+//=============================================================================
+// An ISO 15693 decoder for tag responses (one subcarrier only).
+// Uses cross correlation to identify each bit and EOF.
+// This function is called 8 times per bit (every 2 subcarrier cycles).
+// Subcarrier frequency fs is 424kHz, 1/fs = 2,36us,
+// i.e. function is called every 4,72us
+// LED handling:
+// LED C -> ON once we have received the SOF and are expecting the rest.
+// LED C -> OFF once we have received EOF or are unsynced
+//
+// Returns: true if we received a EOF
+// false if we are still waiting for some more
+//=============================================================================
+
+#define NOISE_THRESHOLD 160 // don't try to correlate noise
+
+typedef struct DecodeTag {
+ enum {
+ STATE_TAG_SOF_LOW,
+ STATE_TAG_SOF_HIGH,
+ STATE_TAG_SOF_HIGH_END,
+ STATE_TAG_RECEIVING_DATA,
+ STATE_TAG_EOF
+ } state;
+ int bitCount;
+ int posCount;
+ enum {
+ LOGIC0,
+ LOGIC1,
+ SOF_PART1,
+ SOF_PART2
+ } lastBit;
+ uint16_t shiftReg;
+ uint16_t max_len;
+ uint8_t *output;
+ int len;
+ int sum1, sum2;
+} DecodeTag_t;
+
+
+static int inline __attribute__((always_inline)) Handle15693SamplesFromTag(uint16_t amplitude, DecodeTag_t *DecodeTag)
+{
+ switch(DecodeTag->state) {
+ case STATE_TAG_SOF_LOW:
+ // waiting for 12 times low (11 times low is accepted as well)
+ if (amplitude < NOISE_THRESHOLD) {
+ DecodeTag->posCount++;
+ } else {
+ if (DecodeTag->posCount > 10) {
+ DecodeTag->posCount = 1;
+ DecodeTag->sum1 = 0;
+ DecodeTag->state = STATE_TAG_SOF_HIGH;
+ } else {
+ DecodeTag->posCount = 0;
+ }
+ }
+ break;
+
+ case STATE_TAG_SOF_HIGH:
+ // waiting for 10 times high. Take average over the last 8
+ if (amplitude > NOISE_THRESHOLD) {
+ DecodeTag->posCount++;
+ if (DecodeTag->posCount > 2) {
+ DecodeTag->sum1 += amplitude; // keep track of average high value
+ }
+ if (DecodeTag->posCount == 10) {
+ DecodeTag->sum1 >>= 4; // calculate half of average high value (8 samples)
+ DecodeTag->state = STATE_TAG_SOF_HIGH_END;
+ }
+ } else { // high phase was too short
+ DecodeTag->posCount = 1;
+ DecodeTag->state = STATE_TAG_SOF_LOW;
+ }
+ break;
+
+ case STATE_TAG_SOF_HIGH_END:
+ // waiting for a falling edge
+ if (amplitude < DecodeTag->sum1) { // signal drops below 50% average high: a falling edge
+ DecodeTag->lastBit = SOF_PART1; // detected 1st part of SOF (12 samples low and 12 samples high)
+ DecodeTag->shiftReg = 0;
+ DecodeTag->bitCount = 0;
+ DecodeTag->len = 0;
+ DecodeTag->sum1 = amplitude;
+ DecodeTag->sum2 = 0;
+ DecodeTag->posCount = 2;
+ DecodeTag->state = STATE_TAG_RECEIVING_DATA;
+ LED_C_ON();
+ } else {
+ DecodeTag->posCount++;
+ if (DecodeTag->posCount > 13) { // high phase too long
+ DecodeTag->posCount = 0;
+ DecodeTag->state = STATE_TAG_SOF_LOW;
+ LED_C_OFF();
+ }
+ }
+ break;
projects / proxmark3-svn / blobdiff