LSF0108: How to make sure the I/O port is "Push-pull" mode or "Open-drain" mode

Nan Li

Intellectual 260 points

Part Number: LSF0108

Hi everyone,

I learned that LSF0108 supports both push-pull and open-drain mode output, my question is:

1) If I add external pullup resistor on one I/O port, so what's the output mode now? push-pull or open-drain and why?

2) If the I/O(output) port mode has relationship with the peripheral part's I/O(input) mode?

Thanks,

Nan

over 3 years ago

0 Clemens Ladisch over 3 years ago

Guru 316780 points

The LSF supports push/pull and open drain only on its inputs. Its outputs always are open drain.

0 Nan Li over 3 years ago in reply to Clemens Ladisch

Intellectual 260 points

Hi Clemens,

I think its output can support push-pull as datasheet shows:

It's wrong?

Regards,

Nan

+1 Emrys Maier over 3 years ago in reply to Nan Li

TI__Guru* 97046 points

Hi Nan,

The datasheet wording is correct, I think you may be misunderstanding it though.

The LSF device is not the one with a push-pull output in the highlighted sentences.

The first talks about the "controller driver."

The second talks about the "peripheral device."

These are the two devices that are connected to the LSF translator. They can have push-pull or open-drain outputs. The LSF remains a passive translator in all cases.

You are correct that the LSF does not exactly have open-drain outputs -- the highlighted case is where the outputs are being clamped to a particular value by the internal pass-FET of the LSF.

There's an FAQ here that explain the device in more detail: [FAQ] How do the LSF translators work?

0 Nan Li over 3 years ago in reply to Emrys Maier

Intellectual 260 points

Hi Emrys,

Thanks, I get it!

I am confused with another point: I need to use LSF0108 for SPI(3-wire) signals, and I see that if I want to make the transfer rate about 100MHz I need add a cap load which is smaller than 30pf,,,

which means that I should connect a serial cap, smaller than 30pf, to the wire? such as below:

Regards,

Nan

0 Nan Li over 3 years ago in reply to Emrys Maier

Intellectual 260 points

Hi Emrys,

I want to test if it can correct transfer data at about 100MHz, the LSF0108 part in my final design like this:

am I right?

Thanks,

Nan

+1 Clemens Ladisch over 3 years ago in reply to Nan Li

Guru 316780 points

That capacitive load just describes the allowed parasitic capacitances of the traces and other devices' inputs. You must not use AC coupling for plain logic signals.

0 Nan Li over 3 years ago in reply to Clemens Ladisch

Intellectual 260 points

Hi Clemens,

uh,,,

So I need to delete these Cap to make sure LSF0108 works normally, and it will support 100MHz data transfer rate, right?

Regards,

Nan

0 Clemens Ladisch over 3 years ago in reply to Nan Li

Guru 316780 points

Yes.

0 Nan Li over 3 years ago in reply to Clemens Ladisch

Intellectual 260 points

Thanks!

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LSF0108: How to make sure the I/O port is "Push-pull" mode or "Open-drain" mode