Nuts & Volts - April 2005

Stamp

er, the BS2. It’s very inexpensive though (especially with OEM parts), and is worth considering for a low-cost, access-control system.

After we’ve recorded our tag IDs, we can put them into a simple EEPROM-based

table for storage. Here’s what my table looks like:

Figure 3. Connecting the reader to a terminal.

Tags:
  EEPROM (“0F0184F20B”)
  EEPROM (“0F01D9D263”)
  EEPROM (“04129C1B43”)
  EEPROM (“0000000000”)
  EEPROM (“0000000000”)

Now, you’ll see what I was just talking about with the BS1 not being quite as convenient as the BS2. There is no STR modifier with the BS1, and its memory cannot be treated like an array. Everything must be done one byte at a time.

Main:
  LOW Enable
  SERIN RX, T2400, ($0A),

➔ tag0, tag1, tag2, tag3, tag4, ➔ tag5, tag6, tag7, tag8, tag9

HIGH Enable

Note that the SERIN line is really long, but needs to be on one line to make sure that all 11 bytes (header plus 10) are received properly. For publication, I’ve split the line, but you’ll find it all together in the downloadable version of the code. With the tag ID stored in the BS1’s RAM, we can compare it to our table entries to determine whether the tag presented is a match or not. Admittedly, this looks a little hairy, but it’s really not that bad. To keep things concise, I’m only showing the first and last bytes, but the same code is required for all 10 elements of the RFID tag string.

Remember that your table will be different, as the RFID tag strings are unique. By using the Memory Map feature of the Stamp IDE, I found I had room for five tags. Since I only had three to work with, I padded the table for the unused positions. A program constant will prevent us from searching past the known tag strings.

The next step is to set up our control outputs. For this security program, we’ll turn off the reader and lock the door.

Reset:
  HIGH Enable
  LOW Latch

After basic setup, we drop into the heart of the program where the reader is activated and wait on a tag ID string.

Check_List:
  FOR tagNum = 0 TO LastTag
 pntr = tagNum 10 + 0
 READ pntr, char
 IF char <> tag0 THEN Bad_Char

‘ removed for clarity

 pntr = tagNum 10 + 9
 READ pntr, char
 IF char <> tag9 THEN Bad_Char
 GOTO Tag_Found
Bad_Char:
  NEXT

Figure 4. Tag strings displayed in a terminal.

As you can see, comparing each byte of the RFID string against a table entry requires three steps that are placed in a loop: 1) We create a pointer to the corresponding position of the table entry, 2) we read the character from the table, and then 3) we compare the two bytes. If they don’t match, the program jumps to the label called “Bad_Char” and we’ll either move to the next table entry or, if we’re at the end of the table, we’ll fall through the loop. Let’s say we do have a match. When that occurs, we will jump to the label called “Tag_Found” and execute the door-opening code:

F o r

E l e c t r o n i c s

NUTS & VOLTS E v e r y t h i n g

Tag_Found:
  DEBUG “Entry: “, #tagNum, CR
  HIGH Latch
  SOUND Spkr, (114, 165)
  LOW Latch
  GOTO Main

80

The latch is activated (to allow entry) and a beep is played through a piezo speaker of the amplifier circuit to alert the user. The beep stops after two seconds and the door relocks. Then, we go back to the top of the program.