Implement basic radio TX on fixed channel

radio_init() configures RADIO in NRF_1Mbit proprietary mode: 8-bit
length field, 4-byte address (BASE0+PREFIX0), 2-byte CRC-16/CCITT,
0 dBm TX power, channel 20 (2420 MHz). READY->START and END->DISABLE
shortcuts let radio_tx() trigger the full ramp-up/TX/disable sequence
by writing TASKS_TXEN once, then polling EVENTS_END.

main.c sends a fixed 4-byte test frame on every button press.
FHSS and AES remain compiled but are not called.

New bitfield unions in regs.h: radio_pcnf0_t, radio_pcnf1_t,
radio_crccnf_t, radio_shorts_t.

src/main.c

@@ -1,15 +1,16 @@
 #include "radio.h"
-#include "fhss.h"
 #include "power.h"
+#include <stdint.h>
+
+static const uint8_t test_frame[] = { 0xDE, 0xAD, 0xBE, 0xEF };
 
 int main(void)
 {
     power_init();
     radio_init();
-    fhss_init();
 
     while (1) {
         power_sleep_until_button();
-        radio_tx_burst();
+        radio_tx(test_frame, sizeof(test_frame));
     }
 }

src/radio.c

@@ -1,11 +1,73 @@
 #include "radio.h"
-#include "fhss.h"
 #include "regs.h"
 #include <nrf52840.h>
+#include <string.h>
+
+/*
+ * Packet buffer layout (S0=1B, LENGTH=8-bit, S1=0, payload up to 255B):
+ *   [0]        LENGTH  - payload byte count (written by radio_tx)
+ *   [1..1+len] payload - caller-supplied data
+ *
+ * RADIO is configured for NRF_1Mbit proprietary mode, fixed channel,
+ * no data whitening, 2-byte CRC over payload only.
+ * TX is synchronous: function returns after EVENTS_END fires.
+ */
+
+#define MAX_PAYLOAD  255u
+#define BUF_SIZE     (1u + MAX_PAYLOAD)
+
+static uint8_t pkt_buf[BUF_SIZE];
+
+/* Logical channel 20 -> 2400 + 20 = 2420 MHz (MAP=0) */
+#define DEFAULT_CHANNEL 20u
+
+/* 4-byte base address (3-byte BALEN field means 3+1=4 total address bytes) */
+#define RADIO_BASE0   0x12345678u
+#define RADIO_PREFIX0 0xABu     /* logical address 0: RADIO_BASE0 + RADIO_PREFIX0[7:0] */
 
 void radio_init(void)
 {
-    /* TODO: configure RADIO peripheral (MODE, PCNF0/1, BASE/PREFIX, CRC) */
+    NRF_RADIO->MODE = (radio_mode_t){
+        .bit = { .MODE = RADIO_MODE_NRF_1MBIT }
+    }.reg;
+
+    /* 8-bit LENGTH field, no S0/S1, 8-bit preamble, CRC not part of LENGTH */
+    NRF_RADIO->PCNF0 = (radio_pcnf0_t){
+        .bit = { .LFLEN = 8, .S0LEN = 0, .S1LEN = 0, .PLEN = 0, .CRCINC = 0 }
+    }.reg;
+
+    /* max 255-byte payload, 3-byte base address, little-endian, no whitening */
+    NRF_RADIO->PCNF1 = (radio_pcnf1_t){
+        .bit = { .MAXLEN = MAX_PAYLOAD, .STATLEN = 0, .BALEN = 3,
+                 .ENDIAN = 0, .WHITEEN = 0 }
+    }.reg;
+
+    NRF_RADIO->BASE0   = RADIO_BASE0;
+    NRF_RADIO->PREFIX0 = RADIO_PREFIX0;
+    NRF_RADIO->TXADDRESS   = 0;     /* transmit on logical address 0 */
+    NRF_RADIO->RXADDRESSES = 1;     /* receive on logical address 0 */
+
+    /* 2-byte CRC, skip address field */
+    NRF_RADIO->CRCCNF = (radio_crccnf_t){
+        .bit = { .LEN = RADIO_CRCCNF_LEN_TWO, .SKIPADDR = 1 }
+    }.reg;
+    NRF_RADIO->CRCPOLY = 0x11021u;  /* CRC-16/CCITT */
+    NRF_RADIO->CRCINIT = 0xFFFFu;
+
+    NRF_RADIO->TXPOWER = (radio_txpower_t){
+        .bit = { .TXPOWER = 0 }     /* 0 dBm */
+    }.reg;
+
+    NRF_RADIO->FREQUENCY = (radio_frequency_t){
+        .bit = { .FREQUENCY = DEFAULT_CHANNEL, .MAP = RADIO_MAP_DEFAULT }
+    }.reg;
+
+    /* READY -> START and END -> DISABLE shortcuts so CPU only triggers TXEN */
+    NRF_RADIO->SHORTS = (radio_shorts_t){
+        .bit = { .READY_START = 1, .END_DISABLE = 1 }
+    }.reg;
+
+    NRF_RADIO->PACKETPTR = (uint32_t)pkt_buf;
 }
 
 void radio_set_channel(uint8_t ch)
@@ -17,11 +79,22 @@ void radio_set_channel(uint8_t ch)
 
 void radio_tx(const uint8_t *data, uint8_t len)
 {
-    (void)data; (void)len;
-    /* TODO: load packet, enable TX, wait for END event, disable */
+    /* len is uint8_t so it cannot exceed MAX_PAYLOAD=255 by type guarantee */
+    pkt_buf[0] = len;
+    memcpy(&pkt_buf[1], data, len);
+
+    NRF_RADIO->EVENTS_END = 0;
+    NRF_RADIO->TASKS_TXEN = 1;
+
+    /* Busy-wait for END event (ramp-up ~40us + TX time, total <1ms for short frames) */
+    while (!NRF_RADIO->EVENTS_END)
+        ;
+
+    NRF_RADIO->EVENTS_END = 0;
+    /* RADIO is now DISABLED via the END_DISABLE shortcut */
 }
 
 void radio_tx_burst(void)
 {
-    /* TODO: hop + TX loop driven by fhss_next_channel() */
+    /* placeholder: FHSS TX loop not yet implemented */
 }