SN74LVT125: Behavior with Hi-z input at startup

Daisuke Higa

Part Number: SN74LVT125
Other Parts Discussed in Thread: SN74LVTH125

Hi support team.

I received the following some questions from my customer.

1. Does this mean that pull-down / pull-up resistors are not recommended for both input (A) and output (Y)?
2. If I use a pull-up / pull-down resistor for input or output, will it have a bad effect?
3. If I start the power supply with the inputs and outputs floating, will the outputs be kept at L?
4. Is it correct to understand that if the input source becomes Hi-z when the power is turned on, a pull-up resistor is required for the input when you want to obtain H for the output?

Could you tell me regarding above questions?

Best regards,

Higa

over 3 years ago

0 Clemens Ladisch over 3 years ago

Guru 243220 points

The inputs must never float (see [FAQ] How does a slow or floating input affect a CMOS device?). If the input is not driven during power up, you need a pull-up or pull-down resistor.

As for the output, the SN74LVT125 itself does not care if you have a pull-up or pull-down. You need it if your circuit expects a valid signal even when the SN74LVT125 is powered down or disabled.

0 user3605066 over 3 years ago in reply to Clemens Ladisch

Prodigy 110 points

I just came across this topic and have one more question, Ladisch.

The device is said to have "bus-hold circuitry" in the datasheet - does this mean this device automatically outputs the last state at the last shutdown state when in the very next wakeup state?

Thanks,

Yukihiro

0 Daisuke Higa over 3 years ago in reply to Clemens Ladisch

Genius 3475 points

Hi Ladisch-san

Thank you for your reply.

I have an additional question.

The SN74LVT125 has a bus hold function, but does the input still require a pull-up or pull-down resistor? (My question is about startup.)
If so, is it because the output may oscillate?

Best regards,

Higa

0 Chad Crosby over 3 years ago in reply to Daisuke Higa

TI__Expert 6680 points

Hi Higa,

Daisuke Higa said:
The SN74LVT125 has a bus hold function, but does the input still require a pull-up or pull-down resistor? (My question is about startup.)

Take a look at section 6.3 of this app note: https://www.ti.com/lit/an/scla015b/scla015b.pdf

Thanks!

Chad Crosby

0 Daisuke Higa over 3 years ago in reply to Chad Crosby

Genius 3475 points

Hi Crosby-san

Thank you for your reply.

>The inputs must never float (see [FAQ] How does a slow or floating input affect a CMOS device?). If the input is not driven during power up, you need a pull-up or pull-down resistor.

Is it wrong to say that Ladisch-san should not make the input floating when he first answered?

Is the formula for calculating the resistance value when pulling down correct in the following?

Rd = Vt / IIhold (max)

I can't find the values for Vt and II (hold) max from the datasheet. Vt = 1.5V? II (hold) max = 75uA?

Best regards,

Higa

0 Clemens Ladisch over 3 years ago in reply to Daisuke Higa

Guru 243220 points

Does the SN74LVT125 really have bus hold? Then what is the difference from the SN74LVTH125?

At the moment, I am assuming that the SN74LVT125 datasheet is wrong, and that it does not have bus hold.

0 Chad Crosby over 3 years ago in reply to Clemens Ladisch

TI__Expert 6680 points

Hi Clemens,

I'd have to agree with you, I'm not seeing any other supporting evidence on my end saying that these devices are the same. Interesting this wasn't caught before. I'll look into getting this changed.

Thanks!

Chad Crosby

0 Chad Crosby over 3 years ago in reply to Daisuke Higa

TI__Expert 6680 points

Hi Higa,

If you need the bus-hold function for your application, I'd recommend using the SN74LVTH125 - this one definitely has the bus-hold feature. I'm going to look into testing the SN74LVT125 out for myself to verify if it has the bus-hold or not.

Daisuke Higa said:
Is it wrong to say that Ladisch-san should not make the input floating when he first answered?

I would still recommend having a pull-up on a signal line that is not being driven by another device. The Bus-hold circuit was designed to mitigate against this; however, it won't harm a bus-hold device to have a correctly sized pull-up resistor on it.

Daisuke Higa said:
Is the formula for calculating the resistance value when pulling down correct in the following?

Rd = Vt / IIhold (max)

I'd recommend using the Rp calculation from the app-note I linked earlier:

Daisuke Higa said:
I can't find the values for Vt and II (hold) max from the datasheet. Vt = 1.5V? II (hold) max = 75uA?

The datasheet for the SN74LVTH125 has the II(hold) spec:

We don't define Vt in our datasheets, as the actual threshold voltage will change depending on temperature, wafer lot, and a whole slew of other factors - but - setting Vt = 1/2 * VCC is usually a safe bet.

Thanks!

Chad Crosby

0 Daisuke Higa over 3 years ago in reply to Chad Crosby

Genius 3475 points

Hi Crosby-san Ladisch-san

Thank you for your support.

>If you need the bus-hold function for your application, I'd recommend using the SN74LVTH125 - this one definitely has the bus-hold feature. >I'm going to look into testing the SN74LVT125 out for myself to verify if it has the bus-hold or not.

I understand. I will wait for your verification result.

Best Regards,

Higa

0 Chad Crosby over 3 years ago in reply to Daisuke Higa

TI__Expert 6680 points

Hi Higa,

Please allow me a couple of weeks to get the parts in & test them.

Thanks!

Chad Crosby

0 Daisuke Higa over 3 years ago in reply to Chad Crosby

Genius 3475 points

Hi Crosby-san

I understand.

Best regards,

Higa

0 Daisuke Higa over 3 years ago in reply to Daisuke Higa

Genius 3475 points

Hi Crosby-san

Do you get samples and have any update?

Best regards,

0 Chad Crosby over 3 years ago in reply to Daisuke Higa

TI__Expert 6680 points

Hi Higa,

I just got the parts in yesterday, and haven't had the chance to test them yet. I'll try to get to them today.

Thanks!

Chad Crosby

0 Chad Crosby over 3 years ago in reply to Chad Crosby

TI__Expert 6680 points

Hi Daisuke Higa, Clemens Ladisch,

I completed testing on both of these parts (SN74LVT125 & SN74LVTH125), and both datasheets are correct - these devices both have bus-hold on the inputs.

Hopefully this helps you Higa.

Thanks!

Chad Crosby

0 Daisuke Higa over 3 years ago in reply to Chad Crosby

Genius 3475 points

Hi Crosby-san

Thank you for your confirmation.

I understand, however have some questions.

Are the SN74LVT125 and SN74LVTH125 exactly the same device?
If yes, could you please confirm IIhold (max) again?
There is no IIhold (max) specification in the SN74LVT125 data sheet.
Is it okay to understand that it is the same as SN74LVTH125?

If No, what's the difference?

Best regarsds,

Higa

+1 Chad Crosby over 3 years ago in reply to Daisuke Higa

TI__Expert 6680 points

Hi Higa,

There actually is an II(hold) spec listed in the electrical characteristic table for the SN74LVT125, I think I might've over looked this earlier - sorry about that.

I think it'd be safe to assume that these devices are the same/very similar. I'm not sure what the differences are between these devices - they could've been created with different names for marketing, but again, that's just my best guess.

Thanks!

Chad Crosby

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SN74LVT125: Behavior with Hi-z input at startup