移位操作的疑问

2万 · 2014-4-27 22:04

Code of TMS320F28335 CCS4\v120\DSP2833x_examples\eqep_pos_speed\Example_posspeed.c文件中有这么一段是计算机械角度的：
pos16bval=(unsigned int)EQep1Regs.QPOSCNT;    // capture position once per QA/QB period
   p->theta_raw = pos16bval+ p->cal_angle;       // raw theta = current pos. + ang. offset from QA

// The following lines calculate p->theta_mech ~= QPOSCNT/mech_scaler [current cnt/(total cnt in 1 rev.)]
// where mech_scaler = 4000 cnts/revolution
1    tmp = (long)((long)p->theta_raw*(long)p->mech_scaler); // Q0*Q26 = Q26
2    tmp &= 0x03FFF000;
3    p->theta_mech = (int)(tmp>>11);          // Q26 -> Q15
4    p->theta_mech &= 0x7FFF;

1~4为什么要这样进行位操作，请解释一下，多谢！