TDC7200_15 Datasheet, PDF (14/50 Pages) Texas Instruments – TDC7200 Time-to-Digital Converter for Time-of-Flight Applications in LIDAR,Magnetostrictive and Flow Meters

TDC7200
SNAS647C – FEBRUARY 2015 – REVISED AUGUST 2015
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Feature Description (continued)
8.3.3.3 Clock Counter STOP Mask
The value in the Clock Counter STOP Mask registers define the end of the mask window. The Clock Counter
STOP Mask value will be referred to as CLOCK_CNTR_STOP_MASK = (CLOCK_CNTR_STOP_MASK_H x 2⁸
+ CLOCK_CNTR_STOP_MASK_L).
The Clock Counter is started by the first rising edge of the external CLOCK after the START signal (see
Figure 18). All STOP signals occurring before the value set by the CLOCK_CNTR_STOP_MASK registers will be
ignored. This feature can be used to help suppress wrong or unwanted STOP trigger signals.
For example, assume the following values:
• The first time-of-flight (TOF1), which is defined as the time measurement from the START to the 1^st STOP =
19 μs.
• The second time-of-flight (TOF2), which is defined as the time measurement from the START to the 2^nd
STOP = 119 μs.
• CLOCK = 8 MHz
In this example, the TDC7200 will provide a CLOCK_COUNT1 of approximately 152 (19 μs / t_CLOCK), and
CLOCK_COUNT2 of approximately 952 (119 μs / t_CLOCK). If the user sets CLOCK_CNTR_STOP_MASK
anywhere between 152 and 952, then the 1^st STOP will be ignored and 2^nd STOP will be measured.
The Clock Counter Overflow value (CLOCK_CNTR_OVF_H x 2⁸ + CLOCK_CNTR_OVF_L) should always be
higher than the Clock Counter STOP Mask value ( x 2⁸ + CLOCK_CNTR_STOP_MASK_L). Otherwise, the Clock
Counter Overflow Interrupt will be set before the STOP mask time expires, and the measurement will be halted.
8.3.3.4 ENABLE
The ENABLE pin is used as a reset to all digital circuits in the TDC7200. Therefore, it is essential that the
ENABLE pin sees a positive edge after the device has powered up. It is also important to ensure that there are
no transients (glitches, etc.) on the ENABLE pin; such glitches could cause the device to RESET.
8.4 Device Functional Modes
8.4.1 Calibration
The time measurements performed by the TDC7200 are based on an internal time base which is represented as
the LSB value of the TIME1 to TIME6 results registers. The typical LSB value can be seen in Electrical
Characteristics. However, the actual value of the LSB can vary depending on environmental variables
(temperature, systematic noise, etc.). This variation can introduce significant error into the measurement result.
There is also an offset error in the measurement due to certain internal delays in the device.
In order to compensate for these errors and to calculate the actual LSB value, calibration needs to be performed.
The TDC7200 calibration consists of two measurement cycles of the external CLOCK. The first is a
measurement of a single clock cycle period of the external clock; the second measurement is for the number of
external CLOCK periods set by the CALIBRATION2_PERDIOS in the CONFIG2 register. The results from the
calibration measurements are stored in the CALIBRATION1 and CALIBRATION2 registers.
The two-point calibration is used to determine the actual LSB in real time in order to convert the TIME1 to TIME6
results from number of delays to a real time-of-flight (TOF) number. As discussed in the next sections, the
calibrations will be used for calculating time-of-flight (TOF) in measurement modes 1 and 2.
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