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I'm currently reading through the datasheet for the 74HC165 8-bit asynchronous parallel-to-serial shift register, and I've a handful of questions about the static and dynamic characteristics described therein:

  1. The recommended operating condition for supply voltage (Vcc) states a typical value of 5V (with a minimum and maximum of 2V and 7V, respectively). Why then, when listing the static/dynamic characteristics such as logic voltages and timings, does the datasheet use Vcc = 2, 4.5, and 7 volts as its conditions for min/typ/max and not 5V?

    A few of the timing characteristics have those three, but then they have a Vcc = 5V case which also specifies an output load capacitance (C_L). Why is this, and why for only some of them?

  2. The timings only have minimum and typical periods, but they don't seem to make sense. For example, pulse width (t_W) which as I understand represents the amount of time the pulse needs to be active to allow time for detection, has a minimum of 16 ns and a typical period of 5 ns. The minimum is obviously larger than typical, so what am I supposed to make of this?

Thank you!

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  • \$\begingroup\$ Just on (1): I do agree, but they decided to specify for 2 V and 7 V, being the extremes, and a 1986-typical 5 V regulator was +/-10 % hence the low-end of 4.5 V is used. Don't know about (2), it's an odd way to specify and if there's a logic to it, it's not apparent. There's nothing in the spec' table footnotes. \$\endgroup\$ – TonyM May 25 '17 at 17:16
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    \$\begingroup\$ 7V is not just the maximum, it's the Absolute Maximum. The chip is not designed to operate on this voltage. If run at 7V it would probably have reduced reliability, and could be damaged by spikes that would be OK at lower operating voltage. \$\endgroup\$ – Bruce Abbott May 26 '17 at 5:07
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Vcc typical 5V (with a minimum and maximum of 2V and 7V) ... why use Vcc = 2, 4.5, and 7 volts as its conditions

Probably because the characteristics at 4.5 V are less desirable than those at 5 V. If they specified values at 5 V, then you couldn't use them with a typical "5 V" supply that could be a little below 5 V. They are specifying the characteristics for a slightly worse case than 5 V so that your supply can have a little slop but you can still rely on those specs.

time the pulse needs to be active ... minimum 16 ns ... typical period 5 ns

There is nothing inconsistent here. The minimum is the real spec. You have to hold the value for 16 ns if you want it to be interpreted correctly all the time. It usually takes only 5 ns, but you can't count on that.

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  • \$\begingroup\$ Great response, thank you. I can't say I'm a fan of the min/typ delays though; if 5ns isn't guaranteed to be enough time then it shouldn't even be mentioned in the spec, should it? \$\endgroup\$ – Connor Spangler May 25 '17 at 17:18
  • \$\begingroup\$ This allows you to estimate the effects of a noise spike. \$\endgroup\$ – CL. May 25 '17 at 17:27
  • \$\begingroup\$ Can you elaborate on that? \$\endgroup\$ – Connor Spangler May 25 '17 at 17:29
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    \$\begingroup\$ @Ans: Typical specs are largely useless. It's usually marketing that wants typical specs included, since they look better than the worst case specs. Then they can use those typical specs on flashy sell sheets to get people's attention and make their product look better than competing ones, even though it may not be when you look at the specs more carefully. Really reputable companies, like the old HP, didn't do this. Typical specs were largely forbidden to ensure nobody got the wrong impression. Sleaze is unfortunately more common now. \$\endgroup\$ – Olin Lathrop May 25 '17 at 17:50
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There are two devices in your data sheet.

74HCT165, which has: 4.5V min, 5.0V typical and 5.5V. This is equivalent to your TTL families of days gone by.

74HC165 has: 2V min, 5V typical and 7V max. Extrapolate to get performance at different voltage (3.3V).

This reflects today's dynamic where you have processors operating at different voltages and reducing voltage for power saving. 3.3V micro with voltage dropping to 2V during power saving at lower frequency. We do not want logic to stop at lower voltage/frequency, just go slower to save power.

Specs are listed for each voltage level for the 74HC165, while the 74HCT165 are more generic. Generally, as the voltage increases the 74HC165 is faster, drawing more power.

Some of the data is Static and some are Dynamic. To test dynamic characteristics they attach a load \$C_L = 50pF\$ to represent an attached circuit and run the tests. All will be specified at 2V, 4.5V and 6V for the 74HC165 and 4.5V for the 74HCT165. The 74HC165 at 4.5V will be comparable to the 74HCT165 at 4.5V.

Timings will have Minimum, Typical or Maximum as required. You design for the extremes, but all chip will generally fall in the typical range. This all depends on what is attached. Exceed \$C_L = 50pF\$ and operation will be closer to extremes. Specs are guidelines engineers can check their design against.

At 25\$^{\circ}\$C: 74HC165 at 2V, CP pulse width is 80ns min 17ns typ; 4.5V: 16ns, 6ns; and 6V: 14ns, 5ns. 74HCT165 at 4.5V: 16ns, 6ns. As expected, they are comparable at the same voltage.

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