finally working double-tap trigger code

linker.lds

@@ -1,8 +1,5 @@
 ENTRY( _start )
 
-__stack_size = 512;
-PROVIDE( _stack_size = __stack_size );
-
 MEMORY
 {
     /* Try to squeeze down to this target size */
@@ -17,6 +14,9 @@ MEMORY
 
 SECTIONS
 {
+    PROVIDE(_bootloader_limit = ORIGIN(FLASH) + LENGTH(FLASH));
+    PROVIDE(_bootloader_stack = ORIGIN(USBRAM) + LENGTH(USBRAM));
+
     .vector :
     {
         *(.vector);

startup.S

@@ -1,13 +1,98 @@
+#define R16_BKP_DATAR10                 0x40006C28
+#define BOOT_MAGIC_APP_IMMEDIATELY      0x4170
+#define BOOT_MAGIC_BOOTLOADER           0x624c
+
+#define R32_PWR_CTLR            0x40007000
+
+#define RCC_BASE                0x40021000
+#define R32_RCC_APB1PCENR       0x4002101C
+#define R32_RCC_RSTSCKR         0x40021024
+
+#define SYSTICK_BASE            0xE000F000
+#define STK_CTLR                0xE000F000
+#define STK_SR                  0xE000F004
+#define STK_CMPLR               0xE000F010
+#define WAIT_TIME   (500 /* ms */ * 1000 /* assume 8 MHz system clock, div8 */)
+
 .section .vector,"ax",@progbits
 .align 1
 
 .global _start
 _start:
+    // Use Adafruit(?)-inspired boot entry, namely "double-reset in short succession to enter bootloader"
+    // 1. If the reset wasn't triggered by the nRST pin, go to application
+    // 2. On nRST, if not first reset, go to bootloader
+    // 3. On nRST, if *is* first reset, set magic and wait
+    // 4a. If wait times out, go to application
+    // 4b. If wait doesn't time out but a reset happens again, detect "not first" reset and go to bootloader
+    la a0, RCC_BASE
+    lw a1, (R32_RCC_RSTSCKR-RCC_BASE)(a0)
+    slli a1, a1, (31 - 26)
+    bgez a1, _enter_application_code     // if PINRSTF == 0
+
+    // Enable BKP and PWR clock
+    lw a1, (R32_RCC_APB1PCENR-RCC_BASE)(a0)
+    la a2, (1 << 27) | (1 << 28)
+    or a1, a1, a2
+    sw a1, (R32_RCC_APB1PCENR-RCC_BASE)(a0)
+    // Enable BKP write
+    la a0, R32_PWR_CTLR
+    lw a1, (a0)
+    ori a1, a1, (1 << 8)
+    sw a1, (a0)
+
+    // Check for magic values
+    la a3, R16_BKP_DATAR10
+    lhu a1, (a3)
+    la a2, BOOT_MAGIC_APP_IMMEDIATELY
+    beq a1, a2, _enter_application_code
+    la a2, BOOT_MAGIC_BOOTLOADER
+    beq a1, a2, _enter_bootloader
+
+    // Here we're on the first reset
+    sh a2, (a3)
+    la a0, SYSTICK_BASE
+    la a1, WAIT_TIME
+    sw a1, (STK_CMPLR-SYSTICK_BASE)(a0)
+    la a1, 0b111000
+    sw a1, (STK_CTLR-SYSTICK_BASE)(a0)
+    // Seems to spuriously fire immediately if start bit is set concurrently
+    la a1, 0b111001
+    sw a1, (STK_CTLR-SYSTICK_BASE)(a0)
+1:
+    lw a1, (STK_SR-SYSTICK_BASE)(a0)
+    slli a1, a1, (31 - 0)
+    bgez a1, 1b
+    // Delay done, clear systick and magic
+    sw zero, (STK_CTLR-SYSTICK_BASE)(a0)
+    sw zero, (STK_SR-SYSTICK_BASE)(a0)
+    sh zero, (a3)
+    // If the delay expires, boot the application after all
+
+_enter_application_code:
+    // Clear all of these enables that we had set
+    la a0, R32_PWR_CTLR
+    lw a1, (a0)
+    xori a1, a1, (1 << 8)
+    sw a1, (a0)
+    la a0, R32_RCC_APB1PCENR
+    lw a1, (a0)
+    la a2, (1 << 27) | (1 << 28)
+    xor a1, a1, a2
+    sw a1, (a0)
+
+_wipwipwip:
+    j _wipwipwip
+
+    j _bootloader_limit
+
+_enter_bootloader:
     // Set up global pointer
 .option push
 .option norelax
     la gp, __global_pointer$
 .option pop
+    la sp, _bootloader_stack
 
     // Load data section from flash to RAM
     la a0, _data_lma