Improve arduino due post to make it more journal-like.

1 files changed, 65 insertions, 10 deletions

diff --git a/_log/arduino-due.md b/_log/arduino-due.md
index 8176b3b..1881547 100644
--- a/_log/arduino-due.md
+++ b/_log/arduino-due.md
@@ -4,19 +4,22 @@ date: 2024-09-16
 layout: post
 ---
 
-Bypassing ATSAM3X8E (Due) bootloader via Serial Wire Debug (SWD). 
+Bought an Arduino Due to play with ARM Cortex chips. Locked out of the chip for
+days—no dev tools (Arduino IDE, Microchip Studio) for OpenBSD. Need to find a
+way to bring up the bare-metal ATSAM3X8E.
 
-Toolchain: ST-LINK/V2 programmer, OpenOCD, ARM GNU Compiler Toolchain.
-
-ARM chips boot into 0x00000. GPNVM bits map one of ROM, flash0, flash1 to
-0x00000: 
+Three on-chip memories: ROM, flash0, flash1. ARM Cortex-M chips boot into
+0x00000. GPNVM bits map one of them to 0x00000: 
 
  - GPNVM1=0 → ROM (default).
  - GPNVM1=1 and GPNVM2=0 → flash0.
  - GPNVM1=1 and GPNVM2=1 → flash1.
 
-By default, control jumps to Atmel's SAM-BA bootloader in ROM. To bypass, set
-GPNVM1=1 and place vector table at 0x80000 (flash0).
+On power-up, control jumps to ROM. Without a user program, Atmel's SAM-BA
+bootloader traps execution in ROM. Must remap memory via SWD to force chip to
+boot from flash, bypassing factory bootloader.
+
+Toolchain: ST-LINK/V2 programmer, OpenOCD, ARM GNU Compiler Toolchain.
 
 Connect ST-LINK/v2 to Arduino Due's DEBUG port:
 
@@ -33,7 +36,7 @@ Connect ST-LINK/v2 to Arduino Due's DEBUG port:
   </tr>
 </table>
 
-Remap memory:
+Remap memory using OpenOCD:
 
 ```
 $ openocd -f openocd-due.cfg
@@ -44,9 +47,59 @@ $ telnet localhost 4444
   > at91sam3 gpnvm show
 ```
 
-Full command list is in OpenOCD manual AT91SAM3 (flash driver section).
+Full command list in OpenOCD manual AT91SAM3 (flash driver section).
 
-Compile and upload program:
+At a minimum, vector table must be initialized with stack pointer and reset
+vector:
+
+```
+__attribute__((noreturn)) void _reset(void) 
+{
+    unsigned long *dst, *src;
+    extern unsigned long _sbss, _ebss;
+    extern unsigned long _sdata, _edata, _sidata;
+
+    for (dst = &_sbss; dst < &_ebss; dst++)
+        *dst = 0;
+    for (dst = &_sdata, src = &_sidata; dst < &_edata;)
+        *dst++ = *src++;
+
+    main();
+}
+
+extern const unsigned int _sp;
+
+__attribute__ ((section(".vtor"))) const void* _tab[] = { &_sp, _reset };
+```  
+
+Linker script places vector table at start of flash0 and initializes stack
+pointer to top of RAM (descending stack):
+
+```
+MEMORY
+{
+    rom (rx)  : ORIGIN = 0x00080000, LENGTH = 512K
+    ram (rwx) : ORIGIN = 0x20000000, LENGTH = 96K
+}
+
+SECTIONS
+{
+    .text :
+    {
+        KEEP(*(.vtor))
+        *(.text*)
+        *(.rodata*)
+    } > rom
+}
+
+_sp = ORIGIN(ram) + LENGTH(ram);
+```
+
+Getting linker script right wasn't easy—long hours with the datasheet, OpenOCD
+manual. David Barrass from OpenBSD and folks at OpenOCD mailing lists (bless
+them!) helped generously.
+
+Build and upload program:
 
 ```
 $ arm-none-eabi-gcc -mcpu=cortex-m3 -mthumb -T script.ld \
@@ -56,6 +109,8 @@ $ arm-none-eabi-gcc -mcpu=cortex-m3 -mthumb -T script.ld \
 $ openocd -f openocd-due.cfg -c "program a.elf verify reset exit"
 ```
 
+Chip unlocked. I'm in.
+
 Commit: <a
 href="https://git.asciimx.com/bare-metal-arduino-due/commit/?id=318496925ca76668dd9d63c3d060376f489276f8"
 class="external" target="_blank" rel="noopener noreferrer">3184969</a>.