diff --git a/core_generator/build.rs b/core_generator/build.rs
index 8a1eadb..45980bb 100644
--- a/core_generator/build.rs
+++ b/core_generator/build.rs
@@ -4,7 +4,7 @@ mod gen;
 
 use avr_mcu::*;
 use std::fs::{self, File};
-use std::io;
+use std::{env, io};
 use std::io::prelude::*;
 use std::path::{Path, PathBuf};
 
@@ -12,7 +12,7 @@ use std::path::{Path, PathBuf};
 /// archicectures that are not AVR.
 const DEFAULT_MCU_FOR_NON_AVR_DOCS: &'static str = "atmega328";
 
-const DEFAULT_FREQUENCY_HZ_FOR_NON_AVR_DOCS: &'static str = "16_000_000";
+const DEFAULT_FREQUENCY_HZ_FOR_NON_AVR_DOCS: u64 = 16_000_000;
 
 fn src_path() -> PathBuf {
     Path::new(env!("CARGO_MANIFEST_DIR")).join("src")
@@ -53,8 +53,11 @@ fn generate_config_module() -> Result<(), io::Error> {
     let path = src_path().join("config.rs");
     let mut f = File::create(&path)?;
 
-    let clock = if cfg!(arch = "avr") {
-        env!("AVR_CPU_FREQUENCY_HZ")
+    let clock: u64 = if cfg!(arch = "avr") {
+        env::var("AVR_CPU_FREQUENCY_HZ")
+            .expect("Please set the '$AVR_CPU_FREQUENCY_HZ' environment variable")
+            .parse()
+            .expect("The $AVR_CPU_FREQUENCY_HZ environment variable is not an integer")
     } else {
         DEFAULT_FREQUENCY_HZ_FOR_NON_AVR_DOCS
     };