I am currently working on a project that involves reading known RFID cards using Arduino Uno and an ethernet shield. Since I have to store a lot of UIDs (roughly 600), I am wondering if it is better to store them on a microSD card or whether it is okay to store them in PROGMEM of the Arduino.
Because I have to continuously check whether the scanned UID matches any of the stored UIDs, reading speed on the SD card has to be very fast. Is this possible or does it take too long to retrieve the data?
There are several documented ways to speed up search of an SD file. First is to be sure the UIDs are in ascending sequence. Then you only have to read to the point of discovering the UID is not in your file.
Storing the UIDs like @Paul_KD7HB says opens up for fast search algorithms. The "half way distance" is one. Make an average based on the first and the last UID, and check there. If the UID is smaller than try half of the first halt intervall, and compare.
More sophiticated algorithms like Hast-tagging is an other oprtion.
Think about maintaining the "database", adding or deleting UIDs.
You probably do not need an SD card - which are now not available less than about 8 Gigabytes - to store your IDs. An EEPROM chip would be easy to connect and code for.
Searching 600 records would take less time than lifting your finger!
