playing with Arduino and a 12-bit LFSR

 

The circuit is just a test of the operation of the 12-bit LFSR x^12+x^6+x^4+x+1 used in 188-110A serial tone modems, in view of its full implementation; its software simulation has already been seen in a previous post, to which I refer for further insights. The 12 flip-flops and the 3 XOR gates implement the "one-to-many" configuration, according to the 188-110A standard. Since the 74xx374 does not provide the asynchronous preset / clear inputs, the LFSR is a "free runner" that is, it is not reset to a certain initial state (seed) after n-cycles; therefore all the circuits related to loading the initial pattern (0xBAD) are missing. 

Pinning and logic circuit (connections) are shown in figure 2: the electric connections (+/- 5V power) are omitted, the connections to the pins of the Arduino board are indicated (notice I used AT MEGA 2560 board so you had to change connections and code in case you use a different board).

Fig. 2 - logic connections and pinning

The 220 ohm resistor & the blue led visible in the photo on the clock line, are not reported in the circuit, they just show the clock pulses... obviously in case of a very low clock speed.

The code of the sketch is quite simple, I just used a random number generator to simulate symbols 0-7 from Modified-Gray Decoder (MGD) and a 8x8 array to perform the scrambler: the tribit number supplied from the random number generator is modulo 8 added (mod8[LFSRsymbol][randSymb]) to the three bit value supplied by LFSR (pins 53,51,49). Results are written into a buffer which is then sent to the serial port (figure 3).

Fig. 3

The 74HC374 is an octal positive-edge triggered D-type flip-flop with 3-state outputs (the second 74HC374 is half used, ie it provides only 4 flip-flops). The device features a clock (CP) and output enable (OE) inputs. The flip-flops will store the state of their inndividual D-inputs that meet the setup on the LOW-to-HIGH clock transition. A HIGH level on OE causes the outputs to assume a high impedance OFF state (OE input does not affect the state of the flip-flops). The74HC86 is a quad 2-input EXCLUSIVE-OR gate used to form the feedback chain.

https://disk.yandex.com/d/7gestYtENRF6VQ