I was looking at the data sheet of a counter IC and the connection circuit diagram showed two connections: Vbb and Vtt
What do these mean? Is it related to the differential inputs the if supports?
Electronic – the meaning of Vtt
digital-logic
Related Solutions
Correct, those are normal D flip-flops. The 4017B itself is just some D flip-flops behind a decoder, so just flip-flops wouldn't be enough to replicate the whole counter; you can use a 74xx42 or 74xx45 as the decoder after the flip-flops.
According to TI's "AHC/AHCT Designer's Guide February 2000", Section 2, "DC Characteristics", subsection 1, "Input Circuit", interfaces with lower-speed circuits with a lower slew rate can cause oscillation at 74AHC/T inputs. To prevent this, 74AHC/T input stages contain a feedback loop which causes the input to have hysteresis much as a Schmitt trigger input would without containing all the circuitry for an actual Schmitt trigger input.
However, the guide goes on to say:
Hysteresis in the input circuit is intended only to process reliably signals that have a slew rate of <10 ns/V. With a signal swing of 5 V, this corresponds to rise and fall times of about 50 ns. If signals with considerably longer rise and fall times are processed, the specially developed Schmitt triggers, such as the SN74AHC(T)14, should be used. These components have a considerably larger hysteresis of about 800 mV at VCC = 5 V and, therefore, allow processing of very slow edges without any problems.
Related Topic
- Electronic – “Input high voltage common mode range?”
- Modelling priority/importance in a digital circuit
- What are “consecutive input pulses”
- The meaning of performing logical operations on two numbers
- Digital Logic – Path Analysis D-FLIP-FLOP and SET Explanation
- SN74LS169B – Identifying Errors in SN74LS169B Datasheet
Best Answer
Vbb - reference voltage for ECL used in single-ended input mode or for some types of termination. Usually you bypass to Vcc and tie the differential input that's not being used to Vbb, or tie inputs through back-to-back Schottky diodes to Vbb. Some devices have a Vbb output or it can be generated with an ECL gate.
Vtt - termination voltage- usually a separate supply at Vcc - 2.0V that allows termination with a single resistor rather than a divider.
Much more on ECL termination here:
Edit: From your information regarding the counter part number and related datasheet:
Vtt should be Vcc - 2.0V.
It might be easier to terminate with a Thevenin-equivalent resistor pair.
R1 || R2 = 50\$\Omega\$
\$\frac { R1}{(R1 + R2)} = \frac {V_{CC}-2}{V_{CC}}\$
(you can do the sums depending on your choice of Vcc)