HCS473 Datasheet, PDF (27/68 Pages) Microchip Technology – Code Hopping Encoder and Transponder

HCS473
3.2.3.2 PRESENT TRANSPORT CODE
Prior to modifying the device EEPROM, the correct 32-
bit transport code (password) must be presented to
gain write access. This is done with the PRESENT
TRANSPORT CODE command followed by the 32-bit
transport code and CRC calculated on the 3-bit com-
mand and 32 bits of data.
The HCS473 will return an Acknowledge if the trans-
port code matches the value programmed in produc-
tion; write access has been granted.
The next command (usually a write) must begin TCMD
after the Acknowledge, Figure 3-13.
The present transport code sequence must precede a
write sequence but not necessarily immediately. Per-
haps all four user memory locations will be written and
verified. The present transport code sequence must
precede only the first write to gain write access. The
system may then alternately write and read (verify)
multiple memory locations. Write access remains until
the next time the device returns to Low Power mode -
communication error or TCMD time out without receiving
another command.
FIGURE 3-13:
PRESENT TRANSPORT CODE SEQUENCE (RF RESPONSE EXAMPLE)
Wake Sequence
or
Previous Command
Response
‘001₂’ 32 bits
TCMD
CMD TCODE
2 bits
WRITE Sequence
CRC ACK ^TCMD CMD ADR DATA
32-Bit
Command _Transport
CRC
Write
Command ^Address
16-Bit
Write Data
TSF
Inductive In
(LC Pins)
RF Response
(DATA Pin)
TCMD
ACK
TCMD
TTPACK
TFINH
TCMD
TCMD
5 LFTE
Transport ACK
 2002 Microchip Technology Inc.
Preliminary
DS40035C-page 25