describing the handling of a SD-card

README.md

@@ -60,6 +60,7 @@ Depending on board hardware following features are supported:
 - IF482 (serial) and DCF77 (gpio) time telegram generator
 - Switch external power / battery
 - LED Matrix display (similar to [this 64x16 model](https://www.instructables.com/id/64x16-RED-LED-Marquee/), can be ordered on [Aliexpress](https://www.aliexpress.com/item/P3-75-dot-matrix-led-module-3-75mm-high-clear-top1-for-text-display-304-60mm/32616683948.html))
+- SD-card (see section SD-card here)
 
 Target platform must be selected in [platformio.ini](https://github.com/cyberman54/ESP32-Paxcounter/blob/master/platformio.ini).<br>
 Hardware dependent settings (pinout etc.) are stored in board files in /hal directory. If you want to use a ESP32 board which is not yet supported, use hal file generic.h and tailor pin mappings to your needs. Pull requests for new boards welcome.<br>
@@ -196,6 +197,43 @@ Follow all steps so far for preparing the device, use the packed payload format.
 There in the sensor configuration select "TheThingsNetwork" and set Decoding Profil to "LoRa serialization", enter your TTN Application and Device Id. Decoding option has to be
 	[{"decoder":"latLng"},{"decoder":"uint16","sensor_id":"yoursensorid"}] 
 
+# SD-card
+Data can be stored on an SD-card if one is availabe. Simply choose the file in src/hal and add the following lines to your hal-file:
+
+    #define HAS_SDCARD 1     // this board has an SD-card-reader/writer
+    // Pins for SD-card
+    #define SDCARD_CS   (13) // fill in the correct numbers for your board
+    #define SDCARD_MOSI (15)
+    #define SDCARD_MISO (2)
+    #define SDCARD_SCLK (14)
+
+Please choose the correct number for the connection of the reader/writer.
+
+This is an example of a board with SD-card: https://www.aliexpress.com/item/32990008126.html
+In this case you take the file src/hal/ttgov21new.h and add the lines given above (numbers given are for this board).
+
+Another approach would be this tiny board: https://www.aliexpress.com/item/32424558182.html (needs 5V).
+In this case you choose the correct file for your ESP32-board in the src/hal-directory and add the lines given above to the correct h-file. Please correct the numbers given in the example to the numbers used corresponding to your wiring.
+
+Some hints:
+These cheap devices often handle SD-cards up to 32GB, not bigger ones. They can handle files in the old DOS-way, to say the filenames are in the 8.3-format. And they often cannot handle subdirectories.
+
+The software included here writes data in a file named PAXCOUNT.xx, where xx can range from 00 to 99. The software starts with 00, checks to see if such a file already exists and if yes it will continue with the next number (up to 99 - in this case it will return no sd-card). So an existing file will not be overwritten.
+
+The data is written to the card and after 3 write-operations the data is flushed to the disk. So maybe the last 3 minutes of data get lost when you disconnect the PAXCOUNTER from power.
+
+And finally: this is the data written to the disk:
+
+    date, time, wifi, bluet
+    00.00.1970,00:01:09,2,0
+    00.00.1970,00:02:09,1,0
+    00.00.1970,00:03:09,2,0
+
+Format of the data is CSV, which can easily imported into LibreOffice, Excel, .....
+
+If you want to change this please look into src/sdcard.cpp and include/sdcard.h.
+
+
 # Payload format
 
 You can select different payload formats in [paxcounter.conf](src/paxcounter.conf#L12):