added simple example for mixing C & ASM (assembly version of blink_led)

sw/example/blink_led/blink_led_in_asm.S

@@ -0,0 +1,37 @@
+.file	"blink_led_in_asm.S"
+.section .text
+.balign 4
+.global blink_led_asm
+
+blink_led_asm:
+
+  /* base address of GPIO controller's output port is passed as argument (in a0)*/
+  sw   zero, 0(a0) /* clear output port */
+
+  li   t1, 0 /* initialize counter */
+
+
+blink_loop:
+  andi t1, t1, 255 /* apply 8-bit mask */
+  sw   t1, 0(a0)   /* output current counter value */
+  addi t1, t1, 1   /* increment counter */
+
+  jal  ra, blink_delay /* call delay function */
+
+  j    blink_loop
+
+
+blink_delay:
+  li   t2, 0xfffff /* delay time */
+
+blink_delay_loop:
+  beq  t2, zero, blink_delay_end
+  addi t2, t2, -1
+  nop
+  nop
+  j    blink_delay_loop
+
+blink_delay_end:
+  ret
+
+.end

sw/example/blink_led/main.c

@@ -48,9 +48,17 @@
 /**@{*/
 /** UART BAUD rate */
 #define BAUD_RATE 19200
+/** Use the custom ASM version for blinking the LEDs if != 0 */
+#define USE_ASM_VERSION 0
 /**@}*/
 
 
+/**********************************************************************//**
+ * ASM function to blink LEDs (if enabled)
+ **************************************************************************/
+extern void blink_led_asm(uint32_t gpio_out_addr);
+
+
 /**********************************************************************//**
  * Main function; shows an incrementing 8-bit counter on GPIO.output(7:0).
  *
@@ -76,6 +84,10 @@ int main() {
   // say hello
   neorv32_uart_print("Blinking LED demo program\n");
 
+
+// use C version of LED blinking
+#if (USE_ASM_VERSION == 0)
+
   neorv32_gpio_port_set(0); // clear gpio output put
 
   int cnt = 0;
@@ -85,5 +97,11 @@ int main() {
     neorv32_cpu_delay_ms(200); // wait 200ms using busy wait
   }
 
+// use ASM version of LED blinking (file: blink_led_in_asm.S)
+#else
+
+  blink_led_asm((uint32_t)(&GPIO_OUTPUT));
+
+#endif
   return 0;
 }