TDC-GP2 acam messelectronic gmbh, TDC-GP2 Datasheet - Page 36

• Manufacturer Part Number: TDC-GP2
• Description: Precision Time Interval Measurement
• Manufacturer: acam messelectronic gmbh

TDC-GP2
7.1 Quartz Osci llator Start Up ti m e
7 Bug Report
7.1 Quartz Oscillator Start Up time
Bug:
When using a quartz resonator for the oscillator the oscillation start up time is about 3 ms even with an
optimized circuit. The TDC-GP2's automatic on-time control for the high speed clock is based on 640 µs or
1280 µs delay without any activity. In case of a quartz resonator this delay is too short and will cause
malfunction.
Corrective:
There are several options to deal with.
1. Oscillator permanently on
The possibility of switching off the high speed oscillator is added only for current saving. This oscillator needs
about 270 µA when running continuously. In all application without attention to current consumption we
recommend to run the high speed oscillator continuously (START_CLKHS=1).
2. Ceramic Resonator
With a ceramic resonator the oscillation start up time is about 200 µs. There is no problem with TDC-GP2's
internal delays. We recommend the use of ceramic resonators in all applications targeting ratio
measurements (like ultrasonic flow metering).
In all applications that need a quartz resonator and the switch-off functionality we recommend the following
measures:
3. Lower Clock Frequency at CLK32In
A clock with lower frequency increases the internal timer delays. In this case we recommend about 4 kHz. The
timer delay is increased to 10 ms then. This is enough for a save oscillation start up time of the quartz.
4. Start/Stop Oscillator by Software
Writing a "1" or "0" into Start_CLKHS starts or stops the oscillator by software. This way a µP can start the
oscillator by command and then after 10 ms can send a measurement opcode like START_CYCLE. At the end
of the measurement the µP has to switch off the oscillator by a further command.
7.2 DIS_PHASENOISE
The TDC-GP2 offers a Phase-Noise function that decouples the calibration clock from the fire pulse generator.
This option was implemented to provide the necessary statistics for averaging in case the user is looking for a
resolution much better than 1 LSB (about 65 ps) and has a a very strong coupling between the start and the
reference clock.
Bug:
Because of a design bug the phase noise unit might fail in case of distortions on the power supply. This is
relevant only in measurement range 2.
Corrective:
The phase noise unit has to be switched off by setting DIS_PHASE_NOISE = 1. In case the start is already
asynchronous to the referenceclock this has no effect on the capability of averaging.
acam-messelectronic gmbh - Am Hasenbiel 27 - D-76297 Stutensee-Blankenloch - Germany - www.acam.de
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