TXB0304: SPI Fanout

John Jose

Part Number: TXB0304
Other Parts Discussed in Thread: LSF0204, TXU0304, SN74LVC125A

Hi,

I am designing a system that has lot of slave devices running on a single SPI bus. The architecture of the existing system is represented in the attached image.

So, here the micro controller drives 8 TXB0304 which is used as a buffer with 3.3V on both VCCA and VCCB (marked as RED) feeding to 8 LSF0204 which is doing the level translation from 3.3V to 1.2V (marked as BLUE).

The total length of the trace from micro controller to each TXB0304 will be around 15cm - 25cm and TXB0304 to each LSF0204 will be around 5cm to 15cm

With respect to the same I have a few questions,

1. Will the TXB0304 will be able to operate with 3.3V on both ends?

2. What are the chances/reasons that the above architecture could fail?

3. Is there any better approach to the above problem?

Regards

John

over 2 years ago

0 Clemens Ladisch over 2 years ago

Guru 316640 points

The TXB has a drive strength if 20 µA and cannot be used as a buffer. Also, the LSF does not have enough drive strength for the TXB inputs.

For SPI level shifting, use a buffered translator like the TXU0304. For plain buffering, use a buffer like the SN74LVC125A, but you probably do not need it, or only a single one at the microcontroller.

0 John Jose over 2 years ago in reply to Clemens Ladisch

Prodigy 20 points

Clemens Ladisch So you are suggesting to replace the LSF0204 with TXU0304 (which are pin compatible) and put one buffer SN74LVC125A after the uC to feed 48(as per the design) TXU0304.

0 Clemens Ladisch over 2 years ago in reply to John Jose

Guru 316640 points

8 or 48? For the latter, you might indeed use more buffers.

0 John Jose over 2 years ago in reply to Clemens Ladisch

Prodigy 20 points

48 and how do I estimate the number of buffers needed.

0 Clemens Ladisch over 2 years ago in reply to John Jose

Guru 316640 points

Estimate the total capacitance of all inputs (about 3 pF per input) and traces (about 1 pF/cm), and aim for roughly 100 pF per output.

0 John Jose over 2 years ago in reply to Clemens Ladisch

Prodigy 20 points

On using the SN74LVC125 the datasheet recommends connecting the OE to VCC using a pull up. Is it okay if I permanently pull it down to the GND. The glitches in startup can be tolerated, but doing so will it cause any glitches once the power up is completed.

0 Clemens Ladisch over 2 years ago in reply to John Jose

Guru 316640 points

The datasheet says:

To ensure the high-impedance state during power up or power down, /OE should be tied to V_CC through a pullup resistor.

If you do not care about the state of the outputs during power up or power down, you can connect /OE directly to GND.

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TXB0304: SPI Fanout