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Merge pull request #1104 from cmolson/em4x70-signal

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em4x70: Use fixed threshold for high/low
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Iceman 2020-12-15 00:35:46 +01:00 committed by GitHub
parent 2bc36d5716 334000d444
commit 70a70a91bc
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GPG key ID: 4AEE18F83AFDEB23
1 changed files with 15 additions and 56 deletions
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71
armsrc/em4x70.c
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@ -55,15 +55,18 @@ static bool command_parity = true;
#define EM4X70_COMMAND_WRITE 0x05
#define EM4X70_COMMAND_UM2 0x07
static uint8_t gHigh = 0;
static uint8_t gLow = 0;
// Constants used to determing high/low state of signal
#define EM4X70_NOISE_THRESHOLD 13 // May depend on noise in environment
#define HIGH_SIGNAL_THRESHOLD (127 + EM4X70_NOISE_THRESHOLD)
#define LOW_SIGNAL_THRESHOLD (127 - EM4X70_NOISE_THRESHOLD)
#define IS_HIGH(sample) (sample>gLow ? true : false)
#define IS_LOW(sample) (sample<gHigh ? true : false)
#define IS_HIGH(sample) (sample > LOW_SIGNAL_THRESHOLD ? true : false)
#define IS_LOW(sample) (sample < HIGH_SIGNAL_THRESHOLD ? true : false)
// Timing related macros
#define IS_TIMEOUT(timeout_ticks) (GetTicks() > timeout_ticks)
#define TICKS_ELAPSED(start_ticks) (GetTicks() - start_ticks)
static uint8_t bits2byte(const uint8_t *bits, int length);
static void bits2bytes(const uint8_t *bits, int length, uint8_t *out);
static int em4x70_receive(uint8_t *bits);
@ -106,64 +109,20 @@ static void EM4170_setup_read(void) {
static bool get_signalproperties(void) {
// calculate signal properties (mean amplitudes) from measured data:
// 32 amplitudes (maximum values) -> mean amplitude value -> gHigh -> gLow
bool signal_found = false;
int no_periods = 32, pct = 50, noise = 140; // pct originally 75, found 50 was working better for me
uint8_t sample_ref = 127;
uint8_t sample_max_mean = 0;
uint8_t sample_max[no_periods];
uint32_t sample_max_sum = 0;
memset(sample_max, 0x00, sizeof(sample_max));
// Simple check to ensure we see a signal above the noise threshold
uint32_t no_periods = 32;
// wait until signal/noise > 1 (max. 32 periods)
for (int i = 0; i < TICKS_PER_FC * EM4X70_T_TAG_FULL_PERIOD * no_periods; i++) {
for (int i = 0; i < EM4X70_T_TAG_FULL_PERIOD * no_periods; i++) {
// about 2 samples per bit period
WaitTicks(TICKS_PER_FC * EM4X70_T_TAG_HALF_PERIOD);
WaitTicks(EM4X70_T_TAG_HALF_PERIOD);
if (AT91C_BASE_SSC->SSC_RHR > noise) {
signal_found = true;
break;
if (AT91C_BASE_SSC->SSC_RHR > HIGH_SIGNAL_THRESHOLD) {
return true;
}
}
if (signal_found == false)
return false;
// calculate mean maximum value of 32 periods, each period has a length of
// 3 single "full periods" to eliminate the influence of a listen window
for (int i = 0; i < no_periods; i++) {
uint32_t start_ticks = GetTicks();
//AT91C_BASE_TC0->TC_CCR = AT91C_TC_SWTRG;
while (TICKS_ELAPSED(start_ticks) < TICKS_PER_FC * 3 * EM4X70_T_TAG_FULL_PERIOD) {
volatile uint8_t sample = (uint8_t)AT91C_BASE_SSC->SSC_RHR;
if (sample > sample_max[i])
sample_max[i] = sample;
}
sample_max_sum += sample_max[i];
}
sample_max_mean = sample_max_sum / no_periods;
// set global envelope variables
gHigh = sample_ref + pct * (sample_max_mean - sample_ref) / 100;
gLow = sample_ref - pct * (sample_max_mean - sample_ref) / 100;
// Basic sanity check
if (gHigh - gLow < EM4X70_MIN_AMPLITUDE) {
return false;
}
Dbprintf("%s: gHigh %d gLow: %d", __func__, gHigh, gLow);
return true;
return false;
}
/**