Added README file of SPI loopback example design.

Author: jakubcabal

README.md

@@ -15,6 +15,14 @@ SPI SLAVE | 24 | 15 | 0 | 318.0 MHz
 
 *Synthesis have been performed using Quartus Prime 17 Lite Edition for FPGA Altera Cyclone IV with these settings: CLK_FREQ = 50 MHz, SCLK_FREQ = 2 MHz, SLAVE_COUNT = 1.*
 
+## The SPI loopback example design:
+
+The SPI loopback example design is for testing data transfer between SPI master and SPI slave over external wires.
+
+Please read [README file of SPI loopback example design](example/README.md).
+
+[![Video of SPI loopback example design](https://img.youtube.com/vi/-TbtB6Sm2Xk/0.jpg)](https://youtu.be/-TbtB6Sm2Xk)
+
 ## License:
 
 The SPI master and SPI slave controllers are available under the GNU LESSER GENERAL PUBLIC LICENSE Version 3.

example/README.md

@@ -0,0 +1,26 @@
+# SPI LOOPBACK EXAMPLE
+
+The SPI loopback example design is for testing data transfer between SPI master and SPI slave over external wires.
+I use it on my FPGA board ([EP4CE6 Starter Board](http://www.ebay.com/itm/111975895262) with Altera FPGA Cyclone IV EP4CE6E22C8 for $45) with few buttons and a seven-segment display (four digit).
+
+There is video of the SPI loopback example design: https://youtu.be/-TbtB6Sm2Xk
+
+[![Video of SPI loopback example design](https://img.youtube.com/vi/-TbtB6Sm2Xk/0.jpg)](https://youtu.be/-TbtB6Sm2Xk)
+
+## Control of SPI loopback example design:
+
+**Display description (from right on board in video):**
+
+* Digit0 = value on SPI slave input
+* Digit1 = value on SPI slave output
+* Digit2 = value on SPI master input
+* Digit3 = value on SPI master output
+
+**Buttons description (from right on board in video):**
+
+* BTN_ACTION (in mode0) = setup value on SPI slave input
+* BTN_ACTION (in mode1) = write (set valid) of SPI slave input value
+* BTN_ACTION (in mode2) = setup value on SPI master input
+* BTN_ACTION (in mode3) = write (set valid) of SPI slave input value and start transfer between SPI master and SPI slave
+* BTN_MODE = switch between modes (mode0 = light decimal point on digit0,...)
+* BTN_RESET = reset FPGA design