and a different fall time, so pick the faster of the two. The reason why the switching time from low to high or high to low will differ in some devices depend on the internal structure of the device. Some ICs may have a different value of internal resistor between the Vcc line to output and a different value between the the output and ground. Normally the resistance is due to the on-resistance of a FET.
The table below provides data for propagation delay and slew rate for each of the families listed. Don't allow digital logic slew rates to be slower than what is specified by the data sheet. All digital logic families will oscillate with slow rise times, destroying the input gate structure of the receiving gate IC.

Use an IC with a Schmitt Trigger input to over come slow rise times.
The Schmitt Trigger will compensate for noisy inputs and incoming slow
rise times by providing switching limits or hystersis to the input.
The graphic is the logic symbol for a NOT gate with a Schmitt Trigger input.

**IC Device Slew Rate**
Device	F	ALS	ABT	AC	HC	AHC	AHC	LVT	ALVC	LVC	LV
Propagation Delay {nS}	4	6	2.7	5	13	5.5	8.3	2.4	2.0	4.5	10
Voltage Swing {V}	3	3	3	4.8	4.8	4.8	3	3	3	3	3
Slew Rate {V/nS}	1.3	1.0	1.0	2.0	0.9	0.8	0.5	1.2	1.3	0.9	0.7
-	V_CC = 5.0v						V_CC = 3.3v

Slew rate, and rise times is also effected by what is connected to the devices output. An IC's rise time may slow as the number of devices the IC has to drive increases. There really are no logic families with very long rise times. Slow rise times these days are considered to be longer then 50nS. However; Slow rise times could be produced by 3-state outputs which are not pulled high [for example], or devices which need to drive large capacitive loads. The 3-state outputs produce slow slew rates, because the floating receiver charges up based on its leakage current (10uA to 20uA). Large capacitive loads cause slow rise times because the driver needs to charge up the capacitor. The rise time is then based on the RC time constant of the circuit, and the amount of current the IC can deliver. Typical rise and fall times for most logic devices will range from between 1nS and 4nS.
Many FPGA's will have a selectable slew rate for it's outputs. The Xilinx 20KE FPGA may be configured for a Fast slew rate of 0.4nS rise and fall time, or slow slew rate of 1.1nS rise time and 2.04nS fall time. In addition, GTL / GTLP devices also offer an adjustable slew rate for their GTL interfaces.

For a review of Noise Margin numbers and a short description of many of the IC logic families , refer back to the Noise Margin page. A graph for Standard voltage devices resides on the Logic Voltage Threshold page. An additional chart of Interface bus threshold levels is provided on the Interface Threshold Voltage Level page.

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	Interface Bus Logic Thresholds	Glue Logic Speed x Power Chart	How to Termination Traces	Resistor Pull-up

Back to the main Logic Design Page.

Electronic Design Key words: Slew Rate, Propagation, Rise Time, Fall Time, Floating input, Glue Logic Families, CMOS, TTL, ECL, Speed, IC, Integrated Circuits,
Propagation delay, Intermittent failure, Random, Logic Types, logic switching levels, Output voltage, Low-Voltage, Threshold Voltage, 74xx, 74AC00, 74HC, 74AHC00,
Digital Devices, Description, Definition for Logic family, Logic Terms, TTL: Transistor Transistor Logic, ECL: Emitter Coupled Logic, CMOS:
Complementary Metal-Oxide Semiconductor, BICMOS: Bipolar Complementary Metal Oxide Semiconductor, Electronic Devices, formula, Calculation


Home


Distributors	Components	Equipment	Software	Standards	Buses	Design	Reference