Make access control with RFID readers

[filjoa]

Hi

I think make a project about access control with RFID readers.

I like get some opinion about better way to start it.

My first question is, where is better for save IDs? If I save IDs on EEPROM, how I can calculate number of tags witch I can register?

best regards

[temtronic]

quick, simple answer for # of tags...

take the size of your EEPROM available and divide by the # of bytes each RFID tag ID number requires.

for example: IF you have 1024 bytes of EEPROM available and each tag needs 4 bytes, them 1024/4=256 tags could be stored.However, you also need a byte to use as a 'flag' to say that ID# is valid and can be used so now the qty is 1024/5= 208 tags.

hth
jay

[Ttelmah]

Remember also, that a huge amount depends on how good the 'security' needs to be.

Normally a reasonable system, will use a 'rolling forward' encryption, where the base sends a key, which is then combined on the tag, using the code number on the tag, and a rolling counter, and replies. The base then looks at the reply, and decodes both the tag ID, and the counter. This is only accepted as valid once. The next time, since the counter has moved forward, the 'real' tag will reply using the next count forwards, but a clone tag, or an attempt to duplicate a reply, won't be accepted, since this count has already been used. The base will normally accept up to two or three counts 'forward', so if one or two replies are missed, the real tag will still be accepted.

Best Wishes

[gpsmikey]

You may want to also give some consideration to having a real time clock (RTC) as part of the system and each tag has space to allow you to set the valid access times (many systems do that these days - access is only valid during certain hours (which could be all 24)) In that case, you would need space for the tag number and start / stop times. As an improved version, you may also want to consider having a limited number of tags in memory but have the ability to query a remote system if the tag is not on the local system so the "big (remote) system" could have as many as needed as well as logging etc. (many systems also log in/out times for each tag).

mikey
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3