TXS0108E: the signal is distorted after tx0108's translation

chunjie wang

Part Number: TXS0108E
Other Parts Discussed in Thread: SN74AXC8T245, SN74AXC4T774, SN74LVC8T245, SN74LV4T125

Dear expert,

In the design, we use the TXS0108 to translate the SPI CLK from 1.8 V to 3.3 V, running at about 625KHz. The signal path is shown in the following picture. And there is no pull-up resistor in the 3.3V SCLK.

When probing the 3.3V clock, it's found that the signal is distorted, shown as the last picture. And the probe is shown in the following picture. Actually, the clock frequency is just 625KHZ, a low clock rate.

Could you explain why he 3.3V clock is distorted, and how to fix it?

Thanks ahead

Regards,

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 244020 points

This looks as if the one-shot times out before the rising edge has reached the high level. I'd guess that the capactive load on the 3.3 V line is too high; are there long traces, cables, or connectors? (See section 9.3.3 of the datasheet.)

0 chunjie wang over 4 years ago in reply to Clemens Ladisch

Intellectual 270 points

Dear Clemens,

I check the SPI device's spec, which tells that the input load is 2 pF and there is a 30KR pull-down resistor internally.

On the board, there is no cable and connectors, and the trace is 3818mil. Is the trace beyond the expectation?

Moreover, could you recommend some unidirectional level translation chips which have a strong drive capability?

Thanks

0 Karan Kotadia over 4 years ago in reply to chunjie wang

TI__Genius 16725 points

Hi,

Yes, having long trace length will add too much capacitance load for the TXS passive translators to handle.

Do you really require an open drain device? If not, then I can recommend you use the SN74AXC8T245. This will have a much stronger drive strength that you require.

Thanks!

-Karan

0 ShreyasRao over 4 years ago in reply to chunjie wang

TI__Guru 56100 points

Chunjie,

The SN74AXC4T774 is specifically built for the SPI lines in a single chip.

Please refer to the video here below:

training.ti.com/tlm-spi-trans

0 chunjie wang over 4 years ago in reply to Karan Kotadia

Intellectual 270 points

Hi Karan，

Many thanks for your inform.

Could you recommend a device used for logic translation, not for the case of open drain, from 3.3V to 5V with the supportive switching rate being beyond 5MHz?

Thanks again.

-Chunjue

0 chunjie wang over 4 years ago in reply to ShreyasRao

Intellectual 270 points

Hi Shreyas,

Wonderful! Thanks a lot!

-chunjie

0 Clemens Ladisch over 4 years ago in reply to chunjie wang

Guru 244020 points

SN74LVC8T245 or 2× SN74LV4T125

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TXS0108E: the signal is distorted after tx0108's translation