signal_generator: fix config conversion, clarify

src/hardware/signal_generator.rs

@@ -55,28 +55,43 @@ impl Default for BasicConfig {
 pub enum Error {
     /// The provided amplitude is out-of-range.
     InvalidAmplitude,
+    /// The provided symmetry is out of range.
+    InvalidSymmetry,
+    /// The provided frequency is out of range.
+    InvalidFrequency,
 }
 
 impl TryFrom<BasicConfig> for Config {
     type Error = Error;
 
     fn try_from(config: BasicConfig) -> Result<Config, Error> {
-        // Calculate the frequency tuning words
+        // Validate symmetry
-        let frequency_tuning_word: [u32; 2] = {
+        if config.symmetry < 0.0 || config.symmetry > 1.0 {
+            return Err(Error::InvalidSymmetry);
+        }
+
         const LSB_PER_HERTZ: f32 = (1u64 << (31 + ADC_SAMPLE_TICKS_LOG2))
             as f32
             / (TIMER_FREQUENCY.0 * 1_000_000) as f32;
         let ftw = config.frequency * LSB_PER_HERTZ;
 
-            if config.symmetry <= ftw / u32::MAX as f32 {
+        // Validate base frequency tuning word to be below Nyquist.
-                [1u32 << 31, ftw as u32]
+        const NYQUIST: f32 = (1u32 << 31) as _;
-            } else if 1. - config.symmetry <= ftw / u32::MAX as f32 {
+        if 2.0 * ftw > NYQUIST {
-                [ftw as u32, 1u32 << 31]
+            return Err(Error::InvalidFrequency);
+        }
+
+        // Calculate the frequency tuning words.
+        let frequency_tuning_word = {
+            let ftws = [ftw / config.symmetry, ftw / (1.0 - config.symmetry)];
+
+            // Clip both frequency tuning words to within Nyquist before rounding.
+            if ftws[0] > NYQUIST {
+                [1u32 << 31, ftws[1] as u32]
+            } else if ftws[1] > NYQUIST {
+                [ftws[0] as u32, 1u32 << 31]
             } else {
-                [
+                [ftws[0] as u32, ftws[1] as u32]
-                    (ftw / config.symmetry) as u32,
-                    (ftw / (1.0 - config.symmetry)) as u32,
-                ]
             }
         };