RfidResearchGroup/proxmark3
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change: reduced demodulator to bare minimum

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The initial code added complexity without improving reading distance.
Thankfully the peak detection signal path has a low noise floor.
This commit is contained in:
Andreas Dröscher 2018-08-09 17:01:28 +02:00
parent 0d0b651246
commit 9ba20b590a
1 changed files with 8 additions and 25 deletions
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33
armsrc/legicrf.c
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@ -19,7 +19,6 @@
static uint8_t* legic_mem; /* card memory, used for read, write and sim */
static legic_card_select_t card;/* metadata of currently selected card */
static crc_t legic_crc;
static int32_t input_threshold; /* values > threshold are 1 else 0 */
//-----------------------------------------------------------------------------
// Frame timing and pseudorandom number generator
@ -50,6 +49,9 @@ static uint32_t last_frame_end; /* ts of last bit of previews rx or tx frame */
#define LEGIC_CARD_MEMSIZE 1024 /* The largest Legic Prime card is 1k */
#define WRITE_LOWERLIMIT 4 /* UID and MCC are not writable */
#define INPUT_THRESHOLD 8 /* heuristically determined, lower values */
/* lead to detecting false ack during write */
//-----------------------------------------------------------------------------
// I/O interface abstraction (FPGA -> ARM)
//-----------------------------------------------------------------------------
@ -91,26 +93,18 @@ static inline int32_t sample_power() {
// Returns a demedulated bit
//
// An aproximated power measurement is available every 18.9us. The bit time
// is 100us. The code samples 5 times and uses samples 3 and 4.
// is 100us. The code samples 5 times and uses the last (most stable) sample.
//
// Note: The demodulator would be drifting (18.9us * 5 != 100us), rx_frame
// has a delay loop that aligns rx_bit calls to the TAG tx timeslots.
static inline bool rx_bit() {
static int32_t p[5];
int32_t power;
for(size_t i = 0; i<5; ++i) {
p[i] = sample_power();
power = sample_power();
}
if((p[2] > input_threshold) && (p[3] > input_threshold)) {
return true;
}
if((p[2] < input_threshold) && (p[3] < input_threshold)) {
return false;
}
Dbprintf("rx_bit failed %i vs %i (threshold %i)", p[2], p[3], input_threshold);
return false;
return (power > INPUT_THRESHOLD);
}
//-----------------------------------------------------------------------------
@ -315,18 +309,7 @@ static uint32_t setup_phase_reader(uint8_t iv) {
// Switch on carrier and let the card charge for 5ms.
last_frame_end += 7500;
// Use the time to calibrate the treshhold.
input_threshold = 8; // heuristically determined
do {
int32_t sample = sample_power();
if(sample > input_threshold) {
input_threshold = sample;
}
} while(GET_TICKS < last_frame_end);
// Set threshold to noise floor * 2
input_threshold <<= 1;
while(GET_TICKS < last_frame_end) { };
legic_prng_init(0);
tx_frame(iv, 7);