GD32F1 Core. Added enhancements from @victor_pv to disable USB prescaler and re-enable so that the USB prescaler can be changed by the core during startup

Author: rogerclarkmelbourne

GD32F1/cores/maple/libmaple/rcc_f1.c

@@ -162,3 +162,12 @@ void rcc_set_prescaler(rcc_prescaler prescaler, uint32 divider) {
     };
     rcc_do_set_prescaler(masks, prescaler, divider);
 }
+
+void rcc_clk_disable(rcc_clk_id id) {
+    static __io uint32* enable_regs[] = {
+        [APB1] = &RCC_BASE->APB1ENR,
+        [APB2] = &RCC_BASE->APB2ENR,
+        [AHB] = &RCC_BASE->AHBENR,
+    };
+    rcc_do_clk_disable(enable_regs, id);
+}

GD32F1/system/libmaple/include/libmaple/rcc.h

@@ -169,6 +169,12 @@ static inline void rcc_disable_css() {
     RCC_BASE->CR &= ~RCC_CR_CSSON;
 }
 
+/**
+ * @brief Turn off the clock line on a peripheral
+ * @param id Clock ID of the peripheral to turn on.
+ */
+extern void rcc_clk_disable(rcc_clk_id id);
+
 #ifdef __cplusplus
 } // extern "C"
 #endif

GD32F1/system/libmaple/rcc_private.h

@@ -64,4 +64,11 @@ static inline void rcc_do_set_prescaler(const uint32 *masks,
     RCC_BASE->CFGR = cfgr;
 }
 
+static inline void rcc_do_clk_disable(__io uint32** enable_regs,
+                                     rcc_clk_id id) {
+    __io uint32 *enable_reg = enable_regs[rcc_dev_clk(id)];
+    uint8 line_num = rcc_dev_table[id].line_num;
+    bb_peri_set_bit(enable_reg, line_num, 0);
+}
+
 #endif

GD32F1/variants/generic_gd32f103c/wirish/boards_setup.cpp

@@ -79,12 +79,15 @@ namespace wirish {
             rcc_set_prescaler(RCC_PRESCALER_AHB, RCC_AHB_SYSCLK_DIV_1);
             rcc_set_prescaler(RCC_PRESCALER_APB1, RCC_APB1_HCLK_DIV_2);
             rcc_set_prescaler(RCC_PRESCALER_APB2, RCC_APB2_HCLK_DIV_1);
+			rcc_clk_disable(RCC_USB);
 			#if F_CPU == 120000000
 			rcc_set_prescaler(RCC_PRESCALER_USB, RCC_USB_SYSCLK_DIV_2_5);
 			#elif F_CPU == 96000000
 			rcc_set_prescaler(RCC_PRESCALER_USB, RCC_USB_SYSCLK_DIV_2);
 			#elif F_CPU == 72000000
 			rcc_set_prescaler(RCC_PRESCALER_USB, RCC_USB_SYSCLK_DIV_1_5);
+			#elif F_CPU == 48000000
+			rcc_set_prescaler(RCC_PRESCALER_USB, RCC_USB_SYSCLK_DIV_1_5);			
 			#endif
         }