diff --git a/README.md b/README.md
index 47a8fe5..8ef73c5 100644
--- a/README.md
+++ b/README.md
@@ -75,6 +75,7 @@ Connect the peripherals the following way (peripheral signal; peripheral signal;
   - S0; 10; PB3; GPIO; to select output
   - S1; 11; PB4; GPIO; to select output
   - S2; 14; PB5; GPIO; to select output
+  - S3; 13; ; GND;
   - I0; 9; 18; CLK; TM1637 for 
   - I1; 8; 18; CLK; TM1637 for 
   - I2; 7; 18; CLK; TM1637 for