stm: Make pendsv_nlr_jump work when debugging is enabled.

Stack layout is different when -g used, so need to handle the pendsv
jump differently.  Addresses Issue #315.

stm/Makefile

@@ -29,7 +29,7 @@ CFLAGS += -Iboards/$(BOARD)
 
 #Debugging/Optimization
 ifeq ($(DEBUG), 1)
-CFLAGS += -g
+CFLAGS += -g -DPENDSV_DEBUG
 COPT = -O0
 else
 COPT += -Os -DNDEBUG

stm/led.c

@@ -103,8 +103,8 @@ void led_toggle(pyb_led_t led) {
             return;
     }
 
-    // XXX this assumes LED is driven by a low MCU output (true for PYBv3, false for PYBv4)
-    if (!(port->ODR & pin)) {
+    // XXX this assumes LED is driven by a high MCU output (false for PYBv3, true for PYBv4)
+    if (port->ODR & pin) {
         // turn LED off
         PYB_LED_OFF(port, pin);
     } else {

stm/main.c

@@ -355,7 +355,7 @@ int main(void) {
     storage_init();
 
     // uncomment these 2 lines if you want REPL on USART_6 (or another usart) as well as on USB VCP
-    //pyb_usart_global_debug = PYB_USART_3;
+    //pyb_usart_global_debug = PYB_USART_YA;
     //usart_init(pyb_usart_global_debug, 115200);
 
     int first_soft_reset = true;

stm/pendsv.c

@@ -22,15 +22,51 @@ void pendsv_nlr_jump(mp_obj_t o) {
     SCB->ICSR = SCB_ICSR_PENDSVSET_Msk;
 }
 
+// since we play tricks with the stack, the compiler must not generate a
+// prelude for this function
+void pendsv_isr_handler(void) __attribute__((naked));
+
 void pendsv_isr_handler(void) {
     // re-jig the stack so that when we return from this interrupt handler
     // it returns instead to nlr_jump with argument pendsv_object
+    // note that stack has a different layout if DEBUG is enabled
+    //
+    // on entry to this (naked) function, stack has the following layout:
+    //
+    // stack layout with DEBUG disabled:
+    //   sp[6]: pc
+    //   sp[5]: ?
+    //   sp[4]: ?
+    //   sp[3]: ?
+    //   sp[2]: ?
+    //   sp[1]: ?
+    //   sp[0]: r0
+    //
+    // stack layout with DEBUG enabled:
+    //   sp[8]: pc
+    //   sp[7]: lr
+    //   sp[6]: ?
+    //   sp[5]: ?
+    //   sp[4]: ?
+    //   sp[3]: ?
+    //   sp[2]: r0
+    //   sp[1]: 0xfffffff9
+    //   sp[0]: ?
+
     __asm volatile (
         "ldr r0, pendsv_object_ptr\n"
         "ldr r0, [r0]\n"
+#if defined(PENDSV_DEBUG)
+        "str r0, [sp, #8]\n"
+#else
         "str r0, [sp, #0]\n"
+#endif
         "ldr r0, nlr_jump_ptr\n"
+#if defined(PENDSV_DEBUG)
+        "str r0, [sp, #32]\n"
+#else
         "str r0, [sp, #24]\n"
+#endif
         "bx lr\n"
         ".align 2\n"
         "pendsv_object_ptr: .word pendsv_object\n"
@@ -38,12 +74,10 @@ void pendsv_isr_handler(void) {
     );
 
     /*
-    sp[0] = r0 = exception to raise
-    sp[6] = pc = nlr_jump
     uint32_t x[2] = {0x424242, 0xdeaddead};
     printf("PendSV: %p\n", x);
     for (uint32_t *p = (uint32_t*)(((uint32_t)x - 15) & 0xfffffff0), i = 64; i > 0; p += 4, i -= 4) {
-        printf(" %p: %08x %08x %08x %08x\n", p, p[0], p[1], p[2], p[3]);
+        printf(" %p: %08x %08x %08x %08x\n", p, (uint)p[0], (uint)p[1], (uint)p[2], (uint)p[3]);
     }
     */
 }

stm/pyexec.c

@@ -245,6 +245,24 @@ void pyexec_repl(void) {
     stdout_tx_str("Micro Python build <git hash> on 25/1/2014; " MICROPY_HW_BOARD_NAME " with STM32F405RG\r\n");
     stdout_tx_str("Type \"help()\" for more information.\r\n");
 
+    // to test ctrl-C
+    /*
+    {
+        uint32_t x[4] = {0x424242, 0xdeaddead, 0x242424, 0xdeadbeef};
+        for (;;) {
+            nlr_buf_t nlr;
+            printf("pyexec_repl: %p\n", x);
+            usb_vcp_set_interrupt_char(VCP_CHAR_CTRL_C);
+            if (nlr_push(&nlr) == 0) {
+                for (;;) {
+                }
+            } else {
+                printf("break\n");
+            }
+        }
+    }
+    */
+
     vstr_t line;
     vstr_init(&line, 32);