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+# A Seven Segment Counter
+
+## Description
+
+In this project, I reused an old "seven segment display" salvaged from a microwave oven to build a counter. The project is based on an Arduino Nano. 
+
+## How it works
+
+The salvaged display is a made of four digits that can be individually selected by its cathode pin. Each segment is lit up by the corresponding anode pin. The display is in a common cathode arrangement. 
+
+This means you can display only one digit at a time and thus multiplexing is required. 
+
+The multi-plexing is achieved by selecting each digit's cathode in turn and for each digit, selecting the anodes of every segment to light up.  
+
+The cathode is set to HIGH while the anodes are set to LOW. 
+When a cathode is not in use, its pin is set to LOW.
+When an anode is not in use, its pin is set to HIGH. 
+
+When done fast enough (50Hz minimum), the persistence of vision let the user see all digits. 
+
+## Drawback
+
+In a first version of this project (check the initial commit of this repo), I used the digitalWrite function to drives the output pins. [But it was way to slow!](https://www.peterbeard.co/blog/post/why-is-arduino-digitalwrite-so-slow/).
+
+So started using the PORT/DDR registers, which turned to be very fast. But I had to find a port that had at least seven available pins to connect all the anodes. 
+
+PORTD was the only one having this configuration. Unfortunately, the first two pins (0 and 1) are also used for the UART opertion (TX/RX). 
+
+Although I succeeded in re-purposing those pins as general purpose output pins, it seems they cannot sink enough current to light up a LED. So, the matching two segments do not light up. 
+
+## References 
+ - https://www.peterbeard.co/blog/post/why-is-arduino-digitalwrite-so-slow/
+ - https://www.arduino.cc/en/Reference/PortManipulation
+ - http://www.pighixxx.com/test/pinouts/boards/nano.pdf
+ - http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-2545-8-bit-AVR-Microcontroller-ATmega48-88-168_Datasheet.pdf
+ - https://www.arduino.cc/reference/en/language/functions/external-interrupts/attachinterrupt/ 
+
+## Further improvements
+
+- Use a 74HC595 multiplexer
+- or drive the two segments connected on TX/RX with a small mosfet
+
+