SN74LVC8T245 SPI Conversion Issue

vineetrvce

Hi All,

Please find attached the circuit diagram in this mail.

Here the VCCB voltage can be either of the three values. When the VCCB is 3.3V, then my SPI communication is working fine. But when I change that to 1.8V or 5V, the SPI communication is not working. Any ideas what is going wrong?

Regards,

Vineet.

over 8 years ago

0 Emrys Maier over 8 years ago

TI__Guru* 96591 points

Vineet,
Thanks for your post. I am moving this to the Voltage Translation Forum for the best support.

0 Jennifer Joseph over 8 years ago

TI__Intellectual 2530 points

Hi Vineet

I will support you on this.

1)Please help me clarify this- On the same issue, but posted in different link you mentioned "I am able to make the level shifters work with VCCB = 3.3V and 5V. But for some reason, the 1.8V is just not working". The above says that only 3.3V works. Which one is correct?

2) Which one comes up first - Vcca or Vccb?

I will look into this further and provide you with update. Thanks.

0 vineetrvce over 8 years ago in reply to Jennifer Joseph

Intellectual 565 points

Hi Jennifer,

Initially with VCCA = 3.3V and VCCB = 3.3V was only working. Then I changed my wiring a little bit, then I was able to make the VCCB = 5V work. So to summarise:

VCCA = 3.3V and VCCB = 3.3V : Working

VCCA = 3.3V and VCCB = 5V : Working

VCCA = 3.3V and VCCB = 1.8V : Not Working.

VCCA is generated by a 3.3V regulator the output of which is going to the 1.8V regulator. So I guess VCCA is coming up first.

Regards,

Vineet.

0 vineetrvce over 8 years ago in reply to vineetrvce

Intellectual 565 points

Hi Jennifer,
I also tried to power the VCCA first and after VCCB. No luck @ VCCB = 1.8V.

0 Jennifer Joseph over 8 years ago in reply to vineetrvce

TI__Intellectual 2530 points

Hi Vineet

I have asked for design help to see if there is any issues with Vccb being lower, there doesn't seem to be an issue that stands out though.

What was the wiring that you changed that helped with the 5V?

Thanks

Logic

Logic forum

SN74LVC8T245 SPI Conversion Issue