I have been reading more and more about bandwidths and frequencies to try to solve the following problem, but I am only ending up more confused. (I have limited electrical/signals background.)

I wish to communicate with an IC via SPI. The device has a max clock frequency of 8 MHz. I also need to use a logic level converter to convert between 5V micro-controller and the 3.3V device. I found a Logic Shifter online that states "the bandwidth on the individual signal channels can range from 20Mbps up to 100Mbps... suitable for higher speed signals such as SPI."

I am confused how to interpret the bandwidth of 20Mbps to 100Mbps in terms of my clock frequency for SPI.

Does the Mbps limit my clock frequency choices, and if so, why? From what I have read, bits-per-second and cycles-per-second measure different things, yet then I wonder why the manufacturer even bothered stating the Mbps range if it doesn't affect my SPI clock.


1 Answer 1


SPI is a fully synchronous serial protocol. For every clock cycle one bit is transferred.

There is, therefore. a 1:1 relationship between bits per second and hertz.

A 20MHz SPI bus runs at 20Mbps.

  • \$\begingroup\$ Oh okay. I was misunderstanding the 1 to 1 relationship. Just to be clear, is it safe to conclude then that the shifter I found (with a minimum of 20 Mbps) is too large for my 8 MHz device? \$\endgroup\$ Jul 23, 2014 at 17:26
  • \$\begingroup\$ I have no idea. I can't say that I have ever seen a shifter with a lower limit on speed, that makes no sense. Do you have a datasheet? Without that I am just stabbing in the dark. \$\endgroup\$
    – Majenko
    Jul 23, 2014 at 17:27
  • 1
    \$\begingroup\$ If you refer to your datasheet P6, you have 4 tables providing different maximum bandwidths ranging from 40 to 100 Mbps. Right above each table there is a "Condition" that indicate in which case the information applies. In your case, that condition is Vcca voltage. What you see is typical: the lower the voltage, the lower the bandwidth. \$\endgroup\$
    – Mishyoshi
    Jul 23, 2014 at 17:35
  • 3
    \$\begingroup\$ @MrHappyAsthma At 8 Mbps, you could use whatever voltage you want and the IC would still be fast enough. \$\endgroup\$
    – Mishyoshi
    Jul 23, 2014 at 17:36
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    \$\begingroup\$ The TXB0104 does not have a minimum speed. Even though it is capable of running at 20 Mbps (or, in certain special conditions, 100 Mbps), it can also run much slower -- it will also work fine at 8 Mbps, 8 kbps, 8 bits per second, 1 bps, etc. \$\endgroup\$
    – davidcary
    Jul 24, 2014 at 1:35

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