TXB0106: For SPI 1.8V to1.2V level shift ,Input 20M,output is 10M or some cycle lost.

?? ?

Part Number: TXB0106
Other Parts Discussed in Thread: TXU0304, TXB0304, SN74AVC4T774

Dear,

I use TXB0106 for SPI level shift from 1.8V to 1.2V.When clock data rate is 20M,out waveform is incorrect

The out waveforms are as follow.

input is 20M,output is10M,the output voltage is only about 1V not 1.2V. on the first waveform ,the green is input, the blue is output, others waveform green is output.

input is 20M output is only 10M,and output voltage is only about1V

sometimes, output is 20M but some cycles are not correct.

if I disconnect the loading ,the output is correct now.the loading is just long pcb trace but the pcb trace has stub, nothing is connect.

So, could you tell why the output is not correct?

1.Is it capacitor too big？

2.Does the TXB0106 have issue on switching directions in my case?

Thanks!

3 months ago

0 Clemens Ladisch 3 months ago

Guru 260250 points

Please show the schematic. Are there any pull-up/-down resistors?

Are you using SPI or QSPI? In the first case, better use the TXU0304.

0 ?? ? 3 months ago in reply to Clemens Ladisch

Prodigy 10 points

For schematic,there are no pull-up/-down resistors. just from 1.8V to 5V,then 5V to SNR_DOVDD minimum is 1.2V.

I have tried only use one level shift ,just from 1.8V to 1.2V,but nothing help.

I just use SPI in my case.

0 Clemens Ladisch 3 months ago in reply to ?? ?

Guru 260250 points

TXB I/Os must not be connected to each other.

Do you have any long traces, connectors, or cables?

Why are you using 5 V?

0 ?? ? 3 months ago in reply to Clemens Ladisch

Prodigy 10 points

The loading has long trace and stub,now I move the stub on pcb ,output waveform is ok.

So it seem that it's a SI issue.

I use 5V because of snr_dovdd is from1.2V to3.6V.

0 Clemens Ladisch 3 months ago in reply to ?? ?

Guru 260250 points

TXU translators do not have restrictions on which of VCCA/VCCB must be smaller. I strongly recommend that you use TXU translators. (There are four-channel devices TXU0x04, and two-channel devices TXU0x02; choose whatever matches the directions of your signals.)

0 Jack Guan 3 months ago

TI__Mastermind 20701 points

Hi,

The TXB translators are designed to be weak (very little drive strength) in order to support auto-bidirectional signals at the I/Os. Because of this, the TXB will not be able to drive signals through high cap. loads/ long PCB and cables and therefore suggest cap. loads to be under 70pF to ensure proper SI.

Since SPI signals are commonly fixed directional, you may reference the fixed direction translators that will provide buffering as Clemens' has mentioned.

Regards,

Jack

0 ?? ? 3 months ago in reply to Jack Guan

Prodigy 10 points

Thanks！

0 ?? ? 3 months ago in reply to Jack Guan

Prodigy 10 points

Hi Jack,

BTW,could you please recommend a level shift device.So it can work more than 20M Inout is from 1.8V to 3.6V output is from 1.2V to 1.8V.

TXB0304 cannot reach 20M under 1.8V to 1.2V

Thanks!

0 Jack Guan 3 months ago in reply to ?? ?

TI__Mastermind 20701 points

Hello,

The TXU0x04 can be used in order down translate your SPI signals from 1.8/3.6V to 1.2/1.8V. It has been tested previously up to 60Mbps at this voltage node, though you may also see the below EVM & IBIS model for further testing as needed:

https://www.ti.com/product/TXU0304#design-tools-simulation

https://www.ti.com/tool/TXU-EVM

Regards,

Jack

0 ?? ? 3 months ago in reply to Jack Guan

Prodigy 10 points

Hi Jack,

Thanks ! But I see that The minimum speed of TXU0x04 is only 10M under 1.8V to 1.2V in datasheet.maybe It is special case.

So,I find that SN74AVC4T774 may be better than TXU0x04 ,but I see that chanel skew was about max 11ns,is there any typical value?

And VCCB must not bigger than VCCA, right?

0 Jack Guan 3 months ago in reply to ?? ?

TI__Mastermind 20701 points

Hello,

Skew was not measured at the time of design of various translators, with AVC4T774 being one of them. More details can be found here in this FAQ.

There are no specific biasing requirements with the AVC4T774 (VCCA can be less then, equal to, or greater then VCCB).

In any case of doubt, it is best to bench with the device EVM to further confirm the characteristics of the these translators under your system load conditions.

Regards,

Jack

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TXB0106: For SPI 1.8V to1.2V level shift ,Input 20M,output is 10M or some cycle lost.