HCS473_13 Datasheet, PDF (28/68 Pages) Microchip Technology – KEELOQ® 3-Axis Transcoder

HCS473
3.2.3.5 WRITE Command
The transponder reader follows the 3-bit WRITE com-
mand with one of eight possible 3-bit address options
indicating which 16-bit EEPROM word to write to
(Table 3-5) and a 2-bit CRC calculated over the com-
mand, address and data bits.
The HCS473 will attempt to write the value into
EEPROM, responding with an Acknowledge sequence
if successful (Figure 3-15).
The following locations are available to write:
• The 64-bit general purpose user EEPROM.
• The 32-bit serial number.
• The16-bit Device Identifiers #1 and #2.
A Transport Code, write access password, protects the
memory locations from undesired modification. The
reader must precede the Write sequence with a suc-
cessful PRESENT TRANSPORT CODE sequence.
Only a correct match with the transport code pro-
grammed during production will allow write access to
the memory locations.
The next command must begin TCMD after the write
Acknowledge.
The PRESENT TRANSPORT CODE sequence must
precede a WRITE sequence but not necessarily imme-
diately. Perhaps all four user memory locations will be
written and verified. The PRESENT TRANSPORT
CODE sequence must precede only the first write. The
system may then alternately write and read (verify)
multiple memory locations. Write access status
remains until the next time the device returns to sleep -
communication error or TCMD without receiving another
command.
FIGURE 3-16:
WRITE SEQUENCE (RF RESPONSE EXAMPLE)
Previous Command
Sequence
‘101₂’ 3 bits 16 bits 2 bits
TCMD
CMD ADR DATA CRC
Previous Command
Sequence
Inductive In
(LC Pins)
Command Address
16-Bit
Write Data
RF Response
(DATA Pin)
TCMD
TCMD
TWRT
CRC
TCMD
ACK
Optional Next
Command
Next
Command
LF must remain on if following
with consecutive command
or if waiting for LF response
TFINH
TWRT
TCMD
TCMD
5 LFTE
Write ACK
DS40035D-page 28
Preliminary
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