SN74LV123A: Minimum retrigger time

Hi Nishie-san,

Yuta Nishie said:

1. Which of tMIR (= 0.3 x tw), tMIR (tPLH + tw) and trr is the minimum retrigger time?

t_rr is the pulse retrigger time:

This value is shown in the timing diagram here:

In the applications section, the datasheet also defines t_MIR as the minimum input retriggering time, which is the same as the minimum t_rr value. See image below and compare to above.

The difference is that t_rr is the retrigger time, which can be a minimum, typical, or maximum, while t_MIR is only the minimum value.

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Yuta Nishie said:

2. The data sheet "10.2.1.3 Retriggering Data" states "the two adjacent input signals must be tMIR apart, where tMIR = 0.30 x tw.".

That is correct. The time between the two input trigger events must be at least t_MIR apart. In the below case, the trigger event is the falling edge:

The amount of time that is added from retriggering will be the output pulse width (K*R*C) plus the propagation delay to the output (t_PLH). In long pulses this is not a major issue due to the propagation delay being only a few nanoseconds, but with short pulses, this can be signficant.

Yuta Nishie said:

Which of "7.7 Timing Requirements, VCC = 3.3 V ± 0.3 V" and "7.10 Switching Characteristics, VCC = 3.3 V ± 0.3 V" is this tw?

The t_w they are referencing here is the output pulse width (as shown in Figure 10)

This value will vary depending on your selected timing components.

To be clear:

Timing Requirements explains what limitations there are on inputs from the designer / system.

Switching Characteristics describes measured outputs of the device.

The output pulse width, t_w, is thus in the Switching Characteristics table -- assuming you are using one of the values shown for your timing components:

The datasheet also indicates that the output pulse will be active 15ns after the trigger input --  it is providing a typical delay. Again, with longer pulse widths, this won't have much impact, but with short pulses this could cause the output to retrigger after the timer expires, creating a double pulse (as shown above) instead of a single pulse as desired.