DATE CREATED: Dec 2015
Horus telemetry layer 2 processing. Takes an array of 8 bit payload
- data, generates parity bits for (23,12) Golay, interleaves data and
- parity bits, pre-pends a Unique Word for modem sync. Caller is
- responsible for providing storage for output packet.
+ data, generates parity bits for a (23,12) Golay code, interleaves
+ data and parity bits, pre-pends a Unique Word for modem sync.
+ Caller is responsible for providing storage for output packet.
[X] work out number of golay codewords rqd
[X] function to return storgage rqd for output packet
$ gcc horus_l2.c -c -Wall
-\*---------------------------------------------------------------------------*/
-
-#ifdef HORUS_L2_UNITTEST
-#define HORUS_L2_RX
-#endif
+ By default the RX side is #ifdef-ed out, leaving the minimal amount
+ of code for tx.
-#define RUN_TIME_TABLES
+\*---------------------------------------------------------------------------*/
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
+#include <stdint.h>
#include "horus_l2.h"
+#ifdef HORUS_L2_UNITTEST
+#define HORUS_L2_RX
+#endif
+
+#define RUN_TIME_TABLES
+
static char uw[] = {'$','$'};
-int get_syndrome(int pattern);
+int32_t get_syndrome(int32_t pattern);
void golay23_init(void);
int golay23_decode(int received_codeword);
+/*
+ We are using a Golay (23,12) code which has a codeword 23 bits
+ long. The tx packet format is:
+
+ | Unique Word | payload data bits | parity bits |
+
+ This function works out how much storage the caller of
+ horus_l2_encode_tx_packet() will need to store the tx packet
+ */
+
int horus_l2_get_num_tx_data_bytes(num_payload_data_bytes) {
int num_payload_data_bits, num_golay_codewords;
int num_tx_data_bits, num_tx_data_bytes;
num_payload_data_bits = num_payload_data_bytes*8;
num_golay_codewords = num_payload_data_bits/12;
- if (num_payload_data_bits % 12)
+ if (num_payload_data_bits % 12) /* round up to 12 bits, may mean some unused bits */
num_golay_codewords++;
num_tx_data_bits = sizeof(uw)*8 + num_payload_data_bits + num_golay_codewords*11;
num_tx_data_bytes = num_tx_data_bits/8;
- if (num_tx_data_bits % 8)
+ if (num_tx_data_bits % 8) /* round up to nearest byte, may mean some unused bits */
num_tx_data_bytes++;
#ifdef DEBUG0
/*
+ Takes an array of payload data bytes, prepends a unique word and appends
+ parity bits.
+
The encoder will run on the payload on a small 8-bit uC. As we are
memory constrained so we do a lot of burrowing for bits out of
packed arrays, and don't use a LUT for Golay encoding. Hopefully it
{
int num_tx_data_bytes, num_payload_data_bits;
unsigned char *pout = output_tx_data;
- int ninbit, ingolay, ningolay, paritybyte, nparitybits;
- int ninbyte, shift, inbit, golayparity, golayparitybit, i;
+ int ninbit, ningolay, nparitybits;
+ int32_t ingolay, paritybyte, inbit, golayparity;
+ int ninbyte, shift, golayparitybit, i;
num_tx_data_bytes = horus_l2_get_num_tx_data_bytes(num_payload_data_bytes);
memcpy(pout, uw, sizeof(uw)); pout += sizeof(uw);
#ifdef HORUS_L2_UNITTEST
-/* unit test designd to run on a Host PC */
+
+/*
+ Test function to construct a packet of payload data, encode, add
+ some bit errors, decode, count errors.
+*/
int test_sending_bytes(int nbytes, float ber) {
unsigned char input_payload[nbytes];
return nerr;
}
+/* unit test designed to run on a PC */
+
int main(void) {
printf("test 0: 22 bytes of payload data BER: 0.00 errors: %d\n", test_sending_bytes(22, 0.0));
printf("test 0: 22 bytes of payload data BER: 0.01 errors: %d\n", test_sending_bytes(22, 0.01));
#include "golaydectable.h"
#endif
-#ifdef GOLAY23_UNITTEST
-static int position[23] = { 0x00000001, 0x00000002, 0x00000004, 0x00000008,
- 0x00000010, 0x00000020, 0x00000040, 0x00000080,
- 0x00000100, 0x00000200, 0x00000400, 0x00000800,
- 0x00001000, 0x00002000, 0x00004000, 0x00008000,
- 0x00010000, 0x00020000, 0x00040000, 0x00080000,
- 0x00100000, 0x00200000, 0x00400000 };
-#endif
-
#ifdef RUN_TIME_TABLES
static int arr2int(int a[], int r)
/*
}
#endif
+#ifdef HORUS_L2_RX
void nextcomb(int n, int r, int a[])
/*
* Calculate next r-combination of an n-set.
a[i] = a[j] + i - j + 1;
return;
}
+#endif
-int get_syndrome(int pattern)
+int32_t get_syndrome(int32_t pattern)
/*
* Compute the syndrome corresponding to the given pattern, i.e., the
* remainder after dividing the pattern (when considering it as the vector
* obtain its syndrome in decoding.
*/
{
- int aux = X22;
+ int32_t aux = X22;
if (pattern >= X11)
while (pattern & MASK12) {
return(pattern);
}
+#ifdef HORUS_L2_RX
+
/*---------------------------------------------------------------------------*\
FUNCTION....: golay23_init()
return errors;
}
-
-#ifdef GOLAY23_UNITTEST
-
-static int golay23_test(int error_pattern) {
- int data;
- int codeword;
- int recd;
- int pattern;
- int decerror;
- int i, tests;
-
- decerror = 0;
- tests = 0;
-
- for (data = 0; data<(1<<12); data++) {
-
- codeword = golay23_encode(data);
- recd = codeword ^ error_pattern;
- recd = golay23_decode(recd);
- pattern = (recd ^ codeword) >> 11;
- for (i=0; i<12; i++)
- if (pattern & position[i])
- decerror++;
- if (decerror) {
- printf("data: 0x%x codeword: 0x%x recd: 0x%x\n", data, codeword, recd);
- printf("there were %d decoding errors\n", decerror);
- exit(1);
- }
- tests++;
- }
-
- return tests;
-}
-
-int main(void)
-{
- int i;
- int tests;
- int a[4];
- int error_pattern;
-
- golay23_init();
-
- /* ---------------------------------------------------------------------
- * Generate DATA
- * ---------------------------------------------------------------------
- */
-
- /* Test all combinations of data and 1,2 or 3 errors */
-
- tests = 0;
- error_pattern = 1;
- for (i=0; i< 23; i++) {
- //printf("error_pattern: 0x%x\n", error_pattern);
- tests += golay23_test(error_pattern);
- error_pattern *= 2;
- }
- printf("%d 1 bit error tests performed OK!\n", tests);
-
- tests = 0;
- a[1] = 1; a[2] = 2;
- error_pattern = arr2int(a,2);
- tests += golay23_test(error_pattern);
- for (i=1; i<253; i++) {
- nextcomb(23,2,a);
- error_pattern = arr2int(a,2);
- //printf("error_pattern: 0x%x\n", error_pattern);
- tests += golay23_test(error_pattern);
- }
- printf("%d 2 bit error tests performed OK!\n", tests);
-
- tests = 0;
- a[1] = 1; a[2] = 2; a[3] = 3;
- error_pattern = arr2int(a,3);
- tests += golay23_test(error_pattern);
- for (i=1; i<1771; i++) {
- nextcomb(23,3,a);
- error_pattern = arr2int(a,3);
- //printf("error_pattern: 0x%x\n", error_pattern);
- tests += golay23_test(error_pattern);
- }
- printf("%d 3 bit error tests performed OK!\n", tests);
-
- return 0;
-}
-#endif
-
-#ifdef GOLAY23_MAKETABLES
-int main(int argc, char *argv[]) {
- FILE *f;
- int i;
-
- golay23_init();
-
- f=fopen("golayenctable.h","wt");
- assert(f != NULL);
-
- fprintf(f,"/* Generated by golay23.c -DGOLAY23_MAKETABLE */\n\n");
- fprintf(f,"const int static encoding_table[]={\n");
-
- for (i=0; i<4095; i++)
- fprintf(f," 0x%x,\n", encoding_table[i]);
- fprintf(f, " 0x%x\n};\n", encoding_table[i]);
- fclose(f);
-
- f=fopen("golaydectable.h","wt");
- assert(f != NULL);
-
- fprintf(f,"/* Generated by golay23.c -DGOLAY23_MAKETABLE */\n\n");
- fprintf(f,"const int static decoding_table[]={\n");
-
- for (i=0; i<2047; i++)
- fprintf(f," 0x%x,\n", decoding_table[i]);
- fprintf(f, " 0x%x\n};\n", decoding_table[i]);
- fclose(f);
-
- return 0;
-}
-
#endif
-
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