UCR4_TCEN
if ((ucr4 & UCR4_TCEN) == 0)
ucr4 &= ~UCR4_TCEN;
if ((!(usr2 & USR2_TXDC)) && (ucr4 & UCR4_TCEN)) {
ucr4 &= ~UCR4_TCEN;
ucr4 |= UCR4_TCEN;
ucr4 &= ~UCR4_TCEN;
ucr4 |= UCR4_TCEN;