I have a project where I have three pins to use and I just realized it would be better to use a memory card than replacing the code each time.

I'm building a Arduino with a push button. Press the button and it sends keystrokes to the connected computer with login details.
The login details need to change every three months so having that on a memory card will be easier.

So since I use a digispark Attiny 85 with a total of 6 pins (where two is used to send the keystrokes).
I have one to the push button, and three pins left.

Is there any card reader that can work with three pins? I only need to read if that makes a difference.
The ones I find use four pins (+ vcc and gnd).

Pretty much any memory card type is ok.

2 Answers 2


Not with an ATTiny85, and not with so few pins, no.

However, all is not lost.

The ATTint85 has 512 bytes of EEPROM inside it where you can store your login details. All you need is some way of telling setting that data in your sketch from the PC - and the simplest way is probably through a USB serial connection.

If you don't have a serial connection available (I think Digispark has a custom interface they use) then you can use whatever interface you have available to do the job.

  • 512 bytes is enough! Interesting! Is this fool proof? It's supposed to be normal people that does this. Not programmers or in any way tech savvy people.
    – Andreas
    Nov 29, 2019 at 18:55
  • Yes, it's foolproof. Not sur ewhat libraries are available for the attiny85, but the Arduino has the EEPROM library bundled with it...
    – Majenko
    Nov 29, 2019 at 19:08
  • I found this youtu.be/U9HEov6dDB8 not sure if it will work with digispark boards, but I will try it when I get them.
    – Andreas
    Nov 30, 2019 at 5:55

SD cards work with SPI. This needs only one pin per device (SlaveSelect) but 3 pins for the other common signals ( MISO / MOSI / SCK )

Card reader modules simply do some voltage level shifting (if at all).

But, you need a lot of RAM to work with SD cards, as data transfer always happens in 512 byte "sectors". With a file system, you need at least two such blocks in parallel. An attiny85 is simply too small.

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