TDC7200: Delay factor for Time-of-Flight

Hi Nagata,

1)  It takes between 1-2 micro-seconds after the last STOP for the INTB interrupt to generate. This is the first available indicator that the measurement is complete, and should be used to determine measurement completion. An experiment using the host MCU could be run to approximate the latency between INTB and the register population. 

2) The ToF calculation example in the datasheet does not include any overhead time for setting INTB, calibration, or SPI communication. Please see datasheet section 8.4.2 for more details. 

Regards,

Gabriel

Nagata,

- There are two calibrations, one occurs just before the trigger pulse is sent (between START_MEAS and TRIGG). The second calibration occurs at the very end of the measurement, before INTB is driven low. 

- The time taken for calibration varies depending on the external clock frequency and the number of clock periods specified for calibration. Calibration 1 is always over one clock period, and calibration 2 can be done over 2, 10, 20, or 40 clock periods. Measure the time between START_MEAS and TRIGG to estimate the calibration time. For calibration 2, what we know from other customers is that it completes and INTB goes low in under 2 micro-seconds typically. 

- The time breakdown for a measurement can be taken from the measurement sequence description in the datasheet, steps 2 - 8: 

There will be a latency between setting the START_MEAS bit and when the START pulse is seen by the TDC7200 (Tpre-meas), and there is also latency time between STOP and INTB (Tpos-meas). The time between START and STOP would just be the ToF. 

Tpre-meas = Tspi + T2trigg + Ten + T2start

Tpos-meas = Tcal2 + T2intB

Tspi = SPI communication time, which can be estimated as you have shown or by measuring the time CSB is low. 

T2trigg = time between START_MEAS is set and the TRIGG pulse is asserted, including the time taken to complete calibration 1

Ten = time for the TDC7200 to enable the START input pin. 

T2start = time between TRIGG received by the AFE and the START is received by the TDC7200

Tcal2 = time for calibration 2 to complete

T2intB = time between cal2 completion and INTB going low, overhead time for measurement registers and status registers to be set, etc. 

As soon as INTB is low, the host controller is able to retrieve the data via SPI. The overall time depends on user selections, the host controller, and clock speeds. I recommend that customers take measurements with their own system when accurate timing information is needed, but if this option is not available, this information can be used to estimate measurement latencies. 

Regards,

Gabriel