Electronic – Cheapest way to translate 5V SPI signal to 3V SPI

digital-logicmicrocontrollerspi

I have a MCU that operates at 5V, and a display that recommends a 3V logic level. They are connected via SPI that can operate up to 1.5Mhz. What is the best way and the cheapest way (not necessarily the same), to convert the 5V to 3V? The SPI connection is one way (SCK, MOSI, CS only; no need for the LCD to communicate to the MCU, so the translation is from 5V to 3V only).

Thanks,
Mike

Best Answer

As Matt said, the simplest is a resistor divider. The drawback is that it will slow edges a little. You'll probably be OK with the 1 kΩ and 2 kΩ he shows but it would be a good idea to check the waveform with a scope.

When speed is a issue, like it most likely would be if you were running the SPI bus at 10 MHz, then a explicit level converter would probably be better. These chips have two power supplies and usually a direction input, which you could permanently tie in one direction in your case.

Related Solutions

Configuring SPI on PIC16LF1827

As I've just done this I feel I should add to Olin's answer with some code. SPI is terribly simple but you do have to wait for it! There are also application notes warning you to read the buffer and, if you don't need the byte, don't use it. I believe that might be PIC dependent.

Before this code starts, I clear any possible errors by clearing SSPOV, WCOL and BF. Just in case!

    ; == This is an SPI transmit from FSR1 ==
    ;
    ;   nbytes contains the 8 bit number of bytes to send
    ;   We are not interested in the received bytes and we
    ;   poll until completion (SPI is very fast)

Xfer_transmit:

    banksel     SSP1BUF
    moviw       FSR1++
    movwf       SSP1BUF                 ; ssp1buf = *fsr1++

    ifbclr      SSP1STAT, BF            ; Wait for transfer to complete
      jmp       $-1

    movf        SSP1BUF, W              ; Clear BF. Discard data.

    banksel     nbytes
    decfsz      nbytes, F
      jmp       Xfer_transmit

    ; Transmit complete.

Electrical – Multiple SPI devices with different logic levels

The 74LVC245A has tri state outputs; taking the \$\overline {OE}\$ pin low enables communication from A to B or B to A (depending on the level on the DIR pin).

The outputs can have a state of High, Low and High impedance (Z).

You can therefore not tie the outputs together as these are push-pull outputs when enabled.

You could follow the '245A with a multiplexer to use a single pin at the micro.

Best Answer

Related Solutions

Configuring SPI on PIC16LF1827

Electrical – Multiple SPI devices with different logic levels

Related Topic