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.

There are many images somewhat like this:

Such plots are more approximate than they may seem.

While this is certainly close enough for hobbying, but there are lot of subtle details that are impossible to catch in a single graph.

Part of those extra levels defined level comes from acknowledgement of real-world details, such as that

• levels are rarely fully rail to rail, due to the way outputs drive, and inputs protect
• the real world has voltage losses and noise, and you probably want some guaranteed leeway for stability

These put some extra numbers to that, more specifically defining some goals and guarantees:

• V_OH - output high min - the minimum voltage and output guarantees to be produce for a high

and in generally, devices will produce higher

• V_IH - input high min - the minimum voltage that an input recognized as high

• V_IL - input low max - the maximum that will still be recognized as low
• V_OL - outut low max - the maximum output allowed to be driven that for low

Notes:

• the spec's V_OH is higher than V_IH, and V_OL is lower than V_IL, in part for leeway reasons:

you can have lower gate voltage, voltage losses, and noise, and less than ~0.4V of it (or however much it is for what you have/combine) won't bother anything - it will still have well defined logic levels

• Mames like 'CMOS' is a isn't a family - it contains several subfamilies

diagrams like the below like to smush that into just one, in part because it is expected you know that you have to look at the details anyway

• In a way V_IH, V_OH, V_IL, and V_OL can be seen as family goals.

They are technically specs,

but even but some of these (especially V_OHand V_OL, near the rails) may

vary between subfamilies (but each will still have strong patterns within families/subfamilies), e.g. visible in the 7400 series, this being part of the difference of those middle letters (the logic family name)

might vary per device (e.g. theATMega328 used in Arduinos is like 5V CMOS but has a more lenient V_IH, V_OH, and V_OL[1])

plus the actual output levels(verify) will vary a little with the amount of current it's sinking

• You may see that CMOS has the idea that V_OL is at 10%, V_IL at 30%, V_IH at 50%, and V_OH at 70% of Vcc

The numbers may not be precise (especially at lower voltage CMOS)

...but it is still a good reminder that in CMOS, these levels may vary when Vcc varies - while most other systems do not(verify)

• LVTTL has the same levels as 3.3V CMOS - the largest difference is that those levels don't shift those levels when Vcc changed

(something else not that easy to show in a plot)

• 2.5V CMOS and 2.5 TTL differ slightly in definition

• if you are using digital input and outputs this will rately bite you, because outputs will be as low and high as capable, and inputs are permissive

• The behaviour in the zone between low and high (V_IL to V_IH) is not defined

what it does here may be well behaved enough in certain devices in certain situations, but not required to -- so do not design to use this, even if it seems to work

• Vt is an indication where the transistor may start to transition', but it is not there for a logic threshold, more to model switching speed and timing(verify), and as an indication of behaviour at lower voltages(verify)

Serial

✎ 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.

Simple serial

• "Classic" 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 terms of the communicated bits,

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 with its higher voltages, and not connect it directly to the latter. (Doing so may function today, but will also everntually burn out the 5V side)

• RS485

differential signalling between two lines:

devices should not be damaged by ~6V differential (assumed to be unloaded level) (verify)

and when loaded should probably stay over 1.5V (verify), recommended over 2V(verify)

devices are expected to detect differences as small as 200mV

(those levels should also be within −7V to +12V of common - which is not used in communication but required for function and to avoid damage. This is a detail that matters mostly in larger setups.

impedance (verify) https://electronics.stackexchange.com/questions/77630/what-is-1-8-unit-load-in-rs485-communication

drivers required to tristate so that every device can send

• RS422

−6V to +6V differential

100 Ohm impedance

largely like RS485

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_-_technical_side#Voltages_and_impedances

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