IC-PMX Datasheet, PDF (19/27 Pages) IC-Haus GmbH – ENERGY HARVESTING MULTITURN COUNTER/ENCODER

iC-PMX
ENERGY HARVESTING MULTITURN COUNTER/ENCODER
WIEGAND MODULE EXCITATION
Rev A3, Page 19/27
A Wiegand wire is a ferromagnetic material with hystere-
sis, so its behavior during the current ramp is slightly
different depending on the previously applied magnetic
field. The slew rate of the current ramp accentuates
this nonlinearity. Figure 11 shows such a nonlinearity
excited by a high current slew rate.
tant concern during a power loss event. The brownout
protection is used to continue the current ramp and the
capacitor CB is calculated for the worst case: Power
loss at the start of the current ramp. The voltage drop
can be calculated based on the electrical characteris-
tics, item no. 103, item no. 301 and item no. 304.
Calculation of CB for a Wiegand module which needs
RAMP(3:2)="10" and WI(1:0)="01":
min(CB) = 1400 µs * (0.5 * 14mA + 2mA) = 11.2 µF
Hysteresis effects of
the Wiegand wire
The capacitor CD between Pin CP and Pin CN should
also be larger than min(CB).
0 us
t_gap
Figure 11: Nonlinearity of V(VWC) During a Current
Ramp
End of Current Ramp Detection
Opcode 0x29 can be used to read the status of the cur-
rent ramp generator. The result is equal to the values
in table 15 if the current ramp is active. The bits 4 to 6
of the status are set to zero when the current ramp has
finished.
However, since the current ramp parameters are usu-
ally calculated based on the resistance of the Wiegand
module, some margin should be provided to compen-
sate for the nonlinearity (see also iC-PMX App Note
2).
Current Ramp Parameters
Opcode 0x21 is used to start a Wiegand wire excitation.
Valid parameters are described in table 14 and table
15. This function is used to determine if the Wiegand
wire is precharged. A current ramp is applied to the coil
around the Wiegand wire. This generates an additional
magnetic field, which can trigger a Wiegand pulse if
the Wiegand wire is precharged. The current direction
determines the polarity of the additional magnetic field
(see Fig. 12).
The duration of the current ramp and the maximum
current must be selected suitable for the chosen Wie-
gand module. The voltage slew rate during the current
ramp is determined by the above parameters and the
Wiegand module’s electrical properties. Its worst case
value should be below the allowed slew rate during
current ramps (see page 8, item no. 204).
Recommended Values for the Capacitors CB and
CD
The capacitor CB between Pin VDD and Pin GND
should be large enough to ensure that the voltage drop
during the current ramp is less than 1V. That’s an impor-
WI(1:0)
Bit
1...0
Description
Select max. current (see page 9, item no. 301),
default value: 0
Table 14: Current Ramp Parameters 1/2
RAMP(7:0)
Bit
Description
7..4
0x7: Start current ramp, default mode
0xF: Start current ramp with disabled Hall sensor
evaluation, value of MP is defined by bit 1
3...2
Select duration of current ramp (0...3) (see page 9,
item 304)
1
Value of MP if bit 7 = 1
0
0: Set current direction to test for a negative
Wiegand pulse (PP = low)
1: Set current direction to test for a positive Wiegand
pulse (PP = high)
Table 15: Current Ramp Parameters 2/2
Wiegand
Module
Coil around
Wiegand Wire
iC-PMX
W1
W2
RAMP(0)=0
Wiegand
Module
Coil around
Wiegand Wire
iC-PMX
W1
W2
RAMP(0)=1
Figure 12: Current Ramp Direction