chore: rewrite to modbus rtu

systick.c

@@ -0,0 +1,50 @@
+#include "systick.h"
+
+#include "ch32v003fun.h"
+
+// Global variable definition
+volatile uint32_t systick_millis;
+
+/*
+ * Initialises the SysTick to trigger an IRQ with auto-reload, using HCLK/1 as
+ * its clock source
+ */
+void systick_init(void) {
+  // Reset any pre-existing configuration
+  SysTick->CTLR = 0x0000;
+
+  // Set the compare register to trigger once per millisecond
+  SysTick->CMP = SYSTICK_ONE_MILLISECOND - 1;
+
+  // Reset the Count Register, and the global millis counter to 0
+  SysTick->CNT = 0x00000000;
+  systick_millis = 0x00000000;
+
+  // Set the SysTick Configuration
+  // NOTE: By not setting SYSTICK_CTLR_STRE, we maintain compatibility with
+  // busywait delay funtions used by ch32v003_fun.
+  SysTick->CTLR |= SYSTICK_CTLR_STE |   // Enable Counter
+                   SYSTICK_CTLR_STIE |  // Enable Interrupts
+                   SYSTICK_CTLR_STCLK;  // Set Clock Source to HCLK/1
+
+  // Enable the SysTick IRQ
+  NVIC_EnableIRQ(SysTicK_IRQn);
+}
+
+/*
+ * SysTick ISR - must be lightweight to prevent the CPU from bogging down.
+ * Increments Compare Register and systick_millis when triggered (every 1ms)
+ */
+void SysTick_Handler(void) {
+  // Increment the Compare Register for the next trigger
+  // If more than this number of ticks elapse before the trigger is reset,
+  // you may miss your next interrupt trigger
+  // (Make sure the IQR is lightweight and CMP value is reasonable)
+  SysTick->CMP += SYSTICK_ONE_MILLISECOND;
+
+  // Clear the trigger state for the next IRQ
+  SysTick->SR = 0x00000000;
+
+  // Increment the milliseconds count
+  systick_millis++;
+}