- int i, j, iold;
- int isum, qsum;
- int outOfWeakAt;
- bool negateI, negateQ;
-
- uint8_t data[256];
- int dataLen = 0;
-
- // As received, the samples are pairs, correlations against I and Q
- // square waves. So estimate angle of initial carrier (or just
- // quadrant, actually), and then do the demod.
-
- // First, estimate where the tag starts modulating.
- for (i = 0; i < GraphTraceLen; i += 2) {
- if (abs(GraphBuffer[i]) + abs(GraphBuffer[i + 1]) > 40) {
- break;
- }
- }
- if (i >= GraphTraceLen) {
- PrintAndLog("too weak to sync");
- return 0;
- }
- PrintAndLog("out of weak at %d", i);
- outOfWeakAt = i;
-
- // Now, estimate the phase in the initial modulation of the tag
- isum = 0;
- qsum = 0;
- for (; i < (outOfWeakAt + 16); i += 2) {
- isum += GraphBuffer[i + 0];
- qsum += GraphBuffer[i + 1];
- }
- negateI = (isum < 0);
- negateQ = (qsum < 0);
-
- // Turn the correlation pairs into soft decisions on the bit.
- j = 0;
- for (i = 0; i < GraphTraceLen / 2; i++) {
- int si = GraphBuffer[j];
- int sq = GraphBuffer[j + 1];
- if (negateI) si = -si;
- if (negateQ) sq = -sq;
- GraphBuffer[i] = si + sq;
- j += 2;
- }
- GraphTraceLen = i;
-
- i = outOfWeakAt / 2;
- while (GraphBuffer[i] > 0 && i < GraphTraceLen)
- i++;
- if (i >= GraphTraceLen) goto demodError;
-
- iold = i;
- while (GraphBuffer[i] < 0 && i < GraphTraceLen)
- i++;
- if (i >= GraphTraceLen) goto demodError;
- if ((i - iold) > 23) goto demodError;
-
- PrintAndLog("make it to demod loop");
-
- for (;;) {
- iold = i;
- while (GraphBuffer[i] >= 0 && i < GraphTraceLen)
- i++;
- if (i >= GraphTraceLen) goto demodError;
- if ((i - iold) > 6) goto demodError;
-
- uint16_t shiftReg = 0;
- if (i + 20 >= GraphTraceLen) goto demodError;
-
- for (j = 0; j < 10; j++) {
- int soft = GraphBuffer[i] + GraphBuffer[i + 1];
-
- if (abs(soft) < (abs(isum) + abs(qsum)) / 20) {
- PrintAndLog("weak bit");
- }
-
- shiftReg >>= 1;
- if(GraphBuffer[i] + GraphBuffer[i+1] >= 0) {
- shiftReg |= 0x200;
- }
-
- i+= 2;
- }