TDC-GP2 acam messelectronic gmbh, TDC-GP2 Datasheet - Page 26

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

5. Details and Special Functions
5.1 Oscillator
The TDC-GP2 uses up to 2 clock signals depending on
the operating mode:
5.1.1 High-Speed Oscillator
Generally the TDC-GP2 needs a 2 to 8 MHz high-speed
clock for calibration. Operating in measurement range
2 the TDC-GP2 needs the high-speed clock signal also
as a part of the time measuring unit.
The oscillator takes an average current of 260 µA when
running all the time.
Because it is needed only during the time measurement
the TDC-GP2 has the capability to control the on-time
by itself. The high-speed clock can be switched on
automatically with a INIT opcode only for the period of
time measurement. An additional delay cares for the
settling time of the oscillator. The settings are done in
register 0, Bits 10 & 11, START_CLKHS:
START_CLKHS
The programmable delay guarantees that the oscillator
has settled before the measurement starts. For
ceramic resonators 640 µs will be sufficient.
Note: Notice the Bug report in section 7 when
using Start_CLKHS with quartz oscillators.
for the TDC measuring unit in measurement range
2
functions.
High-speed clock for calibration and as a predivider
32 kHz clock signal used for internal timer
acam-messelectronic gmbh - Am Hasenbiel 27 - D-76297 Stutensee-Blankenloch - Germany - www.acam.de
15pF
= 0
= 1
= 2
= 3
Figure 13
560K
Oscillator off
Oscillator on
with 640 µs delay.
delay
The measurement is started
same as ‘2’, but with 1280 µs
15pF
4 MHz
26
By this measure the average current consumption can
be drastically reduced.
Example:
At one ToF measurement in an ultrasonic flowmeter
(forth/back) per second the high-speed oscillator is
active only for about 2 ms. The average current
consumption is 260 µAs * 2 ms = 0.52 µA.
5.1.2 32.768 kHz Oscillator
The TDC-GP2 needs a 32.768 kHz reference for the
start-up control of the high-speed clock and the clock
calibration. It therefore offers a an integrated driver:
There is no need for the 32.768 kHz clock if the high
speed clock is permanently running (START_CLKSHS =
1) and the high speed clock calibration is not used (e.g.
in case of a quartz oscillator).
The 32.768 kHz oscillator is permanently running and
has a current consumption of about 4.5 µA at 3.6 V. It
is also possible to provide an external low-frequency
rectangular clock at the CLK32Out pin (3.6 V max.).
This signal could be generated by an external
microprocessor. It will reduce the current consumption
down to 1.2 µA. For this reason the 32 kHz oscillator
should only be used if no external low-current 32.768
kHz clock is available. The settling time of this oscillator
is about 2 s.
The external circuit is necessary only if the 32.768 kHz
oscillator is used. Otherwise CLK32In has to be
connected to GND.
5.1.3 Calibrating a Ceramic High-
speed Oscillator
Using a ceramic oscillator for the 2 to 8 MHz clock will
be attractive because it is of low cost and has a fast
settling time. Unfortunately it has a poor tolerance of
0.3 to 0.5 % and shows a temperature drift. For this
32,768 kHz
15pF
Figure 14
10M
TDC-GP2
15pF
5.1 Oscillator