Nuts & Volts - April 2005

Stamp

When we’re using a BS2, BS2e, or BS2sx, the tag string is stored in our variable buffer; otherwise, it gets stuffed into the first 10 bytes of the Scratchpad. See how much easier this is? The STR and SPSTR modifiers are huge time-savers here.

With the tag string in RAM, we can compare it against the table, and again, it’s much easier with the BS2-family.

Check_List:
  FOR tagNum = 1 TO LastTag
 FOR idx = 0 TO 9
 READ (tagNum - 1 10 + idx), char
 #IF __No_SPRAM #THEN
 IF (char <> buf(idx)) THEN Bad_Char
 #ELSE
 GET idx, chkChar
 IF (char <> chkChar) THEN Bad_Char
 #ENDIF
 NEXT
 GOTO Tag_Found

Bad_Char:
  NEXT

byte from the reader. Our conditional symbol sets up the code to make the comparison against a byte in the variable array or against a byte from the Scratchpad. Note that the Scratchpad cannot be treated like an array, so we are forced to use GET to access the appropriate byte.

Moving on to a good tag, we will do the same as before: sound the beeper (with FREQOUT) and disable the security lock. Remember that FREQOUT is one of those instructions that differs from Stamp to Stamp, so conditional compilation constants are used to keep the timing (two seconds) and tone (880 Hz) the same, no matter which module we use.

Tag_Found:
  GOSUB Show_Name
  HIGH Latch
  FREQOUT Spkr, 2000 */ TmAdj, 880 */ FrAdj
  LOW Latch
  GOTO Main

As with the BS1, a loop is used to work through the known tag strings. What we’re able to do here, however, is use a second loop to test each byte of the received string. The inner loop reads the appropriate byte from the current tag data and compares it against the corresponding tag

We’ve also added the ability to display the name of the person who is assigned to the valid tag. A simple loop will send the characters of the name to a display. We’ll keep it easy and use the Debug Terminal.

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

Show_Name:
  DEBUG DEC tagNum, “: “
  LOOKUP tagNum,
 [Name0, Name1, Name2, Name3], idx

DO
  READ idx, char
  IF (char = 0) THEN EXIT
  DEBUG char
  idx = idx + 1
LOOP
RETURN

The result of the tag search (in the variable tagNum) will be from one to the number of known tags if the tag string is valid. If not, the search will result in zero. The “Show_Name” routine uses the result of the tag search to LOOKUP the first character of the corresponding name. After that, each character is printed in a loop until the zero-terminator is encountered. We could very easily change the DEBUG line to SEROUT for a serial LCD or to LCD-OUT if we’re using a BS2p or BS2pe and have a parallel LCD connected as required.

Well, that’s it. That was pretty simple, wasn’t it? I think so and I’m having a lot of fun with the RFID reader. Be sure to check out the resources listed, as there is lots of interesting information on RFID technology. Joe Grand’s website has some really cool stuff (if you’re into hardware hacking, you’ll love his books).