sm_par clock time

While the parameter sm_par.f32_dwell_msec records the per-pixel dwell time that the user requested, the actual dwell time might be somewhat different. To calculate the actual dwell time used per pixel, one must look at two parameters: sm_par.i32_fast_device, and sm_par.f32_clock_hertz.

• In scans where sm_par.i32_fast_device is either SM_PAR_XPZT_DEVICE or SM_PAR_YPZT_DEVICE, the dwell time per pixel is rigidly fixed and we do not need to record the second scaler channel at every pixel, so there will be no entry named Clock in sm_par.pi8_pixel_channel_names. In this case, sm_par.f32_clock_hertz records the clock rate at which pixels are recorded, and the dwell time in seconds for each pixel is simply given by the inverse of sm_par.f32_clock_hertz.

• In all other scans (where the stage can accelerate and decelerate, thus altering the pixel dwell time), the parameter sm_par.f32_clock_hertz records the clock rate used (typically either 100 kHz or 20 MHz), while the channel named Clock in sm_par.pi8_pixel_channel_names stores the counts of this clock per pixel. In this case, the dwell time in seconds for a particular pixel is given by the ratio (pixel clock counts)/sm_par.f32_clock_hertz.