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Clean up PRBS9 implementation.

new_P1
Rob Riggs 2021-08-17 20:30:37 -05:00
rodzic 33af8c4d82
commit ab54d373b1
4 zmienionych plików z 52 dodań i 53 usunięć
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4
apps/m17-demod.cpp
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@ -244,7 +244,7 @@ bool decode_bert(mobilinkd::M17FrameDecoder::bert_buffer_t const& bert)
for (int j = 0; j != 24; ++j) {
auto b = bert[j];
for (int i = 0; i != 8; ++i) {
prbs(b & 0x80);
prbs.validate(b & 0x80);
b <<= 1;
}
}
@ -252,7 +252,7 @@ bool decode_bert(mobilinkd::M17FrameDecoder::bert_buffer_t const& bert)
auto b = bert[24];
for (int i = 0; i != 5; ++i)
{
prbs(b & 0x80);
prbs.validate(b & 0x80);
b <<= 1;
}

4
apps/m17-mod.cpp
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@ -436,7 +436,7 @@ bitstream_t make_bert_frame(PRBS& prbs)
uint8_t byte = 0;
for (int i = 0; i != 8; ++i) {
byte <<= 1;
byte |= prbs();
byte |= prbs.generate();
}
data[i] = byte;
}
@ -444,7 +444,7 @@ bitstream_t make_bert_frame(PRBS& prbs)
uint8_t byte = 0;
for (int i = 0; i != 5; ++i) {
byte <<= 1;
byte |= prbs();
byte |= prbs.generate();
}
byte <<= 3;
data[24] = byte;

91
include/m17cxx/Util.h
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@ -322,6 +322,7 @@ struct PRBS9
static constexpr uint8_t TAP_1 = 8; // Bit 9
static constexpr uint8_t TAP_2 = 4; // Bit 5
static constexpr uint8_t LOCK_COUNT = 18; // 18 consecutive good bits.
static constexpr uint8_t UNLOCK_COUNT = 25; // 18 consecutive good bits.
uint16_t state = 1;
bool synced = false;
@ -332,69 +333,67 @@ struct PRBS9
size_t hist_count = 0;
size_t hist_pos = 0;
void count_errors(bool error)
{
bit_count += 1;
hist_count -= (history[hist_pos >> 3] & (1 << (hist_pos & 7))) != 0;
if (error) {
err_count += 1;
hist_count += 1;
history[hist_pos >> 3] |= (1 << (hist_pos & 7));
if (hist_count >= UNLOCK_COUNT) synced = false;
} else {
history[hist_pos >> 3] &= ~(1 << (hist_pos & 7));
}
if (++hist_pos == 128) hist_pos = 0;
}
// PRBS generator.
bool operator()()
bool generate()
{
bool result = ((state >> TAP_1) ^ (state >> TAP_2)) & 1;
state = ((state << 1) | result) & MASK;
return result;
}
// PRBS validator.
bool operator()(bool bit)
// PRBS Syncronizer. Returns 0 if the bit matches the PRBS, otherwise 1.
// When synchronizing the LFSR used in the PRBS, a single bad input bit will
// result in 3 error bits being emitted.
bool synchronize(bool bit)
{
bool result = (bit ^ (state >> TAP_1) ^ (state >> TAP_2)) & 1;
state = ((state << 1) | bit) & MASK;
if (result) {
sync_count = 0; // error
} else {
if (++sync_count == LOCK_COUNT) {
synced = true;
bit_count += LOCK_COUNT;
history.fill(0);
hist_count = 0;
hist_pos = 0;
sync_count = 0;
}
}
return result;
}
// PRBS validator. Returns 0 if the bit matches the PRBS, otherwise 1.
// The results are only valid when sync() returns true;
bool validate(bool bit)
{
bool result;
if (!synced) {
// Need to sync the PRBS with the incoming data.
result = (bit ^ (state >> TAP_1) ^ (state >> TAP_2)) & 1;
state = ((state << 1) | bit) & MASK;
if (result) {
sync_count = 0; // error
} else {
if (++sync_count == LOCK_COUNT) {
synced = true;
err_count = 0;
bit_count = LOCK_COUNT;
history.fill(0);
hist_count = 0;
hist_pos = 0;
sync_count = 0;
}
}
} else if (hist_count > 25) {
result = (bit ^ (state >> TAP_1) ^ (state >> TAP_2)) & 1;
state = ((state << 1) | bit) & MASK;
if (result) {
sync_count = 0;
} else {
if (++sync_count == LOCK_COUNT) {
history.fill(0);
hist_count = 0;
hist_pos = 0;
sync_count = 0;
}
}
result = synchronize(bit);
} else {
// PRBS is now free-running.
result = ((state >> TAP_1) ^ (state >> TAP_2)) & 1;
state = ((state << 1) | result) & MASK;
bit_count += 1;
hist_count -= (history[hist_pos >> 3] & (1 << (hist_pos & 7))) != 0;
if (result != bit) {
err_count += 1;
hist_count += 1;
history[hist_pos >> 3] |= (1 << (hist_pos & 7));
} else {
history[hist_pos >> 3] &= ~(1 << (hist_pos & 7));
}
if (++hist_pos == 128) hist_pos = 0;
result = bit != generate();
count_errors(result);
}
return result;
}
bool sync() const { return synced; }
uint32_t errors() const { assert(synced); return err_count; }
uint32_t bits() const { assert(synced); return bit_count; }

6
tests/UtilTest.cpp
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@ -212,7 +212,7 @@ TEST_F(UtilTest, PRBS9)
for (size_t i = 0; i != 511; ++i) {
lfsr = ((__builtin_popcount(lfsr & 0x11) & 1) << 8) | (lfsr >> 1);
bool p = (lfsr & 0x100) == 0x100;
bool n = prbs();
bool n = prbs.generate();
EXPECT_EQ(p,n) << "i = " << i;
}
}
@ -227,7 +227,7 @@ TEST_F(UtilTest, PRBS9_FULL)
uint16_t byte = 0;
uint16_t bits = 0;
for (size_t i = 0; i != 1000; ++i) {
bool n = prbs_generator();
bool n = prbs_generator.generate();
byte <<= 1;
byte |= n;
if (++bits == 8) {
@ -238,7 +238,7 @@ TEST_F(UtilTest, PRBS9_FULL)
if (i == 499) n = !n;
else if (i == 510) n = !n;
prbs_validator(n);
prbs_validator.validate(n);
}
std::cout << std::endl;