Electronics project notes/Device voltage and impedance

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Digital logic voltage levels

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

An image search like this may be the best answer or at least good indicator of the variation out there.

I prefer diagrams like this one: http://www.jsykora.info/2014/05/logic-voltage-levels/

...because these summarize and compare many more, and some related conventions, like the voltages in LVDS communication[1].

The ones you'll probably run into most (in home DIY context):

5V TTL

5V TTL: (V_cc is ideally between 4.75V and 5.25V)

• V_OL: 0.4V
• V_IL: 0.8V
• V_IH: 2V
• V_OH: 2.4V (can be slightly higher in some logic families(verify))
• V_CC: 5V

A more intuitive view on that is:

on inputs

low is 0V to 0.8V

high is 2V to 5V

on outputs

low is 0V to 0.4V

high is 2.4V to 5V

The difference between input and output is mainly there so that you can tie such components together, and noise voltages lower than ~0.4V introduced between them won't bother anything.

CMOS

CMOS defines levels as a percentage of V_CC, which can itself be 5, 3.3, 2.5, 1.8, 1.5, 1.2V. (there have also been higher-voltage CMOS systems but this is fairly rare now(verify))

• • V_OL: 10%
  • V_IL: 30%
  • V_IH: 50%
  • V_OH: 70%

Where you may want to controlling a 5V Led string from a 3.3V device,

you can control 5V TTL but not 5V CMOS, because 5V TLL is high above ~2.4V and 5V CMOS is high above 70%*Vcc = ~3.5V (all the highness happens above 3.3V so it'l never go high). For completeness:

3.3V CMOS means (OL=0.33V, IL=1V, IH=1.65V, OH=2.3V) and matches 5V TTL well enough

5V CMOS means (OL=0.5V, IL=1.5V, IH=2.5V, OH=3.5V)

5V TTL means (OL=0.4V, IL=0.8V, IH=2V, OH=2.4V)

LVTTL

LVTTL: 0 to 3.3V. All threshold levels are identical to the above, and only the V_OH to V_CC region is smaller.

Note that the above means that 3.3 and 5V doesn't always imply the same thresholds unless you know the logic family.

Serial

Simple serial

• RS232

space between +3..+15V, mark between -3V..-15V

mostly replaced by 5V or 3V level, same protocol -- but you still see this bipolar, higher-voltage variation (mostly on non-PC devices?)

• "TTL serial" is RS-232-style in communication,

0 and whatever the IC's Vcc is, usually 5V or 3.3V

common on modern boards and ICs that do serial

Seeing a DE-9 connector, you probably want to use a multimeter to check that it is oldschool RS232, and not connect it directly to the latter. Doing so may work, but will also everntually burn out the 5V side.

• RS485

• RS422

See Electronics notes/IO and wired communication#Common_serial_port_variants for more functional-level details.

Other notes

See also:

• http://en.wikipedia.org/wiki/Digital_signal#Logic_voltage_levels

• http://images.google.com/images?q=voltage+levels
  • http://www.interfacebus.com/voltage_threshold.html

• https://learn.sparkfun.com/tutorials/logic-levels/ttl-logic-levels

Note:

• V_CC and Gnd tend to be correlated with BJT transistors and TLL logic

(Vcc seems to have originally meant "the voltage common to all BJT collector pins", which is often just the positive voltage supply)

• V_DD and V_SS tend to refer to FET style ICs and boards ('source', 'drain')
• V+ and V- are more generic, seen e.g. on board power connectors

• ...but people use them fuzzily, and they may mean little more than "the higher and lower voltage that the power supply puts out"

• Also, while V-, Vss, and Gnd in many cases are

what you'd call 0V and/or

used as a voltage reference and/or

the lowest voltage around in the circuit

...but there are even more footnotes to that one.

• I'm assuming 2-level logic here

there are more-level logic out there, but most are specialized/niche [2]

and that three-state logic that you get from tristating is understood as something distinct

Audio

See Music_-_studio_and_stage_notes#Analog_audio_stuff

Semi-sorted

On a circuit board schematics, and possibly its silkscreen, you see labels like:

• V_CC - positive supply, BJT
• V_EE - negative supply, BJT (may be Gnd)

• V_DD - positive supply, FET
• V_SS - negative supply, FET (may be Gnd)

See also https://en.wikipedia.org/wiki/IC_power-supply_pin

In more general use you might use

V+ and V-

V_S for supply voltage

Schematics often mention a rail's voltage, e.g. +12V

...though people have their habits