Logic Levels and Families
There are a bunch of different voltages that are used in designing circuits. The most common a hobbyist would run into are 5V and 3.3V. But, they're not really 5V and 3.3V, but instead a combination of thresholds that make up the family. For the purposes of this, we're going to talk about the families that are common in the 74xx series of logic circuits.
What is a Logic Low? A High?
We have to start by defining the various levels. There's a few, but they should all make sense, taken from lowest voltage to highest:
- GND. The ground reference potential. Typically this is zero volts. This could also be referred to as VEE or VSS.
- VOL. The target output level for the transmitter to send a logic low. The level must be between GND and VOL.
- VIL. The target input level for the receiver to interpret a logic low. Anything between GND and VIL will be interpreted as logic low.
- Vt. The transition voltage, which is the hard line between logic high and low. This is the minimum potential to cause current to flow within the device's internal transistors.
- VIH The target input level for a receiver to interpret a logic high. Anything between VIH and VCC should be considered a high.
- VOH. The target output level for a transmitter to send a logic high. Anything between VOH and VCC is acceptable.
- VCC. The main positive voltage for the design. This can also be called VDD.
The reason there are different points for input and output thresholds is to provide a clear noise margin to account for noise in the circuit, as well as potential voltage drop over the wire/trace.
Families of Logic
All of the logic out there typically falls into one of a couple of families (mostly defined by JEDEC):
- 5V TTL
- 5V CMOS
- 3.3V CMOS or TTL (typically identical)
- 2.5V CMOS
- 1.8V CMOS
For each of those, they have the following levels in volts:
Voltage Tolerance and Compliance
A device that is voltage tolerant can withstand a voltage greater than its VCC on its I/O pins. For example, if a device has a VCC of 2.5 V and can accept inputs equal to 3.3V and can withstand 3.3V on its outputs, the 2.5V device is called 3.3V tolerant. A device which is voltage compliant can receive signals from and transmit signals to a device which is operated at a voltage greater than its own VCC. For example, if a device has a 2.5V VCC and can transmit and receive signals to and from a 3.3V device, the 2.5V device is said to be 3.3V compliant.
74-Series Logic Families
I should probably write an entire thing on 74-series logic, as it's really the foundation of so many things historically, and even still in the most advanced systems, you'll find a smattering of "glue logic" implemented in 74-series logic. For now, I'm just going to wave my hand at it, and discuss the families.
If we convert this to 74-series logic, we can see how they various logic families (those crazy letters in the middle of part numbers) fit together. For example, for TI, this is just some of the families, and really you have to pull apart what exactly you need:
|ABT, AC/ACT, AHC, AHCT, ALS, AS, BCT, F, LV, LV1T, LV-A, LS, S, TTL, FCT2
|AC, AHC, ALB, ALVC, ALVT, AUP, AVC, LV, LV-A, LVC, LVT, LV1T, AUP1T
|ALVC, ALVT, AUC, AUP, AVC, LV, LV1T, LV-A, LVC
|ALVC, AUC, AUP, AVC, LVC, LV1T
Note that the CD4000 family typically fits under the 5V heading. In addition, there are families with even lower voltage levels available, but they're unlikely to show up anywhere near a hobbyist.
For me, I typically will use LVC or AHC for 3.3V and ACT for 5V.
Comments or Questions?
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