The maximum clock frequency of the STM32F103C8T6 is 72 MHz, but it can be overclocked to 90 MHz with some limitations.

The clock settings can be configured using the Clock Control Register (RCC_CR) and the Flash Control Register (FLASH_ACR). You need to enable the HSE (High-Speed External) clock, configure the PLL (Phase-Locked Loop) and set the clock prescalers.

The BOOT0 and BOOT1 pins are used to select the boot mode of the microcontroller. BOOT0 is used to select between the internal flash and the system memory, while BOOT1 is used to select between the internal flash and the external memory.

To use the USART peripheral, you need to enable the USART clock, configure the USART registers (USART_CR1, USART_CR2, etc.), set the baud rate, and enable the transmitter and receiver.

The NVIC is used to handle interrupts generated by the peripherals. It allows you to prioritize interrupts, enable and disable interrupts, and handle interrupt requests.