PCLK
tconst = (PCLK + speed) / (2 * speed) - 2;
sm->sm_baudrate = PCLK / (2 * ((int)tconst + 2));
tconst = (PCLK + speed) / (2 * speed) - 2;
speed = PCLK / (2 * ((int)tconst + 2));
#define ZSPEED(n) ZSTimeConst(PCLK, n)