>>>Longan Nano GD32VF103<<<
Illustration 1: Longan Nano test bench
1Introduction
The blink example went up quite painlessly, and I was also able to compile and execute the Bad Apple example that takes a stream of bitmap images from the SD Card and shows them on the screen.
With the basics out of the way, I focused on the interrupts.
1.1Arduino Framework Compatibility
The product page claims compatibility with the Arduino Framework, so I wanted to try the Arduino like interrupt handling.
Early on it was obvious that the Arduino framework has not been completed as the time of writing, with basic HAL functions like the digitalRead() empty. There was no hope of getting Arduino style interrupts to work without doing the framework HAL myself, so I dropped the thing.
2Interrupts
The Risc-V ISA specifies the interrupt handling as part of the core. Details about the ECLIC can be found in this excellent post from Kevin Sangeelee.
The GD32VF103 support an interrupt vector table defined as weak symbols in Start.S in the GD32 framework, it supports tiered priority where high priority interrupts can interrupt low priority ones and has hardware acceleration for them inside the MCU, on top of the special instructions and registers inside the core, as per Risc-V ISA specifications.
The GD32VF103 provides special interrupts from all sort of sources, including a reset vector, an error vector and a watchdog vector, on top of interrupts from almost every peripheral.
2.1Difficulties
Getting the interrupts to work was not easy.
In theory, all I had to do was to define a function with the same name as the symbol in start.S, and the C++ linker should have automagically linked the definitions by overriding the default .weak symbols.
After lots of testing, stack overflow came through with the answer: I need to use the keyword extern “C” to tell the C++ compiler not to change the name of the function and allow the linker to do its job and link the address of the interrupt service routine to the interrupt vector table.
3Example: PA8 EXTI
The Boot button is also wired to the PA8 pin. The EXTI in combination with the ECLIC is used to sense an edge on PA8 and trigger an ISR.
4Example: RTC Interrupt
The RTC timer is initialized to emit a “second” interrupt every 250ms. The interrupt toggles the RED LED.
5Conclusions
Interrupts are fundamental in an hard real time MCU application and the GD32VF103 is equipped with a refined system to handle interrupts.
This document shows how to generate interrupts from the EXTI and RTC. interrupts can be generated from all sort of sources, and there are even sources for reset, watchdog, errors, etc...
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