Implement full FHSS TX/RX, real AES key, button read, slot sync

radio_tx_burst(): captures current slot, calls fhss_next_channel() to
advance the sequence, sets channel, transmits ptt_frame_t, then holds
the dwell window via TIMER0 so the receiver has the full 2 ms to listen.

radio_rx_burst(): advances to the same channel, enables RX, waits for
EVENTS_END or TIMER0 COMPARE0 expiry. On a valid CRC the ptt_frame_t
is returned; caller syncs FHSS with fhss_set_slot(frame.slot + 1).

TIMER0: configured once in radio_init() (MODE=Timer, BITMODE=16-bit,
PRESCALER=4 -> 1 MHz tick, CC[0]=2000 -> 2 ms, COMPARE0_STOP shortcut).

fhss.c: replace zero key with a non-trivial 128-bit value; add
fhss_set_slot() and fhss_get_slot() for RX synchronisation.

power.c: add BUTTON_ACTIVE_LOW flag (default 1 = pull-up + GND button);
configure PIN_CNF accordingly; implement power_button_pressed() via
NRF_P0->IN.

regs.h: add gpio_pin_cnf_t with GPIO_PULL_* constants.

main.c: tight PTT loop -- radio_tx_burst() while button held, otherwise
radio_rx_burst() with slot sync on reception. No WFI in RX mode.

include/fhss.h

@@ -10,18 +10,29 @@
 #pragma once
 #include <stdint.h>
 
+/** Dwell time per channel in milliseconds. */
+#define FHSS_DWELL_MS 2u
+
+/** Number of channels in the hopping sequence. */
+#define FHSS_CHANNELS 40u
+
 /** @brief Reset the slot counter to zero. */
 void fhss_init(void);
 
 /**
- * @brief Return the next channel in the hopping sequence.
- *
- * Encrypts the current slot counter big-endian with AES-128-ECB, returns
- * @c block[0] % 40, and advances the slot counter.
- *
+ * @brief Return the next channel in the hopping sequence and advance the slot.
  * @return Channel index in [0, 39].
  */
 uint8_t fhss_next_channel(void);
 
-/** @brief Advance the slot counter without transmitting (receiver side). */
+/** @brief Advance the slot counter by one (receiver side, no packet received). */
 void fhss_sync_tick(void);
+
+/**
+ * @brief Force the slot counter to a specific value for RX synchronisation.
+ * @param s New slot value.
+ */
+void fhss_set_slot(uint32_t s);
+
+/** @brief Return the current slot counter value. */
+uint32_t fhss_get_slot(void);

include/power.h

@@ -1,10 +1,11 @@
 /**
  * @file power.h
- * @brief Power management: DC/DC regulator, GPIOTE wakeup, SYSTEM_ON sleep.
+ * @brief Power management: DC/DC regulator, GPIOTE wakeup, WFI sleep.
  */
 #pragma once
+#include <stdbool.h>
 
-/** @brief Enable the DC/DC converter and configure GPIOTE wakeup on the PTT button. */
+/** @brief Enable the DC/DC converter, configure GPIO input and GPIOTE wakeup on the PTT button. */
 void power_init(void);
 
 /**
@@ -14,3 +15,9 @@ void power_init(void);
  * interrupt fires (button press) and resumes from here.
  */
 void power_sleep_until_button(void);
+
+/**
+ * @brief Return the current state of the PTT button.
+ * @return true when the button is pressed.
+ */
+bool power_button_pressed(void);

include/radio.h

@@ -3,9 +3,23 @@
  * @brief RADIO peripheral driver -- NRF_1Mbit proprietary mode.
  */
 #pragma once
+#include <stdbool.h>
 #include <stdint.h>
 
-/** @brief Configure the RADIO peripheral (mode, packet format, address, CRC, power, channel). */
+/**
+ * @brief PTT packet transmitted on every FHSS hop.
+ *
+ * The receiver uses @p slot to resynchronise its FHSS counter after
+ * receiving the first packet.
+ */
+typedef struct __attribute__((packed)) {
+    uint32_t slot; /**< Sender's FHSS slot number at time of transmission. */
+    uint8_t flags; /**< Bitmask: PTT_FLAG_ACTIVE when voice channel is open. */
+} ptt_frame_t;
+
+#define PTT_FLAG_ACTIVE 0x01u /**< PTT button is held on the transmitting side. */
+
+/** @brief Configure the RADIO peripheral (mode, packet format, address, CRC, power). */
 void radio_init(void);
 
 /**
@@ -18,13 +32,30 @@ void radio_set_channel(uint8_t ch);
  * @brief Transmit one packet synchronously.
  *
  * Loads @p data into the internal packet buffer, asserts TASKS_TXEN, and
- * returns after EVENTS_END fires.  The RADIO is DISABLED automatically via
- * the END_DISABLE shortcut before the function returns.
+ * returns after EVENTS_END fires.  RADIO is DISABLED automatically via the
+ * END_DISABLE shortcut before the function returns.
  *
  * @param data  Payload bytes.
  * @param len   Payload length (0-255 bytes).
  */
 void radio_tx(const uint8_t *data, uint8_t len);
 
-/** @brief Transmit a burst with FHSS hopping (not yet implemented). */
+/**
+ * @brief Transmit one FHSS hop: advance channel, send PTT frame, hold dwell time.
+ *
+ * Call repeatedly in a loop while the PTT button is held.  Each call occupies
+ * exactly FHSS_DWELL_MS milliseconds.
+ */
 void radio_tx_burst(void);
+
+/**
+ * @brief Receive one FHSS hop: advance channel, listen for FHSS_DWELL_MS ms.
+ *
+ * If a packet with a valid CRC arrives during the dwell window, @p frame_out
+ * is filled and the function returns true.  The caller should then call
+ * fhss_set_slot(frame_out->slot + 1) to synchronise the hopping sequence.
+ *
+ * @param frame_out  Destination for the received frame (must not be NULL).
+ * @return true if a valid packet was received, false on timeout or CRC error.
+ */
+bool radio_rx_burst(ptt_frame_t *frame_out);

include/regs.h

@@ -153,3 +153,24 @@ typedef union {
     } bit;
     uint32_t reg;
 } radio_shorts_t;
+
+/* GPIO */
+
+/** @brief GPIO PIN_CNF[n]: pin configuration register. */
+typedef union {
+    struct {
+        uint32_t DIR   : 1;  /* [0]     0=Input  1=Output */
+        uint32_t INPUT : 1;  /* [1]     0=Connect  1=Disconnect */
+        uint32_t PULL  : 2;  /* [3:2]   0=Disabled  1=Pulldown  3=Pullup */
+        uint32_t       : 4;  /* [7:4]   reserved */
+        uint32_t DRIVE : 3;  /* [10:8]  0=S0S1 standard drive */
+        uint32_t       : 5;  /* [15:11] reserved */
+        uint32_t SENSE : 2;  /* [17:16] 0=Disabled  2=SenseHigh  3=SenseLow */
+        uint32_t       : 14; /* [31:18] reserved */
+    } bit;
+    uint32_t reg;
+} gpio_pin_cnf_t;
+
+#define GPIO_PULL_DISABLED 0u
+#define GPIO_PULL_PULLDOWN 1u
+#define GPIO_PULL_PULLUP 3u