1 files changed, 49 insertions, 0 deletions
diff --git a/firmware/s1224/include/hdlc.h b/firmware/s1224/include/hdlc.h
new file mode 100644
index 0000000..9b4cb22
--- /dev/null
+++ b/firmware/s1224/include/hdlc.h
@@ -0,0 +1,49 @@
+/**
+ * HDLC frame helpers — flags, bit-stuff, FCS-CCITT (AX.25).
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+#ifndef S1224_HDLC_H
+#define S1224_HDLC_H
+
+#include <stddef.h>
+#include <stdint.h>
+#include <stdbool.h>
+
+#define HDLC_FLAG       0x7Eu
+#define HDLC_FCS_INIT   0xFFFFu
+#define HDLC_FCS_GOOD   0xF0B8u
+#define HDLC_MAX_INFO   512
+
+uint16_t hdlc_fcs_update(uint16_t fcs, uint8_t byte);
+uint16_t hdlc_fcs_final(uint16_t fcs);
+
+/**
+ * Build on-wire bit stream (NRZI already applied by caller after this):
+ * FLAG + stuffed(info + FCS_lo + FCS_hi) + FLAG(+optional trailing flags).
+ * Returns bit count written to bits[] (MSB-first within each stuffed byte stream
+ * is not used — output is a raw bit array, LSB-first per AX.25 bit order).
+ *
+ * bits_cap is capacity in bits. Returns 0 on overflow / bad args.
+ */
+size_t hdlc_encode_bits(const uint8_t *info, size_t info_len,
+                        uint8_t *bits, size_t bits_cap,
+                        unsigned trailing_flags);
+
+/**
+ * Feed one NRZ bit (after NRZI decode). Returns true when a complete good
+ * frame is available in out[] (info only, FCS stripped).
+ */
+typedef struct {
+    uint8_t  buf[HDLC_MAX_INFO + 2];
+    size_t   len;
+    uint8_t  ones;
+    bool     in_frame;
+    bool     stuffing;
+    uint8_t  byte_acc;
+    uint8_t  bit_count;
+} hdlc_rx_t;
+
+void hdlc_rx_init(hdlc_rx_t *rx);
+bool hdlc_rx_bit(hdlc_rx_t *rx, bool bit, uint8_t *out, size_t *out_len);
+
+#endif /* S1224_HDLC_H */