UA1ZBE Custom Firmware v1.0

Features:
- VFO mode with direct frequency input
- POCSAG decoder (512/1200 baud) with BCH(31,21) correction
- Full-screen Spectrum analyzer
- FM Radio receiver
- RSSI signal indicator overlay
- Custom boot splash (UA1ZBE / POCSAG pager / build date)

Architecture:
- app/mode.c — mode dispatcher (VFO/POCSAG/Spectrum/FM)
- app/boot_splash.c — 2-second boot splash
- app/pocsag/ — POCSAG decoder + BCH correction
- app/display_rssi.c — RSSI indicator
- main.c — entry point with custom init
- syscalls.c — bare-metal _sbrk stub

Build: arm-none-eabi-gcc -Os -flto -Wall -Werror -Wextra
Size: 57.9KB Flash / 3.6KB RAM

Controls:
- 0-9: Direct frequency input (VFO)
- SK2: POCSAG mode
- SK1: Spectrum analyzer
- 0: FM Radio
- EXIT: Return to VFO
- F/*: Toggle 512/1200 baud (in POCSAG)

app/display_rssi.h

@@ -0,0 +1,36 @@
+/* UA1ZBE Custom Firmware - RSSI Display Module
+ *
+ * Reads RSSI from BK4819 every 100-200ms, converts to dBm,
+ * and draws an indicator on the display.
+ *
+ * Format: "S:42" or "-87dBm" with mini bar
+ * Shown in upper-right corner of screen.
+ */
+
+#ifndef APP_DISPLAY_RSSI_H
+#define APP_DISPLAY_RSSI_H
+
+#include <stdbool.h>
+#include <stdint.h>
+
+/* Initialize RSSI display module */
+void RSSI_Init(void);
+
+/* Call from 10ms timeslice to update RSSI reading */
+void RSSI_Update(void);
+
+/* Draw RSSI indicator on the screen
+ * x, y: position on screen
+ * with_bar: if true, draw a mini signal bar next to the value */
+void RSSI_Draw(int x, int y, bool with_bar);
+
+/* Get current RSSI value in dBm */
+int16_t RSSI_GetdBm(void);
+
+/* Get raw RSSI value (0-511 from BK4819 REG_67) */
+uint16_t RSSI_GetRaw(void);
+
+/* Get S-meter level (0-9) */
+uint8_t RSSI_GetSLevel(void);
+
+#endif