diff --git a/crates/fbuild-build-mcu/src/ch32v/orchestrator.rs b/crates/fbuild-build-mcu/src/ch32v/orchestrator.rs
index 24342324..3c297f7b 100644
--- a/crates/fbuild-build-mcu/src/ch32v/orchestrator.rs
+++ b/crates/fbuild-build-mcu/src/ch32v/orchestrator.rs
@@ -1,4 +1,4 @@
-//! CH32V build orchestrator — wires together config, packages, compiler, linker.
+//! CH32V build orchestrator — wires together config, packages, compiler, linker.
 //!
 //! Build phases:
 //! 1. Parse platformio.ini
@@ -122,6 +122,12 @@ impl BuildOrchestrator for Ch32vOrchestrator {
         let mut defines = ctx.board.get_defines();
         defines.extend(mcu_config.defines_map());
         defines.insert(system_series.clone(), "1".to_string());
+        let (sysclk_name, sysclk_value) = sysclk_define(
+            &series,
+            &ctx.board.f_cpu,
+            ctx.board.clock_source.as_deref().unwrap_or("hsi+pll"),
+        )?;
+        defines.insert(sysclk_name, sysclk_value);
         // CH32V cores use `#include VARIANT_H` — define it from the variant dir
         if let Some(vh) = resolve_variant_h(&variant_dir, ctx.board.variant_h.as_deref()) {
             defines.insert("VARIANT_H".to_string(), format!("\\\"{}\\\"", vh));
@@ -260,6 +266,52 @@ fn validate_ch32v_framework(framework: Option<&str>) -> fbuild_core::Result<()>
     }
 }
 
+fn sysclk_define(
+    series: &str,
+    f_cpu: &str,
+    clock_source: &str,
+) -> fbuild_core::Result<(String, String)> {
+    let hz = f_cpu.trim_end_matches('L').parse::<u64>().map_err(|_| {
+        fbuild_core::FbuildError::ConfigError(format!(
+            "ch32v: invalid f_cpu `{f_cpu}` for {series}"
+        ))
+    })?;
+    let mhz = hz / 1_000_000;
+    if mhz == 0 || hz % 1_000_000 != 0 {
+        return Err(fbuild_core::FbuildError::ConfigError(format!(
+            "ch32v: unsupported f_cpu `{f_cpu}` for {series}; expected a whole MHz value"
+        )));
+    }
+    let supported: &[u64] = match series {
+        "ch32v003" | "ch32v006" => &[24, 48],
+        "ch32v103" => &[48, 56, 72],
+        "ch32v203" | "ch32v208" | "ch32v303" | "ch32v307" => &[48, 56, 72, 96, 120, 144],
+        "ch32x035" => &[8, 12, 16, 24, 48],
+        "ch32l103" => &[48, 72, 96],
+        _ => &[],
+    };
+    if !supported.contains(&mhz) {
+        return Err(fbuild_core::FbuildError::ConfigError(format!(
+            "ch32v: unsupported f_cpu `{f_cpu}` for {series}; supported values: {supported:?} MHz"
+        )));
+    }
+    let source = match clock_source.trim().to_ascii_lowercase().as_str() {
+        "hsi" | "hsi+pll" => "HSI",
+        "hse" | "hse+pll" => "HSE",
+        other => {
+            return Err(fbuild_core::FbuildError::ConfigError(format!(
+                "ch32v: unsupported clock_source `{other}` for {series}; expected hsi, hsi+pll, hse, or hse+pll"
+            )));
+        }
+    };
+    let unit = if matches!(series, "ch32v003" | "ch32v006") && source == "HSI" {
+        "MHZ"
+    } else {
+        "MHz"
+    };
+    Ok((format!("SYSCLK_FREQ_{mhz}{unit}_{source}"), hz.to_string()))
+}
+
 /// Recursively add subdirectories that contain .h files as include paths.
 fn discover_header_subdirs(dir: &Path, include_dirs: &mut Vec<PathBuf>) {
     if let Ok(entries) = std::fs::read_dir(dir) {
@@ -435,6 +487,27 @@ mod tests {
         assert!(error.to_string().contains("1108"));
     }
 
+    #[test]
+    fn test_sysclk_define_uses_series_spelling_and_clock_source() {
+        assert_eq!(
+            sysclk_define("ch32v203", "144000000L", "hsi+pll").unwrap(),
+            (
+                "SYSCLK_FREQ_144MHz_HSI".to_string(),
+                "144000000".to_string()
+            )
+        );
+        assert_eq!(
+            sysclk_define("ch32v003", "48000000L", "hsi+pll").unwrap(),
+            ("SYSCLK_FREQ_48MHZ_HSI".to_string(), "48000000".to_string())
+        );
+    }
+
+    #[test]
+    fn test_sysclk_define_rejects_unsupported_frequency() {
+        let error = sysclk_define("ch32v203", "8000000L", "hsi").unwrap_err();
+        assert!(error.to_string().contains("supported values"));
+    }
+
     #[test]
     fn test_series_to_system_dir() {
         // CH32V series: last digit replaced with 'x'
diff --git a/crates/fbuild-config/src/board/db.rs b/crates/fbuild-config/src/board/db.rs
index 62528e16..17d99488 100644
--- a/crates/fbuild-config/src/board/db.rs
+++ b/crates/fbuild-config/src/board/db.rs
@@ -200,6 +200,12 @@ fn flatten_board_entry(entry: &serde_json::Value, board_id: &str) -> HashMap<Str
     } else if let Some(f_cpu_str) = build.and_then(|b| b.get("f_cpu")).and_then(|v| v.as_str()) {
         d.insert("f_cpu".into(), f_cpu_str.to_string());
     }
+    if let Some(clock_source) = build
+        .and_then(|b| b.get("clock_source"))
+        .and_then(|v| v.as_str())
+    {
+        d.insert("clock_source".into(), clock_source.to_string());
+    }
 
     // ram/rom: enriched top-level wins; fall back to PIO `upload.maximum_ram_size`
     // and `upload.maximum_size` respectively.
diff --git a/crates/fbuild-config/src/board/loaders.rs b/crates/fbuild-config/src/board/loaders.rs
index 01ea7c7c..416f9a9f 100644
--- a/crates/fbuild-config/src/board/loaders.rs
+++ b/crates/fbuild-config/src/board/loaders.rs
@@ -163,6 +163,7 @@ impl BoardConfig {
             name,
             mcu,
             f_cpu: get("f_cpu").unwrap_or_else(|| "16000000L".to_string()),
+            clock_source: get("clock_source"),
             board: get("board")
                 .or_else(|| props.get("board").cloned())
                 .unwrap_or_else(|| board_id_to_board_define(board_id)),
@@ -319,6 +320,8 @@ impl BoardConfig {
             name: get("name", board_id),
             mcu: get("mcu", "unknown"),
             f_cpu: get("f_cpu", "16000000L"),
+            clock_source: Some(get("clock_source", ""))
+                .filter(|source| !source.is_empty()),
             board: get("board", &board_id_to_board_define(board_id)),
             core: get("core", "arduino"),
             variant: get("variant", "standard"),
diff --git a/crates/fbuild-config/src/board/types.rs b/crates/fbuild-config/src/board/types.rs
index ee3858a6..bc71a204 100644
--- a/crates/fbuild-config/src/board/types.rs
+++ b/crates/fbuild-config/src/board/types.rs
@@ -42,6 +42,9 @@ pub struct BoardConfig {
     pub name: String,
     pub mcu: String,
     pub f_cpu: String,
+    /// Clock source used by the MCU startup code (for example `hsi+pll`).
+    #[serde(default)]
+    pub clock_source: Option<String>,
     pub board: String,
     pub core: String,
     pub variant: String,