hcs500 Microchip Technology Inc., hcs500 Datasheet - Page 9

• Manufacturer Part Number: hcs500
• Description: Keeloq Code Hopping Decoder
• Manufacturer: Microchip Technology Inc.

4.2
4.2.1
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.
FIGURE 4-3:
Decoder
2002 Microchip Technology Inc.
C Data
Data
CLK
Command Mode
A
MICROCONTROLLER COMMAND
MODE ACTIVATION
T
T
Start Command
REQ
RESP
MICROCONTROLLER COMMAND MODE ACTIVATION
T
T
START
ACK
B
LSB
T
CLKL
Command Byte
T
CLKH
MSB
T
CMD
T
DS
C
LSB
4.2.2
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
• The decoder takes the data line high and checks
• If the clock line goes high after 50 s but before
• The microcontroller can then start to clock out the
high. If the clock line is high, the valid transmis-
sion notification is aborted, and the microcontrol-
ler Command mode request is serviced.
that the clock line doesn’t go high within 50 s. If
the clock line goes high, the valid transmission
notification is aborted and the Command mode
request is serviced.
500 ms, the decoder will acknowledge by taking
the data line low.
80-bit data stream of the received transmission.
Address Byte
COLLISION DETECTION
MSB
T
ADDR
D
LSB
Data Byte
HCS500
DS40153C-page 9
MSB
T
DATA
E