KIT33794DWBEVM Datasheet MC33794EKR2, MC33794EK, KIT33794DWBEVM | P10-P12

Sensors

10 Freescale Semiconductor

MC33794

DYNAMIC ELECTRICAL CHARACTERISTICS

ELECTRODE SELECTION

Table 4. Dynamic Electrical Characteristics

Characteristics noted under conditions 9.0 V ≤ VPWR ≤ 18 V, -40°C ≤ T

≤ 85°C unless otherwise noted. Typical values noted

reflect the approximate parameter means at T

= 25°C under normal conditions unless otherwise noted. Voltages are relative

to GND unless otherwise noted.

Characteristic Symbol Min Typ Max Unit

OSC

OSC Frequency Stability

(12),

(13)

STAB

– – 10 %

OSC Center Frequency

R_OSC = 39 kΩ

OSC

– 120 –

kHz

Harmonic Content

(12)

2nd through 4th Harmonic Level

5th and Higher

OSCH

ARM

–

-20

-60

Shield Driver

Shield Driver Maximum Harmonic level below Fundamental

(12)

10 pF ≤ C

LOAD

≤ 500 pF

HARM

– -20 –

Shield Driver Gain Bandwidth Product

(12)

Measured at 120 kHz

GBW

–4.5 –

MHz

POR

POR Time-Out Period

PER

9.0 – 50 ms

Watchdog

Watchdog Time-Out Period

WDPER

50 68 250 ms

Watchdog Reset Hold Time

WDHLD

9.0 – 50 ms

Lamp Driver

Short Circuit to Battery Survival Time

SCB

3.0 – – ms

Notes

12. Verified by design and characterization. Not tested in production.

13. Does not include errors in external reference parts.

Table 5. Electrode Selection

TERMINAL/SIGNAL D C B A

Source (internal)

0 0 0 0

0 0 0 1

0 0 1 0

0 0 1 1

0 1 0 0

0 1 0 1

0 1 1 0

0 1 1 1

1 0 0 0

1 0 0 1

REF_A

1 0 1 0

REF_B

1 0 1 1

Internal OSC

1 1 0 0

Internal OSC after 22 kΩ

1 1 0 1

Internal Ground

1 1 1 0

Reserved

1 1 1 1

Table 5. Electrode Selection (continued)

TERMINAL/SIGNAL D C B A

Sensors

Freescale Semiconductor 11

MC33794

FUNCTIONAL DESCRIPTION

INTRODUCTION

The MC33794 is intended for use in detecting objects

using an electric field. The IC generates a low radio

frequency sine wave. The frequency is set by an external

resistor and is optimized for 120 kHz. The sine wave has very

low harmonic content to reduce potential interference at

higher harmonically related frequencies. The internal

generator produces a nominal 5.0 V peak-to-peak output that

is passed through an internal resistor of about 22 kΩ. An

internal multiplexer routes the signal to one of 11 terminals

under control of the ABCD input terminals. A receiver

multiplexer simultaneously connected to the selected

electrode routes its signal to a detector, which converts the

sine wave to a DC level. This DC level is filtered by an

external capacitor and is multiplied and offset to increase

sensitivity. All of the unselected electrode outputs are

grounded by the device. The current flowing between the

selected electrode and the other grounded electrodes plus

other grounded objects around the electrode causes a

voltage drop across the internal resistance. Objects brought

into or out of the electric field change the current and resulting

voltage at the IC terminal, which in turn reduces the voltage

at LP_CAP and LEVEL.

A shield driver is included to minimize the effect of

capacitance caused by using coaxial cables to connect to

remote electrodes. By driving the coax shield with this signal,

the shield voltage follows that of the center conductor,

significantly reducing the effective capacitance of the coax

and maintaining sensitivity to the capacitance at the

electrode.

The MC33794 is made to work with and support a

microcontroller. It provides two voltage regulators, a Power-

ON-reset/out-of-range voltage detector, watchdog circuit,

lamp driver and sense circuit, and a physical layer ISO-9141

communications interface.

BLOCK DIAGRAM COMPONENTS

Refer to Figure 1, MC33794 Internal Block Diagram,

page 2

, for a graphic representation of the block diagram

information in this section.

OSC

This section generates a high purity sine wave. The center

frequency is controlled by a resistor attached to R_OSC. The

normal operating frequency is around 120 kHz. A square

wave version of the frequency output is available at CLK.

Timing for the Power-ON Reset and watchdog (POR/WD)

circuit are derived from this oscillator’s frequency.

MUX OUT

This circuit directs the output of the sine wave to one of

nine possible electrode outputs or two reference terminals.

All unused terminals are automatically grounded (except the

two reference terminals). The selected output is controlled by

the ABCD inputs.

ELECTRODES E1-E9

These are the electrode terminals. They are connected

either directly or through coaxial cables to the electrodes for

measurements. Every electrode has a 2.8K (

± 20%) resistor

in series with the external pad and the internal electronics.

Only one of these electrodes can be selected at a time for

capacitance measurement. All of the other unselected

electrodes are switched to ground by an internal switch that

has an internal on-resistance of approximately 700

Ω. The

signal at the selected electrode terminal is routed to the

shield driver amplifier by an internal switch. All of the coaxial

cable shields should be isolated from ground and connected

SHIELD.

REF_A & REF_B ELECTRODES

These terminals can be individually selected like E1

through E9. Unlike E1 through E9, these terminals are not

grounded when not selected. Both terminals have a 2.8K

(

± 20%) resistor in series with the external pad and the

internal electronics. The purpose of these terminals is to

allow known capacitors to be measured. By using capacitors

at the low and high end of the expected range, absolute

values for the capacitance on the electrodes can be

computed. These terminals can be used for electrodes E10

and E11 with the only difference is that these two electrodes

will not be grounded when not selected.

SHIELD DRIVE

This circuit provides a buffered version of the returned AC

signal from the electrode. Since it nearly has the same

amplitude and phase as the electrode signal, there is little or

no potential difference between the two signals thereby

cancelling out any electric field. In effect, the shield drives

and isolates the electrode signal from external virtual

grounds. A common application is to connect the Shield Drive

to the shield of a coax cable used to connect an electrode to

the corresponding electrode terminal. Another typical use is

to drive a ground plane that is used behind an array of touch

sensor electrodes in order to cancel out any virtual grounds

that could attenuate the AC signal.

MUX IN

This circuit connects the selected electrode, reference, or

one of two internal nodes to an amplifier/detector. The

selection is controlled by the ABCD inputs and follows the

driven electrode/reference when one is selected.

Sensors

12 Freescale Semiconductor

MC33794

RECT

The rectifier circuit detects the level from MUX IN by

offsetting the midpoint of the sine wave to zero volts and

inverting the waveform when it is below the midpoint. It is

important to avoid DC loading of the signal, which would

cause a shift in the midpoint voltage of the signal.

LPF

The rectified sine wave is filtered by a low pass function

formed by an internal resistance and an external capacitance

attached to LP_CAP. The nominal value of the internal

resistance is 50 k

Ω. The value of the external capacitor is

selected to provide filtering of noise while still allowing the

desired settling time for the detector output. A 10 nF

capacitor would allow 99% settling in less than 5.0 ms. In

practice, it is recommended you wait at least 1.5 ms after

selecting an electrode before reading LEVEL.

GAIN AND OFFSET

This circuit multiplies the detected and filtered signal by a

gain and offsets the result by a DC level. This results in an

output range that covers 1.0 V to 4.0 V for capacitive loading

of the field in the range of 10 pF to 100 pF. This allows higher

sensitivity for a digital-to-analog converter with a 0 V-to-5.0 V

input range.

ATTN

This circuit passes the undetected signal to SIGNAL for

external use.

SHIELD_EN

A logic low on this input disables the shield drive. The

purpose of doing this is to be able to detect that the shield

signal is not working or the connection to the coax shields is

broken. If either of these conditions exists, there will be little

or no change in the capacitance measured when the

SHIELD_EN is changed. If the SHIELD output is working and

properly connected, the capacitance of the coax will not be

cancelled when this terminal is asserted and the measured

capacitance will appear to change by approximately the

capacitance between the center conductor and the shield in

the coax.

LAMP CKT

This section controls the operation of the LAMP_OUT

terminal. When LAMP_CTRL is asserted, LAMP_OUT is

pulled to LAMP_GND. If one side of an indicator lamp or LED

(with appropriate current setting resistor) is connected to a

positive voltage source and the other is connected to

LAMP_OUT, and LAMP_GND is connected to ground, the

lamp will light. This circuit provides current limiting to prevent

damage to itself in the case of a shorted lamp or during a

high-surge condition typical of an incandescent lamp

burnout.

LAMP_GND should always be connected to ground even

if the lamp circuit is not used.

ISO-9141

This circuit connects to an ISO-9141 bus to allow remote

communications. ISO_IN is data from the bus to the MCU

and ISO_OUT is data to drive onto the bus from the MCU.

POR/WD

This circuit is a combined Power-ON Reset and watchdog

timer. The RST output is held low until a certain amount of

time after the V

REG output (V

) has remained above a

minimum operating threshold. If V

falls below the level at

any time, RST is pulled low again and held until the required

time after V

has returned high. An over voltage circuit is

also included, which will force a reset if V

rises above a

maximum voltage. The watchdog function also can force RST

low if too long an interval is allowed to pass between positive

transitions on WD_IN.

INTERNAL V

REGULATOR

This circuit converts an unregulated voltage from VIN to a

regulated 5.0 V source, which is used internally and available

for other components requiring a regulated voltage source.

INTERNAL V

REGULATOR

This is a regulator for analog devices that require more

than 5.0 V. This is used by the device and some current is

available to operate op-amps and other devices. By having

this higher voltage available, some applications can avoid the

need for a rail-to-rail output amplifier and still achieve the 0 V-

to-5.0 V output for a digital-to-analog converter input.

DD_MON

is a divided output from internal V

REG, which

allows a 0.0 V-to-5.0 V ADC to measure V

. Normal value

for V

is 8.5 Volts.

CONTROL LOGIC

This contains the logic that decodes and controls the

MUXes and some of the test modes

APPLICATION INFORMATION

The MC33794 is intended to be used where an object’s

size and proximity are to be determined. This is done by

placing electrodes in the area where the object will be. The

proximity of an object to an electrode can be determined by

the increase in effective capacitance as the object gets closer

to the electrode and modifies the electric field between the

electrode and surrounding electrically common objects. The

shape and size of an object can be determined by using

multiple electrodes over an area and observing the

capacitance change on each of the electrodes. Those that

don’t change have nothing near them, and those that do

change have part of the object near them.

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