04 Radio decoders
RADIO decoders
The more common radio decoders used by remote controls (car, gate opener etc.) are the so-called NATIONAL with MM53200 or UM3750 IC, then the MOTOROLA with MC145026-7-8 IC and finally the Rolling Microchip CODE generated by the HCS200 IC or better HCSxxx.
National (MM53200 - UM3750)
12 BIT encoder (on-off) for a total of 4096 combinations even if the last 2 BIT are in general used for channel signaling. The received signal presents 0X1 for every transmitted BIT , where X is the BIT that interests us. Transmission timing are 11.52mS for the duration of all the 12BIT packages of (WORD) and 0,32 mS for each single BIT intended like 0 X 1) therefore we have a total of 0.96 mS for every BIT decoded. Remember that the time constant RC can be modified. Being by now an older code, it is not easy to find on the net the datasheet, that's why I attached it.
Motorola (MC145026 - MC145027 -MC145028)
9 BIT encoder (on-off-open) every bit has 3 states for a total of 19683 possible codes. Every received signal can be simplified to 11 for every received BIT 1' , 00 for every BIT 0' and 10 if Open, every DATE PERIOD happens in 8 clock (decode time of a single BIT). You can see the timing in the datasheet at page 10. Also in this IC, the choice of RC components is fundamental for the timing of the circuit. Obviously RC of transmitter = RC of the receiver!
Rolling Code Microchip
PREFACE
For those who don't know the meaning of the term "Rolling Code", I'll try to make a very simple example.Assume to transmit a fixed code from an transmitter to an receiver, anyone that is eavesdropping, can receive the code, save it and then reproduce it! In order to avoid this, a variable code is transmitted, that varies depending on an algorithm known only by the receiver. Therefore in order to simplify if I transmit code 25 and the rolling algorithm it's +10, pressing again the remote control, I will transmit new codes 35, 45 and so on. The receiver, after recognizing 25 as a valid code, cancels it from the memory and saves 25+10=35 therefore expecting in the future 35 and so on. At this point a question arises: And if for any other reason I pressed the remote control far away from the receiver (that can not receive) when I press again the code will be 45 instead of the expected 35! The solution is to create a window where TX and RX synchronize, in our case the receiver will expect 35 but also 45,55,65. (we can do it, we have millions of combinations). In the Keeloq coding this is a "little more" complicated!!
The decoding Rolling Code from Microchip is much more complex than the others, it's composed from 66 BIT with transmission 1/3 2/3 (I receive 110 for the BIT 0' and 100 for the BIT 1').
Here are the BIT values:
1 BIT transmitter Battery status
1 BIT repeated Code
4 BIT KEYS
28 BIT Serial Number (practically the fixed part of the rolling code)
32 BIT of Rolling Code (Including discriminating and synchronism)
For further information I recommend the Microchip Center Design with the relative DataSheet and routine.
