THS789 Datasheet, PDF (25/34 Pages) Texas Instruments – THS789 Quad-Channel Time Measurement Unit (TMU)

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THS789
SLOS776A – SEPTEMBER 2012 – REVISED DECEMBER 2015
Table 16. Trigger Polarity Programmability
REGISTER BITS
SYNC_TS_Pol
Pol_X
0
0
0
1
1
0
1
1
TRIGGER POLARITY FROM
SYNC TO EVENT
Pos to Pos
Pos to Neg
Neg to Pos
Neg to Neg
SYNC
SYNC_TS_Pol = 0
Pol_X = 1
SYNC
SYNC_TS_Pol = 1
Pol_X = 0
EVENT
EVENT
T1
T2
T3
T1
SYNC
SYNC_TS_Pol = 1
Pol_X = 1
EVENT
T1
T2
Figure 9. Time-Measurement Examples With Different Edge Polarities
T0431-01
8.2.2.2 Output Clock to Data/Strobe Phasing
The output of each channel is an Rdata and Rstrobe signal. The RCLK for all the channels is a common output.
Operating at 300 MHz, these signals must be handled carefully. Particularly important are the termination and
phase alignment of the signals at the receiving circuitry. Termination has been discussed previously. Phase
alignment is now discussed: The two outputs from each channel are clocked out through identical flip-flops with
the same internal clock. Data and strobe output edges from a particular channel match well (< 50 ps). The match
channel-to-channel is not as good due to the greater wiring distances internal to the TMU. However, the total
time difference is below 125 ps. Because the RClock is a common output, the wiring lengths from the four
channels must be matched and controlled to achieve good setup and hold times at the input to the receiving
circuit. The RClock rising edge is adjusted internal to the TMU to be close to the center of the eye diagram of the
data/strobe signals. (The internal clock has a good 50/50 duty cycle. The rising edge clocks out the data/strobe.
The falling edge is inverted and used as the RClock after appropriate adjustments for the internal propagation
delay times.) The receiving circuitry requirements for setup and hold timing must be carefully examined for the
proper timing. Delays may be added to the PCB microstrips to adjust timing. A good rule is 125 ps of delay per
inch of microstrip length.
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