Other packets are not buffered.
The implementation of the preamble removal unit uses four times oversampling within
each 30 ms period for each pulse. The design uses counters to count the sampling period.
The difference in the sync pulse can be detected by counting for how many samples the
?¬?nal signal is high, 5 for tags and 7 for readers.
11.4.2.3 Manchester Decoder
The Manchester decoder translates the Manchester encoded data immediately following a
valid preamble. It is the block for ?¬?ltering and buffering incoming RFID packets. The
decoder extracts a bit stream of nonreturn to zero (NRZ) data from the encoded data.
The Manchester code combines the concept of clock with synchronous data into a single
serial data stream, as shown in Figure 11.23. In order to enforce synchronization, Manchester
code contains a transition in the middle of each Manchester bit. The Manchester bit
Clock
Data
Manchester
1 3 5
1 0 0
18 ?µs
b1 b2 b3 b4
2 4
0 1
FIGURE 11.23
Example of Manchester encoding.
220 RFID Handbook: Applications, Technology, Security, and Privacy
represents zero (0) NRZ data if this transition is from high to low. Similarly, the Manchester
bit represents one (1) NRZ data if the transition is from low to high. By representing
data with a guaranteed transition for each bit, slight discrepancies of timing can be
tolerated without disrupting the communicated data.
Pages:
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429