feat: implement channel helpers

cel_crsf_channel_us_to_val() and cel_crsf_channel_val_to_us()
convert between microseconds (988-2012) and 11-bit values (172-1811)
with rounding. cel_crsf_channel_default() fills safe/disarmed values.

celrs/crsf.c

@@ -124,28 +124,34 @@ int16_t cel_crsf_channel_clamp(int16_t value) {
 }
 
 int16_t cel_crsf_channel_us_to_val(uint16_t us) {
-    /* TODO: convert microseconds to 11-bit channel value.
-     * Linear mapping: 988us -> 172, 2012us -> 1811.
-     * value = (us - 988) * (1811 - 172) / (2012 - 988) + 172
-     * Clamp result to [CEL_CRSF_CH_MIN, CEL_CRSF_CH_MAX]. */
-    (void)us;
-    return CEL_CRSF_CH_MID;
+    /* Clamp to valid range first to avoid underflow */
+    if (us < 988) us = 988;
+    if (us > 2012) us = 2012;
+    /* Round to nearest: add half the divisor before dividing */
+    int16_t value = (int16_t)(
+        ((us - 988) * (CEL_CRSF_CH_MAX - CEL_CRSF_CH_MIN)
+         + (2012 - 988) / 2) / (2012 - 988)
+        + CEL_CRSF_CH_MIN);
+    return cel_crsf_channel_clamp(value);
 }
 
 uint16_t cel_crsf_channel_val_to_us(int16_t value) {
-    /* TODO: convert 11-bit channel value to microseconds.
-     * Linear mapping: 172 -> 988us, 1811 -> 2012us.
-     * us = (value - 172) * (2012 - 988) / (1811 - 172) + 988 */
-    (void)value;
-    return 1500;
+    value = cel_crsf_channel_clamp(value);
+    return (uint16_t)(
+        ((value - CEL_CRSF_CH_MIN) * (2012 - 988)
+         + (CEL_CRSF_CH_MAX - CEL_CRSF_CH_MIN) / 2)
+        / (CEL_CRSF_CH_MAX - CEL_CRSF_CH_MIN) + 988);
 }
 
 void cel_crsf_channel_default(int16_t channels[16]) {
-    /* TODO: fill with safe/disarmed values.
-     * CH1-Roll, CH2-Pitch, CH4-Yaw = CEL_CRSF_CH_MID (centered)
-     * CH3-Throttle = CEL_CRSF_CH_MIN (zero)
-     * CH5..CH16 (AUX) = CEL_CRSF_CH_MIN (off) */
-    (void)channels;
+    memset(channels, 0, sizeof(int16_t) * 16);
+    channels[0] = CEL_CRSF_CH_MID; /* roll */
+    channels[1] = CEL_CRSF_CH_MID; /* pitch */
+    channels[2] = CEL_CRSF_CH_MIN; /* throttle */
+    channels[3] = CEL_CRSF_CH_MID; /* yaw */
+    for (int i = 4; i < 16; i++) {
+        channels[i] = CEL_CRSF_CH_MIN;
+    }
 }
 
 /* --------------------------------------------------------------------------- */

tests/test_crsf.c

@@ -38,6 +38,60 @@ void test_channel_clamp_mid(void) {
     TEST_ASSERT_EQUAL_INT16(CEL_CRSF_CH_MID, cel_crsf_channel_clamp(CEL_CRSF_CH_MID));
 }
 
+void test_channel_us_to_val_min(void) {
+    TEST_ASSERT_EQUAL_INT16(CEL_CRSF_CH_MIN, cel_crsf_channel_us_to_val(988));
+}
+
+void test_channel_us_to_val_mid(void) {
+    TEST_ASSERT_EQUAL_INT16(CEL_CRSF_CH_MID, cel_crsf_channel_us_to_val(1500));
+}
+
+void test_channel_us_to_val_max(void) {
+    TEST_ASSERT_EQUAL_INT16(CEL_CRSF_CH_MAX, cel_crsf_channel_us_to_val(2012));
+}
+
+void test_channel_us_to_val_below_min(void) {
+    TEST_ASSERT_EQUAL_INT16(CEL_CRSF_CH_MIN, cel_crsf_channel_us_to_val(0));
+}
+
+void test_channel_us_to_val_above_max(void) {
+    TEST_ASSERT_EQUAL_INT16(CEL_CRSF_CH_MAX, cel_crsf_channel_us_to_val(65535));
+}
+
+void test_channel_val_to_us_min(void) {
+    TEST_ASSERT_EQUAL_UINT16(988, cel_crsf_channel_val_to_us(CEL_CRSF_CH_MIN));
+}
+
+void test_channel_val_to_us_mid(void) {
+    TEST_ASSERT_EQUAL_UINT16(1500, cel_crsf_channel_val_to_us(CEL_CRSF_CH_MID));
+}
+
+void test_channel_val_to_us_max(void) {
+    TEST_ASSERT_EQUAL_UINT16(2012, cel_crsf_channel_val_to_us(CEL_CRSF_CH_MAX));
+}
+
+void test_channel_default_throttle_min(void) {
+    int16_t ch[16];
+    cel_crsf_channel_default(ch);
+    TEST_ASSERT_EQUAL_INT16(CEL_CRSF_CH_MIN, ch[2]); /* throttle */
+}
+
+void test_channel_default_centered(void) {
+    int16_t ch[16];
+    cel_crsf_channel_default(ch);
+    TEST_ASSERT_EQUAL_INT16(CEL_CRSF_CH_MID, ch[0]); /* roll */
+    TEST_ASSERT_EQUAL_INT16(CEL_CRSF_CH_MID, ch[1]); /* pitch */
+    TEST_ASSERT_EQUAL_INT16(CEL_CRSF_CH_MID, ch[3]); /* yaw */
+}
+
+void test_channel_default_aux_min(void) {
+    int16_t ch[16];
+    cel_crsf_channel_default(ch);
+    for (int i = 4; i < 16; i++) {
+        TEST_ASSERT_EQUAL_INT16(CEL_CRSF_CH_MIN, ch[i]);
+    }
+}
+
 /* Frame parse tests — ELRS format: [addr][length][type][payload][crc] */
 
 /* Build a valid test frame with known CRC */
@@ -123,6 +177,17 @@ int main(void) {
     RUN_TEST(test_channel_clamp_min);
     RUN_TEST(test_channel_clamp_max);
     RUN_TEST(test_channel_clamp_mid);
+    RUN_TEST(test_channel_us_to_val_min);
+    RUN_TEST(test_channel_us_to_val_mid);
+    RUN_TEST(test_channel_us_to_val_max);
+    RUN_TEST(test_channel_us_to_val_below_min);
+    RUN_TEST(test_channel_us_to_val_above_max);
+    RUN_TEST(test_channel_val_to_us_min);
+    RUN_TEST(test_channel_val_to_us_mid);
+    RUN_TEST(test_channel_val_to_us_max);
+    RUN_TEST(test_channel_default_throttle_min);
+    RUN_TEST(test_channel_default_centered);
+    RUN_TEST(test_channel_default_aux_min);
     RUN_TEST(test_parse_valid_frame);
     RUN_TEST(test_parse_null_frame);
     RUN_TEST(test_parse_null_buf);