Compressive Sensing system for recording of ECoG signals in-vivo

4.1.3. Serial Implementation with two PRBSIn [3] the measurement matrix generation has been carried out by using the combination ofPRBS generators, one of them with 50 Flip-Flops and the other one with 15 Flip-Flops. As it canbe observed in Fig.4.1.3.1, the top block generates 50 different outputs, each of them comingput from a Flip-Flop.The bottom block is settled as one output serial PRBS with randomness period of 2 15 – 1. Inorder to achieve 50 outputs in each clock cycle by keeping uncorrelated each of the 50 bitssequences,the bottom PRBS mixes each of the outputs from the top PRBS by XORing itscurrent output value by all their outputs. In this way, it is possible to accomplish 2 15 – 1 randomsequences which maintain no correlation between each others.Figure 4.1.3.1.Random Generator [3].By keeping this idea in mind [3], a new random matrix generator based on two PRBS has beenachieved in order to generate multiple sequences outputs. As it is depicted above, this design ismuch compact than the parallel one, and simultaneous outputs are achieved by cleverlycrossing the states of the two PRBSs by XORing them. A sketch of how is executed theoperation is included in Fig. 4.1.3.2. In this case, a 4FF-PRBS and a 5FF-PRBS have beencombined in order to achieve 16 parallel outputs [31].For the case of a 16 outputs serial Implementation with two PRBS, the crosses have beenconsidered are shown in Appendix D. Each PRBS is created by an LFSR, so the ‘0’ or ‘1’ valuescirculate sequentially from a FF to the next one without changing till a tap is reached. In order toavoid the same state to propagate in successive clock periods, crosses have been chosen inorder to obtain 16 outputs follow the following criteria: a) the first FF of 4FF-PRBS is XORedwith the four last FFs of 5FF-PRBS, (four outputs are achieved); b) the last FF of 4FF-PRBS isXORed with the four last FFs of 5FF-PRBS, (four outputs are achieved); c) The first FF of 5F-PRBS is XORed with the four first FFs of the 4FF-PRBS, (four outputs are achieved); d) look forfour more outputs by taking into consideration outputs which have not been previously mixed43