I'm working on a circuit where a 3.3v flash memory, which comunicate with SPI protocol, has to be interfaced to an 8-bit PIC microcontroller working at 5v. For the MISO signal there should not be any problem, but for MOSI, SCK and CS there are. So the solution i came across is a voltage divider. I've done this calculations but don't know if they are ok. _ First of all, i've calculated r1 in this way: (5-3.3)/0.001=1700 ohm _ Then i've resolved the equation Vout = Vin * (r2 / (r1+r2)) 3.3 = 5 * (r2/(1700+r2)) ...resolving the equation in order to find r2...

r2=3300 ohm. Is that correct? i have not any tester and i have just one flash memory. i don't want to fry it... Is the method i've used proper (first determinate r1 with the current and then find r2)? Thank you very very much. Greetings from Italy.

  • 1
    \$\begingroup\$ Who knows what R1 is and what R2 is - you didn't show a circuit. This may seem trivial but it isn't. You should also link the data sheet for the flash memory and state what speed you hope to achieve. \$\endgroup\$
    – Andy aka
    Jun 13, 2020 at 12:40
  • \$\begingroup\$ Which PIC model? Have you checked 3.3V is above logic 1 threshold? At what speeds you plan to communicate? Any capacitance (stray, pin) after the divider will round down the edges, and there is a limit how round edges the SPI flash works with. Basically, resistor divider can be used for low speed stuff, but it may have issues with high speed stuff. \$\endgroup\$
    – Justme
    Jun 13, 2020 at 12:47
  • \$\begingroup\$ The microcontroller is a PIC18F46Q10. \$\endgroup\$
    – morov
    Jun 13, 2020 at 13:25
  • \$\begingroup\$ The flash is the SST25VF080B, the clock of sck is at 2 Mhz \$\endgroup\$
    – morov
    Jun 13, 2020 at 13:26
  • \$\begingroup\$ For the circuit take the CS line like an example: from the pin of the microcontroller, first there is R1, after that there is: _R2 which goes to 0v _and the CS of the flash \$\endgroup\$
    – morov
    Jun 13, 2020 at 13:32


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