Interface Threshold Voltage Levels




Interface Devices Voltage Threshold Levels

Comparison of Input and Output [I/O] logic switching levels for:
CMOS [5V], PECL [5V], RS422/485, RS232, LVDS, BTL, & GTL logic families.

Although not shown in the graph above an IC will handle a much larger voltage level than shown as the Voh level.
Being able to support a larger input voltage than what may be driven by an output is common for ICs.
VML [not shown] has a Voh of 1.65v and a Vol of 0.85 volts and an output voltage of 800mV [single ended].
CML [not shown] has a Voh of 1.9v and a Vol of 1.1 volts and an output voltage of 800mV [single ended].





The graph above provides a comparison between the Input and Output [I/O] logic switching levels for a few different Interface bus types.
The important point is the voltage point at which each of the logic families switch at.
The output logic levels above are defined by the Terms section below.
For a review of Noise Margin numbers and a short description of many of the IC logic families , refer to the Logic Family Selection page.
A graph for Low Voltage [LV] devices resides on the LV Logic Threshold page.
An additional chart of Standard Logic Voltage levels is provided on the Logic Threshold Voltage Levels page


Logic Switching Terms -
VCC: The voltage applied to the power pin(s). In most cases it's the 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.
Vtt may also be listed as a '+' or '-' value, via a dot on the input pin.
Vcc is the output drive level, but may be a different voltage than the internal circuitry of the IC.
That is, a few types of ICs have one Vcc pin to power the device, and a separate Vcc line to drive the output pin.


Description of TTL, ECL and CMOS Glue Logic Families





Regardless of an IC being low-voltage, or high-speed, interface ICs are designed to drive large loads.
The load could be a low resistive IC, a high capacitive load, or a long transmission line.
An interface IC needs to provide more current than a normal logic IC.

Back to the Logic Design Page

Related pages on this site:
Logic Family Slew Rate. .Glue Logic Speed x Power Chart. .How to Termination Traces. .Digital Logic Pitfalls.


PC motherboard
Home

Distributor rolodex Electronic Components Electronic Equipment EDA CDROM Software Engineering Standards, BOB card Cabled Computer Bus Electronic Engineering Design Table Conversion DB9-to-DB25.
DistributorsComponents Equipment Software Standards Buses Design Reference

Modified 12/23/11
Copyright © 1998 - 2016 All rights reserved Larry Davis