Posted on February 21, 2014 at 17:20It is probably there to support 7bit data width for the RS232 standard.
Or strip parity, ST's definition of data bit includes the parity bit, the USART_DR can contain up to 9-bits. Masking covers a couple of ill's including stopping some compilers warning as you cast from a 16-bit unsigned int to an 8-bit signed/unsigned char.
There is generally much disagreement as to whether a char is signed or unsigned, many compilers/os have their own ideas, with defaults, or overrides.
