HCS515_11 Datasheet, PDF (9/38 Pages) Microchip Technology – KEELOQ® Code Hopping Decoder stand-alone decoder

HCS515
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 fourth part is the data. The micro-
controller 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
microcontroller 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 clocked 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 HCS515 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 microcon-
troller 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
TCLKL
TCMD
TADDR
TREQ
TSTART
TCLKH
TDS
CLK
TDATA
μC Data
HCS515
Data
TRESP
TACK
LSB
MSB
LSB
MSB
LSB
MSB
START Command
Command Byte
Address Byte
Data Byte
A
B
C
D
E
© 2011 Microchip Technology Inc.
DS40183E-page 9