RTC: Program fuse bits from C source.

firmware/rtc/MacRTC.c

@@ -34,6 +34,19 @@
 #include <avr/interrupt.h>
 #include <avr/sleep.h>
 
+/* Fuse bit programming.  */
+#if defined(__AVR_ATtiny25__) || defined(__AVR_ATtiny45__) || \
+  defined(__AVR_ATtiny85__)
+FUSES = {
+  // Use 8 MHz internal clock, not default 1 MHz internal clock.
+  .low = (LFUSE_DEFAULT | ~FUSE_CKDIV8),
+  // Disable the external RESET pin since it is used for the 1-second
+  // interrupt output.
+  .high = (HFUSE_DEFAULT & FUSE_RSTDISBL),
+  .extended = EFUSE_DEFAULT,
+};
+#endif
+
 /********************************************************************/
 // Simplified Arduino.h definitions.
 typedef enum { false, true } bool; // Compatibility with C++.

firmware/rtc/README.md

@@ -9,11 +9,19 @@ unfortunately cannot be used (efficiently/effectively) by the
 ATTiny85.
 
 The AVR core clock is run from an internal oscillator to generate 8
-MHz, so set the fuse bits accordingly when programming.  See the
-source code of MacRTC.c for more information on electrical
-specifications and the like.
+MHz.  The required fuse bytes setting is included in the generated ELF
+object file in the `.fuse` section, in the order (low, high,
+extended).  See the source code of MacRTC.c for more information on
+electrical specifications and the like.
 
 Reference source, Visited 2020-08-05:
 
 * https://www.reddit.com/r/VintageApple/comments/91e5cf/couldnt_find_a_replacement_for_the_rtcpram_chip/e2xqq60/
 * https://pastebin.com/baPZ4nN4
+
+## Device Programming
+
+Please note: For the ATTiny85 form factor, the external RESET pin must
+be disabled since it is used for the 1-second interrupt output.
+Therefore, after the initial programming, it will only be possible to
+reprogram via high-voltage serial programming.