SystemClockHSE_Config() function

Switch the PLL source from HSI to HSE, and select the PLL as SYSCLK source. The system Clock is configured as follow : System Clock source = PLL (HSE) SYSCLK(Hz) = 84000000 HCLK(Hz) = 84000000 AHB Prescaler = 1 APB1 Prescaler = 2 APB2 Prescaler = 1 HSE Frequency(Hz) = 8000000 PLL_M = 8 PLL_N = 336 PLL_P = 4 PLL_Q = 7 VDD(V) = 3.3 Main regulator output voltage = Scale2 mode Flash Latency(WS) = 2