Add firmware skeleton for RP2040

Complete working firmware including:
- CMakeLists.txt for Pico SDK build
- SSD1306 OLED driver (128x32, I2C)
- High-resolution latency measurement using hardware timer
- Debounced button with short/long press detection
- Three modes: Single, Continuous, Stats
- USB serial debugging output

Includes 8x8 font with numbers and letters for display.

firmware/include/button.h

@@ -0,0 +1,23 @@
+#ifndef BUTTON_H
+#define BUTTON_H
+
+#include <stdbool.h>
+#include <stdint.h>
+
+typedef enum {
+    BTN_NONE,
+    BTN_SHORT_PRESS,    // < 500ms
+    BTN_LONG_PRESS      // >= 500ms
+} button_event_t;
+
+// Initialize button GPIO
+void button_init(void);
+
+// Poll button state (call in main loop)
+// Returns event type when button is released
+button_event_t button_poll(void);
+
+// Check if button is currently pressed
+bool button_is_pressed(void);
+
+#endif // BUTTON_H

firmware/include/config.h

@@ -0,0 +1,32 @@
+#ifndef CONFIG_H
+#define CONFIG_H
+
+// GPIO Pin Assignments (RP2040-Zero)
+#define PIN_I2C_SDA     0   // OLED SDA
+#define PIN_I2C_SCL     1   // OLED SCL
+#define PIN_TRIG_OUT    2   // Trigger output
+#define PIN_RETURN_IN   3   // Return input
+#define PIN_BUTTON      4   // Mode button
+
+// I2C Configuration
+#define I2C_PORT        i2c0
+#define I2C_FREQ        400000  // 400 kHz
+
+// OLED Display (SSD1306 128x32)
+#define OLED_ADDR       0x3C
+#define OLED_WIDTH      128
+#define OLED_HEIGHT     32
+
+// Timing Configuration
+#define TRIGGER_PULSE_MS    5       // Trigger pulse width
+#define MEASURE_TIMEOUT_MS  1000    // Max latency measurement
+#define DEBOUNCE_MS         50      // Button debounce
+#define CONTINUOUS_INTERVAL_MS 500  // Interval for continuous mode
+
+// Display
+#define DISPLAY_UPDATE_MS   100     // Display refresh rate
+
+// Statistics
+#define STATS_SAMPLES       10      // Rolling average sample count
+
+#endif // CONFIG_H

firmware/include/display.h

@@ -0,0 +1,31 @@
+#ifndef DISPLAY_H
+#define DISPLAY_H
+
+#include <stdbool.h>
+#include <stdint.h>
+
+// Initialize the OLED display
+void display_init(void);
+
+// Clear the display
+void display_clear(void);
+
+// Show latency value in milliseconds
+void display_latency(float latency_ms);
+
+// Show "WAITING..." message
+void display_waiting(void);
+
+// Show "TIMEOUT" message
+void display_timeout(void);
+
+// Show statistics (min/max/avg)
+void display_stats(float min_ms, float max_ms, float avg_ms);
+
+// Show mode indicator
+void display_mode(const char* mode);
+
+// Update display (call periodically)
+void display_update(void);
+
+#endif // DISPLAY_H

firmware/include/latency.h

@@ -0,0 +1,38 @@
+#ifndef LATENCY_H
+#define LATENCY_H
+
+#include <stdbool.h>
+#include <stdint.h>
+
+// Measurement result
+typedef struct {
+    bool valid;         // True if measurement succeeded
+    float latency_ms;   // Measured latency in milliseconds
+    uint64_t latency_us; // Raw latency in microseconds
+} measurement_t;
+
+// Statistics
+typedef struct {
+    float min_ms;
+    float max_ms;
+    float avg_ms;
+    uint32_t count;
+} stats_t;
+
+// Initialize latency measurement hardware
+void latency_init(void);
+
+// Perform a single latency measurement
+// Returns result struct with valid flag and latency value
+measurement_t latency_measure(void);
+
+// Get current statistics
+stats_t latency_get_stats(void);
+
+// Reset statistics
+void latency_reset_stats(void);
+
+// Add measurement to statistics
+void latency_add_to_stats(float latency_ms);
+
+#endif // LATENCY_H