HCS500-I Datasheet, PDF (9/34 Pages) Microchip Technology – KEELOQ Code Hopping Decoder

HCS500
4.2 Command Mode
4.2.1
MICROCONTROLLER COMMAND
MODE ACTIVATION
The microcontroller command consists of four parts.
The first part activates the Command mode, the sec-
ond part is the actual command, the third is the address
accessed, and the last part is the data. The microcon-
troller starts the command by taking the clock line high
for up to 500 ms. The decoder acknowledges the start-
up sequence by taking the data line high. The micro-
controller takes the clock line low, after which the
decoder will take the data line low, tri-state the data line
and wait for the command to be clock in. The data must
be set up on the rising edge and will be sampled on the
falling edge of the clock line.
4.2.2
COLLISION DETECTION
The HCS500 uses collision detection to prevent
clashes between the decoder and microcontroller.
Whenever the decoder receives a valid transmission
the following sequence is followed:
• The decoder first checks to see if the clock line is
high. If the clock line is high, the valid transmis-
sion notification is aborted, and the microcontrol-
ler Command mode request is serviced.
• The decoder takes the data line high and checks
that the clock line doesn’t go high within 50 µs. If
the clock line goes high, the valid transmission
notification is aborted and the Command mode
request is serviced.
• If the clock line goes high after 50 µs but before
500 ms, the decoder will acknowledge by taking
the data line low.
• The microcontroller can then start to clock out the
80-bit data stream of the received transmission.
FIGURE 4-3: MICROCONTROLLER COMMAND MODE ACTIVATION
CLK
TREQ
TCLKL
TSTART
TCLKH
TCMD
TDS
TADDR
TDATA
µC Data
TRESP
TACK
LSB
MSB
LSB
MSB
LSB
MSB
Decoder
Data
Start Command
Command Byte
Address Byte
Data Byte
A
B
C
D
E
© 2002 Microchip Technology Inc.
DS40153C-page 9