support 2^n Mhz clock frequencies

Author: nerdralph

README.md

@@ -6,7 +6,5 @@ For example, the Blink example sketch built for the Uno takes 924 bytes of flash
 Download [a release](https://github.com/nerdralph/ArduinoShrink/releases), and extract the files into your Arduino/libraries folder.  Select ArduinoShrink from the Sketch, Include Library menu.  This will replace several Arduino functions with smaller and faster versions.  The Arduino pin number must be known at compile time if you are using this library.
 
 ## Limitations
-ArduinoShrink supports only ATmegax8 MCUs running at 16Mhz
+ArduinoShrink supports only ATmega8 and  ATmegax8 MCUs.  Accurate timing for millis() and delay() requires a freqquency that is a power of two such as 1Mz, 8Mhz, or 16Mhz.
 
-## Development plans
-Future versions will support 8Mhz.

src/wiring.c

@@ -9,9 +9,11 @@ extern void delay_impl();
 void delay(uint32_t msec)
 {
     register uint32_t ms asm ("r24") = msec;
-    asm volatile (
+    asm ( ".equ T0CNT_PER_MS, %0\n" 
         ".global T0CNT_PER_MS\n"
-        ".equ T0CNT_PER_MS, 250\n"
+        // T0 prescaler is 64
+        :: "i" (F_CPU / 1000 / 64) );
+    asm volatile (
         "rcall %x1\n" 
         : "+r"(ms)
         : "i"(delay_impl)