Hi. I am a complete newbie, please don’t assume I know anything 
To start learning I have signed up for a course on EdX (Microcontroller Applications with RISC-V). Unfortunately the discussion board is no longer active so I am not able to post queries and receive response on the course board.
Using an FE310 (Sparkfun dev board) and FreedomStudio. The course describes the requirements and development of code for simple interrupt drive examples and I have unpacked these in detail and am reasonably comfortable with what “should” happen.
Control is via external switches (push buttons).The first of these turns an LED on (on board LED) using an external interrupt and the second turns it OFF by polling the GPIO. I have metered the voltages on the buttons and can confirm that they do pull the signal low (pull up resistors are enabled so they are normally high).
Running in debug mode the code never reaches the ISR. Any pointers as to what I am missing would be greatly appreciated.
Here is the code:
/******************************************************************************
Red-V Thing Plus Interrupt_Demo,
by Eduardo Corpeño
Using the built-in LED and two external buttons to control it.
One button is handled by polling, and the other by interrupts.
Date: June 20, 2022
Developed using Freedom Studio v4.18.0.2021-04-1 on Windows 10
LICENSE: This code is released under the MIT License
(http://opensource.org/licenses/MIT)
******************************************************************************/
#include <metal/gpio.h> //include GPIO library, https://sifive.github.io/freedom-metal-docs/apiref/gpio.html
#include <stdint.h>
#include <metal/csr.h> //Control and Status Registers (CSRs)
// Macro Functions
#define Red_V_enable_output(x) *((uint32_t *) 0x10012008) |= (1<<(x))
#define Red_V_set_pin(x) *((uint32_t *) 0x1001200C) |= (1<<(x))
#define Red_V_clear_pin(x) *((uint32_t *) 0x1001200C) &= ~(1<<(x))
#define Red_V_read_pin(x) (*((uint32_t *) 0x10012000) & (1<<(x)))
#define Red_V_enable_pullup(x) *((uint32_t *) 0x10012010) |= (1<<(x))
#define Red_V_GPIO_set_ie(x) *((uint32_t *) 0x10012020) |= (1<<(x))
#define Red_V_GPIO_clear_flag(x) *((uint32_t *) 0x10012024) |= (1<<(x))
#define Red_V_PLIC_GPIO_set_priority(pin,p) *((uint32_t *) (0x0C000020+4*(pin))) = (p)
#define Red_V_PLIC_clear_ie() *((uint32_t *) 0x0C002000) = 0;\
*((uint32_t *) 0x0C002004) = 0
#define Red_V_PLIC_set_ie1(x) *((uint32_t *) 0x0C002000) |= (1<<(x))
#define Red_V_PLIC_set_ie2(x) *((uint32_t *) 0x0C002004) |= (1<<(x))
#define Red_V_PLIC_claim *((uint32_t *) 0x0C200004)
void gpio_isr(void) __attribute__((interrupt, aligned(64)));
void gpio_isr(){
uint32_t plic_id;
Red_V_set_pin(5); // Turn LED ON
Red_V_GPIO_clear_flag(1); // Level 3: Clear GPIO0_1 flag
plic_id = Red_V_PLIC_claim; // Level 2: Claim GPIO interrupt
Red_V_PLIC_claim = plic_id; //This signals interrupt completion. See 10.8 of the manual.
}
int main (void){
struct metal_gpio *gpio_0; // Make instance of GPIO
gpio_0 = metal_gpio_get_device(0);
// Pins are set when initialized so we must disable it when we use it as an input/output
metal_gpio_disable_input(gpio_0, 5);
// Set as gpio as output
//metal_gpio_enable_output(gpio_0, 5);
Red_V_enable_output(5);
// Pins have more than one function, make sure we disconnect anything connected
metal_gpio_disable_pinmux(gpio_0, 5);
metal_gpio_enable_input(gpio_0, 0); // enable input 0
metal_gpio_enable_input(gpio_0, 1); // enable input 1
metal_gpio_disable_output(gpio_0, 0); // disable output 0
metal_gpio_disable_output(gpio_0, 1); // disable output 1
metal_gpio_disable_pinmux(gpio_0, 0); // disable alternate functions for pin 0
metal_gpio_disable_pinmux(gpio_0, 1); // disable alternate functions for pin 1
Red_V_enable_pullup(0); // enable pull-up for pin 0
Red_V_enable_pullup(1); // enable pull-up for pin 1
// Interrupt Configuration
// Level 3: GPIO0_1 Falling Edge Interrupt Enable
Red_V_GPIO_set_ie(1);
Red_V_GPIO_clear_flag(1);
// Level 2: PLIC Setting for GPIO0
Red_V_PLIC_GPIO_set_priority(1,7); // Pin 1, priority 7
// Level 2: PLIC (IE1, bit 9) for GPIO0_1
Red_V_PLIC_clear_ie(); // Disable all other interrupts
Red_V_PLIC_set_ie1(9); // Enable GPIO0_1
// Level 1: Enable interrupts with MIE in mstatus[3] Sec 8.3 of FE310-G002 manual
volatile uintptr_t saved_config;
METAL_CPU_GET_CSR(mstatus,saved_config);
saved_config |= (0x1U<<3);
METAL_CPU_SET_CSR(mstatus,saved_config);
// Level 1: Set base vector mtvec
METAL_CPU_SET_CSR(mtvec,&gpio_isr); //The & turns a value into a pointer
// Level 1: Enable interrupts with MEIE in mie[11] Sec 8.3 of FE310-G002 manual
METAL_CPU_GET_CSR(mie,saved_config);
saved_config |= (0x1U<<11);
METAL_CPU_SET_CSR(mie,saved_config);
while(1){
if(Red_V_read_pin(0) == 0) // Read input pin 0
Red_V_clear_pin(5); // Turn LED OFF
}
return 0; // Unreachable code
}
