Application Notes

NXP Semiconductors
Application Note
Document Number: AN5253
Rev. 1.0, 3/2016
VRS automatic mode setting
Featuring the MC33813/33814 IC
1
Introduction
This application note examines fundamentals of variable reluctance
sensor (VRS) systems, as well as how to correctly determine the
VRS automatic mode parameters through a single bench
measurement.
The MC33813 and 33814 are engine control analog power ICs
intended for one and two cylinder motorcycle and other small
engine control applications. The MC33813 and 33814 offers an
innovating VRS automatic mode.
NXP analog ICs are manufactured using the SMARTMOS process,
a combinational BiCMOS manufacturing flow, integrating precision
analog, power functions, and dense CMOS logic together on a
single cost-effective die.
2
Description of the VRS
system
The MC33813 and 33814 contain a VRS comparator circuit with
multiple thresholds, programmed via the SPI, allowing the system
to handle different sensors and the wide dynamic range of the VRS
outputs at engine speeds from crank to running. The output of this
circuit is provided on the VRSOUT pin to the MCU.
As specified in the datasheet, two sets of parameters (Input
comparator threshold and Blanking time) could be specified for
engine cranking and engine running conditions. Consider
"thresholds" can be manually set with SPI writes using values from
Table 1.
In typical operation, a low value threshold ~100 mV is used for
cranking while a higher threshold is used when the engine is
running. Alternatively, an innovative Automatic mode can be used
to improve noise immunity at low engine speeds and cranking
without compromising performance at normal engine speeds. The
following sections explain some theory of operation and how to
correctly set the Automatic Mode.
© 2016 NXP B.V.
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
2 Description of the VRS system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
2.1 Theory points. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
2.2 Automatic mode circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
3 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Description of the VRS system
2.1
Theory points
A single tooth system is used to illustrate that only a single measurement is necessary to define the key system characteristic Volt-Time
Constant (VTC).
Figure 1. Single tooth system
2.1.1
Predictive curve
The VRS sensor operates by generating a voltage (e.m.f.), based on the change in magnetic flux through the sensor.
For a single tooth running at a velocity # 1, the generated signal has an amplitude V1, and a period t1.
For velocity #2, which is double the velocity #1, the generated signal has a amplitude V2 and a period t2.
As illustrated in Figure 2, t2 is one half the t1 period, when V2 is double the amplitude V1.
Figure 2. Input signals at two different velocities
In conclusion: t2 = ½ t1, and V2 = 2V1
 V2 X t2 = 2V1 x ( 1/ 2 )x t1
 V2 x t2 = V1 x t1
A linear relation exists between the VRS amplitude and velocity. This predictive curve could be generated based on only one
measurement. As the velocity is proportional to RPM (revolutions per minute), this predictive curve is also valid for the RPM cutoff.
VRS automatic mode setting, Rev. 1.0
2
NXP Semiconductors
Description of the VRS system
Figure 3. Predictive curve
2.1.2
Zero crossing and peak voltage
To generate the predictive curve, the volt-time constant (VTC) of the system requires two data points:
• Voltage: VPEAK is the absolute voltage of the peak signal
• Time: Tau is the timing between the zero crossing and VPEAK (not between two voltage peaks)
Yellow: VRS Input.
Pink: VRS Output
Figure 4. Tau & VPEAK illustration
VRS automatic mode setting, Rev. 1.0
NXP Semiconductors
3
Description of the VRS system
The data pair of V and T allows the user to determine the VTC, and therefore the VRS amplitude vs. Engine Speed Curve
Figure 5. Predictive curve
2.2
2.2.1
Automatic mode circuitry
Automatic mode circuitry principle
The purpose of the automatic mode is to get a VRS system less sensitive to the noise in the cranking area and to reduce software
overhead.
In automatic mode, as soon the VRS Input signal crosses zero, the VRS system selects the highest input comparator threshold (VTMAX
typically, 1.715 V) and decays as 1/t. To accommodate a wide variety of trigger wheels and sensors, the decay rate is programmable via
the SPI VRS Automatic Parameters Configuration register.
The oscilloscope plot in Figure 6 illustrates the VPEAK detection and the Decay circuitry. The VRSP input signal is in blue, the VRSOUT
signal is in yellow, and an internal signal is in pink (accessible only in test mode) reflects the input comparator threshold value (VT).
When VSRP reaches the VT selected, the VRSOUT signal is set to a high level. At this point, the VPEAK detection circuit starts to determine
the magnitude of the positive peak and digitizes it according to SPI VRS Status register definition. When the VRSP signal crosses zero,
the decay circuitry sets the VT threshold to the maximum level, and decays to the VT selected in the appropriate timing. In this way, the
threshold tracks the input signal, allowing to the system to be less sensitive to noise than manual mode with a fixed VT threshold.
Figure 6. Peak detection and decay system illustration
VRS automatic mode setting, Rev. 1.0
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NXP Semiconductors
Description of the VRS system
2.2.2
Mathematica formula
Mantissa and exponent parameters defined in the VRS Automatic mode parameters register set the decay time of the system from the
VTMAX to VTSELECTED.
The Mathematica formula is:
• E (Truncated) = log2 [(VTSELECTED X Tau)/18.1] - 4
• M (Rounded to nearest integer) = {[(VTSELECTED X Tau )/18.1] /2E} - 16
2.2.3
Measurement instructions
Only four steps are necessary to correctly set the automatic mode.
1. Measuring VPEAK & Tau for the minimum RPM
2.
For the minimum system RPM, measure the maximum VPEAK (VPEAK_MAX) and Tau parameters as described in 2.1.1, Predictive
curve, page 2.
Selecting the appropriate input comparator threshold (VTSELECTED)
From the SPI VRS Manual Parameters table, select the appropriate VT (VTSELECTED) for the input comparator, to detect the
VPEAK_MAX.
Note: Adding some noise margin (~100 mV) could be recommended depending on the VPEAK_MAX and VTSELECTED value. As thresholds
are discrete, it could lead to select a VT corresponding to one or more lower LSBs.
Table 1. Peak detector output in the SPI VRS status register
SPI VRS Status Register Bits 7,6,5,4
Peak Values (nominal)
0000
10 mV
0001
14 mV
0010
20 mV
0011
28 mV
0100
40 mV
0101
56 mV
0110
80 mV
0111
110 mV
1000
150 mV
1001
215 mV
1010
300 mV
1011
425 mV
1100
600 mV
1101
850 mV
1110
1.210 V
1111
1.715 V
3.
Mantissa and exponent calculation
4.
Using the Tau measured and the VTSELECTED, customers can calculate the mantissa and exponent according to the
mathematical formula.
Note that Exponent values should be truncated, and the Mantissa value should be rounded to the nearest integer.
SPI register setting
The customer should now select the automatic mode and set the three required parameters (VTSELECTED, Mantissa and
Exponent) with:
• Automatic mode in bit 7 in the SPI Miscellaneous Parameters Configuration register
• Mantissa and Exponent in bit 7-0 in the SPI VRS Automatic Parameters Configuration register
• VTSELECTED in bit 7-4 (as well as the Filter Time parameter) in the SPI VRS Engine Cranking Parameter Control register.
VRS automatic mode setting, Rev. 1.0
NXP Semiconductors
5
Description of the VRS system
Table 2. SPI configuration register
Reg
#
11
12
13
Hex
B
C
D
VRS Engine Running
Parameters
7
6
5
4
3
2
1
0
R/W
Threshold
3
Threshold
2
Threshold
1
Threshold
0
Filter Time
3
Filter Time
2
Filter Time
1
Filter Time
0
Reset
(0)
(1)
(0)
(1)
(0)
(0)
(1)
(1)
R/W
mantiss 8
mantiss 4
mantiss 2
mantiss 1
exponent
8
exponent
4
exponent
2
exponent
1
Reset
(0)
(0)
(1)
(0)
(0)
(0)
(0)
(1)
R/W
Man./Auto
Disable
VRS
x
High/ Low
Ref
De-glitch
Gnd VRSN
Inv Inputs
Disable
2.5 V CM
Reset
(0)
(0)
(0)
(0)
(0)
(0)
(0)
(0)
7
6
5
4
3
2
1
0
R/W
Threshold
3
Threshold
2
Threshold
1
Threshold
0
Filter Time
3
Filter Time
2
Filter Time
1
Filter Time
0
Reset
(0)
(1)
(0)
(1)
(0)
(0)
(1)
(1)
VRS Automatic Parameters
VRS Miscellaneous
Parameters
Table 3. SPI control register
Reg
#
Hex
13
D
VRS Engine Cranking
Parameters
VRS automatic mode setting, Rev. 1.0
NXP Semiconductors
6
References
3
References
The following are URLs where you can obtain information on related NXP products and application solutions:
NXP.com
Support
Pages
Description
URL
MC33813
Data sheet
http://www.nxp.com/files/analog/doc/data_sheet/MC33813.pdf
MC33814
Data sheet
http://www.nxp.com/files/analog/doc/data_sheet/MC33814.pdf
Automotive Products
http://www.nxp.com/products/automotive-products
Power Management Home Page
http://www.nxp.com/products/automotive-products/power-management:POWER-MANAGEMENTAUTO
VRS automatic mode setting, Rev. 1.0
NXP Semiconductors
7
Revision history
4
Revision history
Revision
Date
1.0
3/2016
Description
• Initial release
VRS automatic mode setting, Rev. 1.0
NXP Semiconductors
8
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products herein.
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© 2016 NXP B.V.
Document Number: AN5253
Rev. 1.0
3/2016
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