
MAX1441
Automotive, Two-Channel Proximity and
Touch Sensor
10
Excitation Frequency
To avoid interference, the excitation frequency can be
adjusted to automatically spread within a frequency
range. The lower frequency bound and spread-spectrum
bandwidth registers determine this range. Spread spec-
trum continuously changes the excitation frequency so
that the radiated power is distributed over a frequency
range rather than a single frequency. This lowers the
radiated energy density and thus leads to a cleaner
spectrum. In addition, by changing the excitation fre-
quency, the capacitance measurement becomes more
immune against interference signals.
Offset Capacitance
In case the capacitance measurement permanently
reaches the upper or lower limit, there is a likelihood of
a parasitic capacitance on top of the touch pads. This
could be an ice coating on the door handle for example.
In this case, the offset capacitance is adjusted so that
the capacitance reenters the measurement range. A
parasitic capacitance up to 63pF is compensated for
each channel independently by properly setting the off-
set registers, CO1/CO2.
Single Conversion Mode
The C2D converter can be placed into a single conver-
sion mode. In the single conversion mode, the micro-
controller triggers a single conversion by setting bit SCT.
If the single conversion mode is enabled, the analog
front-end powers up only during the conversion. SCEN
controls the single conversion mode. SCEN = 1 enables
the single conversion mode and SCEN = 0 disables the
single conversion mode. When single conversion mode
is enabled, set bit SCT to trigger a conversion. SCT bit
automatically clears after the conversion is completed.
Standby Control
To save power, the analog front-end can be put in the
standby state. During standby, the conversion rate is
determined by the standby state conversion rate divider.
SB controls the standby mode. SB = 1 enables the stand-
by mode and SB = 0 disables the standby mode. There is
only 1 SB bit common to both channels, so the channels
cannot be placed in the standby state independently.
The following sequence of control register writes to the
PD register is recommended for entering standby mode:
1) Set PD to 06h to put both AFE channels into reset state.
2) Write DSB and SSB2 registers to set standby rate (if
not already set).
3) Set PD to 00h to release AFE reset.
4) Set PD to 01h to enter standby mode.
Standby State Conversion-Rate Divider
The maximum C2D conversion rate is 1.66kHz. Standby
state uses conversion-rate reduction to save power. The
conversion rate divider and conversion rate subdivider
determine the final conversion rate. The DSB divider
DSB[4:0] is common to both sensor channels. The con-
version rate for channel 2 can be further reduced by the
SSB2[4:0] divider. The conversion rate in kHz is deter-
mined by the equation:
conv,channel1
1
f
1.66kHz
D
= ×
conv,channel2
1
f
1.66kHz
D S
=
×
where D is an integer number determined by a 5-bit word
DSB[4:0] and S is an integer number determined by a
5-bit word SSB2[4:0]. The default value of D and S is
one. D > 1 when S > 1.
Table 1. Important AFE Function Registers
REGISTER
FUNCTION
CRNG
Adjust the capacitance range
FEL
Set the frequency of excitation
FEB
Set the bandwidth of the spread-
spectrum modulation
CO1, CO2
Set the capacitance offset
SCT
Put the device in single-conversion
mode
DSB
Set the standby conversion rate
SSB2
Set the channel 2 standby conver-
sion rate subdivider
PD
Put the AFE in power-down mode
(does not affect CPU operation)
WU1, WU2
Select the wakeup criteria (rate-of-
change and/or absolute capacitive
change)
AT1H, AT2H
Set the absolute wake-up thresh-
old
RT1H, RT2H
Set the capacitance rate-of-
change threshold
CRSLT1L, CRSLT1H
Channel 1 conversion result
CRSLT2L, CRSLT2H
Channel 2 conversion result
AFEINTST
Interrupt status of the AFE