Electronics project notes/Device voltage and impedance
Digital logic voltage levels
|This article/section is a stub — probably a pile of half-sorted notes and is probably a first version, is not well-checked, so may have incorrect bits. (Feel free to ignore, or tell me)|
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.
The ones you'll probably run into most (in home DIY context):
5V TTL: (Vcc is ideally between 4.75V and 5.25V)
- VOL: 0.4V
- VIL: 0.8V
- VIH: 2V
- VOH: 2.4V (can be slightly higher in some logic families(verify))
- VCC: 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 defines levels as a percentage of VCC, 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))
- VOL: 10%
- VIL: 30%
- VIH: 50%
- VOH: 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: 0 to 3.3V. All threshold levels are identical to the above, and only the VOH to VCC 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.
- 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.
See Electronics notes/IO and wired communication#Common_serial_port_variants for more functional-level details.
- VCC 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)
- VDD and VSS 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
Theory: Impedance when connecting two things
Output impedance is larger than the load's input impedance
On a circuit board schematics, and possibly its silkscreen, you see labels like:
- VCC - positive supply, BJT
- VEE - negative supply, BJT (may be Gnd)
- VDD - positive supply, FET
- VSS - negative supply, FET (may be Gnd)
In more general use you might use
- V+ and V-
- VS for supply voltage
- Schematics often mention a rail's voltage, e.g. +12V
...though people have their habits