SN65HVS885 SPI MISO Question

Daniel Vamos

Other Parts Discussed in Thread: SN65HVS885EVM, ADS1263

Hello,

I would like to connect SN65HVS885EVM to an existing SPI Bus where an ADS1263 ADC is located. Every time I connect the serializer to the bus, (more precisely connecting the SOP to MISO) I lost the data comming from ADS. When I disconnect the SOP from the bus, it works fine.

I can communicate with both devices separately but when I connect both, the MISO line is down.

Here is a thread similar to my problem: but I cant go on.

Any idea?

over 9 years ago

0 Daniel Vamos over 9 years ago

Genius 3950 points

uC is TM4C123GXL. I suppose the Logic level could be an issue. The LP has 3.3V signals but the serializer require 5V TTL logic levels. What do you think?

0 Casey McCrea over 9 years ago in reply to Daniel Vamos

TI__Guru** 103215 points

Hello Daniel,

Based on the data sheet and user's guide for the device I don't see the SOP being a tri-stated output. Once data is clocked out serially, the SOP pin will remain at the logic level of SIP. So in this case if you have SIP pulled low (or floating), then the SOP output will remain low before and after you clock out the parallel inputs. So if this is the case, then your MISO line will be pulled down and you won't be able to communicate. If you pull SIP high, then at the end the serial clocking the MISO line will be pulled high and the ADC must pull the line low to communicate. The pull up may be too strong for the ADC to pull down though, but you could give it a try as a first option.

Another thing to consider is using a unidirectional level shifter with output enable. This should allow you to put the SOP output into high impedance by disabling output enable. This is probably a good idea anyways since you are using 3.3V logic.

Best Regards,
Casey

0 Daniel Vamos over 9 years ago in reply to Casey McCrea

Genius 3950 points

Could you recommend a device of level shifter?

0 Casey McCrea over 9 years ago in reply to Daniel Vamos

TI__Guru** 103215 points

Hello Daniel,

I'm not very familiar with the level shifter portfolio, but here is the main page for voltage level translation products: www.ti.com/.../voltage-level-translation-overview.page

I believe what you need is a 5V to 3.3V unidirectional level shifter and you must make sure the part has an output enable/disable functionality to allow for putting the output in high impedance. If you need more assistance selecting the level shifter, you could also post a question in the logic forum and explain the functionality you need. The logic forum is here: e2e.ti.com/.../

Best Regards,
Casey

0 Daniel Vamos over 9 years ago in reply to Casey McCrea

Genius 3950 points

Thank you for your reply.

0 Daniel Vamos over 9 years ago in reply to Casey McCrea

Genius 3950 points

Dear Casey,

We have a schematic for Digital Input. Before we go on with board design, may I ask you to review our 1-paged design? If its possible I could send you in a pm.

Thank you in advance,

0 Casey McCrea over 9 years ago in reply to Daniel Vamos

TI__Guru** 103215 points

Hello Daniel,

You can send me an email at c-mccrea@ti.com

Thanks,
Casey

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SN65HVS885 SPI MISO Question