THS788 Datasheet, PDF (26/43 Pages) Texas Instruments – QUAD-CHANNEL TIME MEASUREMENT UNIT (TMU)

THS788
SLOS616B – MARCH 2010 – REVISED JUNE 2011
The worst case for data to be output from serial bus:
T_event + 5(R_clkcycles) + (R_datalength + 3) x R_clkcycles + (R_datalength + 3) x R_clkcycles
The best case for data to be output from serial bus:
T_event + 5(R_clkcycles) + (R_datalength + 3) x R_clkcycles
Where:
T_event = 5 ns (minimum repeat capture time)
5(R_clkcycles) = number cycles for FIFO to ALU to Shift register
R_clkcycles is period of R_CLK data; i.e.300 Mhz, SDR = 3.33 ns
R_datalength = number of results bits
In the case where R_CLK = 300 MHz, with 16-bit serial result:
Min Latency = 5 ns + 17 ns + (16 + 3) x 3.33 ns = 85 ns
Max Latency = 5 ns + 17 ns + (16 + 3) x 3.33 ns + (16 + 3) x 3.33 ns = 149 ns
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NOTE
The THS788 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 THS788 in this fashion, it is important to consider the
uncertainty that is introduced by the load pulse timing. Since 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.
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.
Temperature Sensor
A temperature sensor has been located centrally in the THS788 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.
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