Connect two or more modules sharing the same pins?

Question

I've found some separate instructions for connecting an SD reader/writer and also instructions for connecting a RFID reader and Piezo Buzzer. I would like to use both modules and the piezo buzzer together on the same Arduino. See below for the pin wiring details from each instruction I found:

SD Reader/Writer

• 3.3v goes to 3.3v on the Arduino UNO (for power)
• GND goes to Ground on Arduino UNO
• D0 goes to pin 12 on Arduino UNO
• D1 goes to pin 11 on Arduino UNO
• CLK goes to pin 13 on Arduino UNO
• D3 goes to pin 10 on Arduino UNO

RFID Reader

• SDA - Digital 10
• SCK - Digital 13
• MOSI - Digital 11
• MISO - Digital 12
• IRQ - unconnected
• GND - GND
• RST - Digital 9
• 3.3V - 3.3V

Piezo Buzzer

• Red - Pin 8
• Black - GND

Is using the same pin for more that one module ok?

• chrisl: What RFID reader do you have / what interface does it use? SPI?
• Chris Snow: (1) I'm currently using this RFID reader, but I'm going to change to ID-20LA which has a better range (2) I'm thinking of buying this SD module

Answer 1

pins 11 MOSI, 12 MISO and 13 SCK are SPI bus. You can connect more devices to the bus. A 'slave select' ('cable select') pin determines which device will communicate. In your table both devices use pin 10 for this. You must change one.

I recommend to wire SD reader D3 to pin 4 and in sketch use SD.begin(4).

Note: some cheap SD card adapters block other devices on the bus and can be used only as single device on the bus