THS789 Datasheet, PDF (16/34 Pages) Texas Instruments – THS789 Quad-Channel Time Measurement Unit (TMU)

THS789
SLOS776A – SEPTEMBER 2012 – REVISED DECEMBER 2015
Programming (continued)
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NOTE
The THS789 device was intended for sync-event, event, event, sync-event ... processing.
However, some applications desire the use of a sync pulse that is a fixed period. During a
sync period, there could be multiple events, or no events. The TMU can be used
effectively for this scenario as well.
For applications using the THS789 device in this fashion, it is important to consider the
uncertainty that is introduced by the load pulse timing. Because the load pulse is free
running and asynchronous to any events, the latency will vary based on this timing.
Additionally, the load pulse is the mechanism that will cause the ALU to grab the current
sync value for the result calculation.
If an event is in the FIFO, waiting for the load pulse and a new sync occurs, the ALU will
use the new sync value for calculating the result. In this case, the event would precede the
sync resulting in a negative result. The system could then offset the result by one sync
cycle as the result is negative, indicating that is was captured during a prior sync cycle.
7.5.2.5 TMU Calibration
The TMU calibration process is identical to a normal TMU time-stamp measurement. The process involves
measuring a known interval and calculating the difference between the measured value and the actual value.
The result is then stored into calibration registers inside the TMU. The TMU takes the stored calibration values
and corrects the subsequent time-stamp measurements.
There are four calibration registers for each channel. These are identified as follows:
• A calibration register for positive sync edge and positive event edge
• A calibration register for positive sync edge and negative event edge
• A calibration register for negative sync edge and positive event edge
• A calibration register for negative sync edge and negative event edge
Calibration due to temperature changes following the initial system calibration may be required if temperature
variations are significant.
7.5.2.6 Temperature Sensor
A temperature sensor has been located centrally in the THS789 device for monitoring the die temperature. There
are two monitor outputs for this feature. An analog voltage proportional to the die temperature is presented at the
TEMP pin. Also, an overtemperature alarm output is available at the OT_ALARM pin. The overtemperature alarm
(OT_ALARM) is an open-drain output that is activated when the die temperature reaches 141°C.
The overtemperature alarm sets a register bit (OT_ALM) in the central register and may be accessed through the
serial interface.
The overtemperature alarm initiates an automatic power down to prevent overheating of the device. The digital
blocks remain functional when in automatic power down. Following a power down, the user is required to reset
OT_ALM using the serial interface. A register bit (RST_OT_ALM) is used for this purpose.
The temperature-monitoring function and its associated overtemperarture alarm circuit may be disabled by the
user, using a register bit (OT_EN). The default for the temperature-monitoring function is disabled.
OT_EN = 1: Temperature-monitoring function is enabled.
OT_EN = 0: Temperature-monitoring function is disabled.
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