Fix green LEDs in hello world demo

Since the display is now connected to the bottom 4 bits of the GPIO, I
needed to change the GPIO header to increase the mask to 8 bits.
Afterwards the hello world demo is changed to shift through the top 4
bits of the GPIO. Button 0 is still used as a way to pause the shifting.

sw/c/common/gpio.h

@@ -12,7 +12,8 @@
 #define GPIO_IN_DBNC_REG 0x8
 #define GPIO_OUT_SHIFT_REG 0xC
 
-#define GPIO_OUT_MASK 0xF  // Support 4-bit output
+#define GPIO_OUT_MASK 0xFF  // Support 8-bit output
+#define GPIO_LED_MASK 0xF0  // Top 4 bits are green LEDs
 
 typedef void* gpio_t;
 

sw/c/demo/hello_world/main.c

@@ -33,8 +33,8 @@ int main(void) {
 
   uint64_t last_elapsed_time = get_elapsed_time();
 
-  // Reset green LEDs to off
-  set_outputs(GPIO_OUT, 0x0);
+  // Reset green LEDs to having just one on
+  set_outputs(GPIO_OUT, 0x10); // Bottom 4 bits are LCD control as you can see in top_artya7.sv
 
   // PWM variables
   uint32_t counter    = UINT8_MAX;
@@ -64,12 +64,17 @@ int main(void) {
       // Re-enable interrupts with output complete
       set_global_interrupt_enable(1);
 
-      // Cycling through green LEDs when BTN0 is pressed
+      // Cycling through green LEDs
       if (USE_GPIO_SHIFT_REG) {
+        // Feed value of BTN0 into the shift register
         set_outputs(GPIO_OUT_SHIFT, in_val);
       } else {
+        // Cycle through LEDs unless BTN0 is pressed
         uint32_t out_val = read_gpio(GPIO_OUT);
-        out_val          = ((out_val << 1) & GPIO_OUT_MASK) | (in_val & 0x1);
+        out_val = (out_val << 1) & GPIO_LED_MASK;
+        if ((in_val & 0x1) || (out_val == 0)) {
+          out_val = 0x10;
+        }
         set_outputs(GPIO_OUT, out_val);
       }