Update README.md

README.md

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 # SST39SF040-Programmer
 Arduino Mega 2560 SST39SF040 Flash Chip programmer
+
+This was taken from an example which only wrote to one address. I've added more code to process commands and also allow writing manually in a terminal window.
+
+SST39SF040 Pins               Arduino Mega 2560 Pins
+A0 to A18                     22 to 40
+D0 to D7                      44 to 51
+#WE                           2
+#OE                           3
+#CS                           GND
+Vss                           GND
+Vdd                           +5v
+
+The commands are formatted as follows:
+
+Write a byte: <W,addr,byte>
+This writes a single "decimal" byte (not HEX!) to a decimal address. All writes are checked by reading back the data and messages sent back to the terminal
+
+Read 256 byte block: <R,addr,0>
+Set addr to the first 256 byte boundary that you want to read 256 bytes from. The second parameter is ignored and hence 0.
+
+Erase entire chip: <E,0,0>
+Both parameters are ignored and the whole chip is erased. There is no function to erase a sector which is the minimum you can erase in one go, plus I don't need to erase anything but the whole chip.
+
+At some point I'll write a C# application to flash a whole chip, or you can have a go yourself and let me know and I'll add it to this repo.
+
+Enjoy!
+Dan R