TDC-GP2 acam messelectronic gmbh, TDC-GP2 Datasheet - Page 27

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

reason the TDC-GP2 allows to execute a calibration
measurement that allows to compensate this behavior.
This measurement is based on the very precise 32.768
kHz clock. The TDC-GP2 generates start/stop pulses
from the 32.768 kHz and measures this time interval
with its TDC unit. The result is stored in the result reg-
ister and the interrupt flag is set. The frequency error
of the ceramic resonator can be calculated by the mi-
croprocessor. The calibration is configured by setting
register 0, CALRES# and is started with
"START_Cal_Resonator" - instruction by the microproc-
essor.
The time interval to be measured is set by CALRES#
which defines the number of periods of the 32.768kHz
clock:
CALRES#
The results is given in multiples of the high-speed clock
and (divided by 1, 2 or 4 (DIV_CLKHS)) as 32 bit fixed
point numbers with 16 integer bits and 16 fractional
bits.
The microcontroller can compare this measured value
to the theoretical value and calculate the correction
factor RES_X/RES
Example:
The system shall work with a 4 MHz resonator. With
CLKHSDIV=0 and CALRES#=1 the theoretical result is
122.0703125µs/250ns = 488.28125 (RES_0 =
0x01E84800). If the ceramic resonator in use is not
exactly at 4MHz but only 3.98MHz the calibration
measurement will show 485,83984375 (RES_0 =
1E5D700). The correction factor for the
microcontroller is 1.005.
Note:
During clock calibration the start input has to be
enabled.
5.1.4 How to use Clock Calibration
a. Application
This option is dedicated especially to ultrasonic flow
and heat meters. In those applications the use of ce-
ramic oscillators shows two main advantages: lower
cost and less current consumption. Mainly because of
the short oszillation start up time of the ceramic oscil-
Time
RES
_
acam-messelectronic gmbh - Am Hasenbiel 27 - D-76297 Stutensee-Blankenloch - Germany - www.acam.de
x
T
HSCLK
= 0
= 1
= 2
= 3
theor
2
.
DIV
2 periods = 61.035 µs
4 periods = 122.07 µs
8 periods = 244.14 µs
16 periods = 488.281µs
_
CLKHS
27
lator the operating current can be reduced by several
µA. Referring to 10 years of operation this saves sev-
eral 100 mAh in battery capacitance. There is no nega-
tive effect on the resolution when using this option the
correct way.
b. Jitter of the 32 kHz clock and consequences
The 32 kHz clock is very precise in frequency with only
a few ppm error. However the phase jitter is about 3 to
5 ns peak-peak. For this reason also a calibration
measurement (Start_Cal_Resonator) has this error.
When multiplying a measurement result with the
calibration result, the jitter of the calibration is
transferred to the result by the ratio calibration
measurement time (see CALRES#) to measurement
time. Using a permanently updated calibration value will
add a considerable jitter to the measurement result.
c. Application of this option in ultrasonic flow
meters
A measurement result is always made of two single
time-of-flight measurements in ultrasonic flow meters,
with and against the flow direction. The difference
between those measurements is a measure for the
flow. To avoid an influence of the calibration jitter on
this measurement result it is necessary only to use the
same calibration for both ToF measurements. Following
this the differenc between the two ToF measurements
will be free of the jitter of the clock calibration
measurement. The clock can be calibrated only
between measurements that are not directly
substracted from each other.
5.2 Fire-pulse Generator
5.2.1 General Description
The fire-pulse generator generates a sequence of
pulses which is highly programmable in frequency,
phase and number of pulses. The high-speed oscillator
frequency divided by the factor selected for ClkHSDiv is
used as the basic frequency. This frequency is internally
doubled and can freely be divided by a factor of 2 to 15.
It is possible to generate 1 to 15 pulses. For each
pulse the phase can be adjusted per register
configuration. The fire-pulse generator is activated by
sending opcode Start_Cycle.
The fire-pulse generator provides 2 outputs, Fire1 and
Fire2. The driver strength of each output is 48mA @5V.
These 2 outputs can be paralleled to increase the
driver strength up to 96 mA. Furthermore Fire2 output
signal can be inverted to double the signal amplitude.
The outputs can be set individually high-Z.
5.2 F i re-pul se Generator
TDC-GP2