Switch to 9600 baud for maximum RC oscillator tolerance

Co-authored-by: AlexDev404 <34188635+AlexDev404@users.noreply.github.com>

Author: Copilot

src/reference/quickstart-sim.md

@@ -7,7 +7,7 @@
 ```
 
 ```
-avrdude -CC:\Users\Immanuel\AppData\Local\Arduino15\packages\arduino\tools\avrdude\6.3.0-arduino17/etc/avrdude.conf -v -patmega328p -carduino -PCOM5 -b19200 -D -Uflash:w:C:\Data\zilog\XP_F082A_uART\src\reference\Test.BareMinimum.ino.hex:i
+avrdude -CC:\Users\Immanuel\AppData\Local\Arduino15\packages\arduino\tools\avrdude\6.3.0-arduino17/etc/avrdude.conf -v -patmega328p -carduino -PCOM5 -b9600 -D -Uflash:w:C:\Data\zilog\XP_F082A_uART\src\reference\Test.BareMinimum.ino.hex:i
 ```
 
 # To test (non-spoofed)
@@ -19,14 +19,15 @@ avrdude -CC:\Users\Immanuel\AppData\Local\Arduino15\packages\arduino\tools\avrdu
 ```
 
 ```
-avrdude -CC:\Data\zilog\XP_F082A_uART\src\avrdude.conf -v -pz8f081a_no_nvds -carduino -PCOM5 -b19200 -D -Uflash:w:C:\Data\zilog\XP_F082A_uART\src\reference\Test.BareMinimum.ino.hex:i
+avrdude -CC:\Data\zilog\XP_F082A_uART\src\avrdude.conf -v -pz8f081a_no_nvds -carduino -PCOM5 -b9600 -D -Uflash:w:C:\Data\zilog\XP_F082A_uART\src\reference\Test.BareMinimum.ino.hex:i
 ```
 
 # Notes
 
-- The bootloader uses **19200 baud** for better tolerance with the Z8F082A internal RC oscillator
-- Use `-b19200` with avrdude to match the bootloader baud rate
+- The bootloader uses **9600 baud** for maximum tolerance with the Z8F082A internal RC oscillator
+- Use `-b9600` with avrdude to match the bootloader baud rate
 - If you still get sync errors with random garbage (0xXX responses), verify:
   1. TX/RX are connected correctly (RX→TX, TX→RX)
   2. CTS is connected if required by your FTDI adapter
-  3. The chip's internal oscillator is configured (init_systemclock() is called)
+  3. The chip's internal oscillator is configured (init_systemclock() is called)
+  4. Power supply is stable (3.3V)

src/uart.c

@@ -8,31 +8,31 @@
  * UART Baud Rate Configuration for Z8F082A
  * 
  * The Z8F082A internal RC oscillator varies between parts (~5.5 MHz typical).
- * Using 19200 baud provides better tolerance for clock variation than 115200.
+ * Using 9600 baud provides the BEST tolerance for clock variation.
  * 
  * IMPORTANT: avrdude command must specify matching baud rate:
- *   avrdude -c stk500v1 -b 19200 -P /dev/ttyUSB0 ...
+ *   avrdude -c stk500v1 -b 9600 -P COM3 ...
  * 
- * Clock Frequency Options:
- *   5.5296 MHz (Internal RC): BRG = 18 for 19200 baud (exact)
- *   18.432 MHz (Crystal):     BRG = 60 for 19200 baud (exact)
- *   20.0 MHz (Crystal):       BRG = 65 for 19200 baud (0.16% error)
+ * Clock Frequency Options (BRG values for 9600 baud):
+ *   5.5296 MHz (Internal RC): BRG = 36 for 9600 baud (exact)
+ *   18.432 MHz (Crystal):     BRG = 120 for 9600 baud (exact)
+ *   20.0 MHz (Crystal):       BRG = 130 for 9600 baud (0.16% error)
  */
 
 /* System clock frequency - adjust to match your actual oscillator */
 #define SYSTEM_CLOCK_HZ     5529600UL   /* 5.5296 MHz internal oscillator */
 
-/* Baud rate - using 19200 for better tolerance with RC oscillator */
-#define BAUD_RATE           19200UL
+/* Baud rate - using 9600 for MAXIMUM tolerance with RC oscillator variation */
+#define BAUD_RATE           9600UL
 
 /* Calculate baud rate divisor: BRG = (freq + baud*8) / (baud * 16) */
 #define BRG_VALUE ((SYSTEM_CLOCK_HZ + BAUD_RATE * 8UL) / (BAUD_RATE * 16UL))
 
 void init_uart0(void)
 {
     /* Initialize UART0 for STK500 communication
-     * BRG = (5529600 + 19200*8) / (19200*16) = 5683200 / 307200 = 18
-     * Actual baud = 5529600 / (16 * 18) = 19200 (exact!)
+     * BRG = (5529600 + 9600*8) / (9600*16) = 5606400 / 153600 = 36
+     * Actual baud = 5529600 / (16 * 36) = 9600 (exact!)
      */
 
     /* Set baud rate generator */