HCS473 Datasheet, PDF (15/68 Pages) Microchip Technology – Code Hopping Encoder and Transponder

HCS473
3.1.3
CODE HOPPING MODULATION
FORMAT
The data modulation format is selectable between
Pulse Width Modulation (PWM) and Manchester using
the modulation select (MSEL) configuration option.
Regardless of the modulation format, each code word
contains a leading preamble and a synchronization
header to wake the receiver and provide synchroniza-
tion events for the receive routine. Each code word also
contains a trailing guard time to separate code words.
Manchester encoding further includes a leading data ‘1’
START pulse and closing 1 RFTE STOP pulse around
each data block.
The same code word repeats as long as the same input
pins remain active, until a timeout occurs or a delayed
seed transmission is activated.
The modulated data timing is typically referred to in
multiples of a basic Timing Element (RFTE). ‘RF’ TE
because the DATA pin output is typically sent through a
RF transmitter to the decoder or transponder reader.
RFTE may be selected using the RF Transmission
Baud Rate (RFBSL) configuration option
(Section 3.1.4.13).
FIGURE 3-3:
PWM TRANSMISSION FORMAT (MSEL = 0)
TOTAL TRANSMISSION:
1 CODE WORD
Preamble Sync Encrypt Fixed Guard Preamble Sync Encrypt
TE TE TE
LOGIC "0"
LOGIC "1"
Preamble
Header
Encrypted
Portion
CODE WORD
Fixed Code
Portion
Guard
Time
FIGURE 3-4:
MANCHESTER TRANSMISSION FORMAT (MSEL = 1)
TOTAL TRANSMISSION:
1 CODE WORD
Preamble Sync Encrypt Fixed Guard Preamble Sync Encrypt
TE TE
LOGIC “0”
START bit
bit 0 _{bit 1} bit 2
LOGIC “1”
STOP bit
Preamble Header
Encrypted
Portion
CODE WORD
Fixed Code
Portion
Guard
Time
 2002 Microchip Technology Inc.
Preliminary
DS40035C-page 13