USARTEx Exported Constants

Modules
	USARTEx Word Length
	USARTEx Slave Select Management
	USARTEx Synchronous Slave mode enable
	USART SLAVE mode.
	USARTEx FIFO mode
	USART FIFO mode.
	USARTEx TXFIFO threshold level
	USART TXFIFO level.
	USARTEx RXFIFO threshold level
	USART RXFIFO level.