Question: What can the Contest EVM do?

Answer: The EVM can be used in one of two basic operating modes.

The first and simplest mode is a preprogrammed demonstration mode. The EVM's E1 through E9 electrodes are brought out and arrayed for the area to be covered by the part. A 12-V source is attached to power the EVM. And, a serial communications session is established between a PC running Windows and the MCU's Monitor program. The program displays the levels detected at the MC33794 electrode(s) in an bar-graph format that's easy to understand. Many of the EVM operating parameters also can be set. The user can quickly see the effects of electrode spacing and placement, the effects of distributed ground, and much more.

The second operating mode allows the user to evaluate code written for the MC908QY4 MCU to control the MC33794 operation. Using the MCU's User Mode Monitor Access program, the user can start writing code to control the MC33794 using an appropriate software development environment. The user code is downloaded from the PC through the EVM's serial port and into the MCU's 4-KB flash memory ROM space. The EVM's small Monitor Communication program is protected from accidental or inadvertent erasure. A software development environment suited for the MC908QY4 MCU can be downloaded from P&E Microcomputer Systems, Inc. at www.pemicro.com.

Other development tools are available from a number of sources including Metrowerks (www.metrowerks.com). From the home page, click on the Embedded button, and then the HC08 Tools button.

Question: Is the MC33794 available in production?

Answer: The MC33794 is available in high-volume production today.

Question: What MC33794 development tools are available?

Answer: Two hardware development tools are offered. The KIT33794DEVM E-Field Development Kit is available from Motorola component distributors, and the Contest EVM is offered in limited quantities for Contestants.

The physical difference between the kit and EVM regard the reprogrammability of their MCUs. The KIT33794DEVM MCU is socketed and can be removed for flash memory reprogramming. The Contest EVM uses an MCU in a SOIC packaged that is soldered to the PCB and is not removable for reprogramming.

Software and documentation for the KIT33794DEVM Kit are provided on an included CD-ROM. The Contest EVM equivalent materials are available for downloading.

Question: The part is more complicated than other types of sensors I’ve used. Why?

Answer: Unlike other sensors, the MC33794 is an electric field sensor plus a System-on-a-Chip—having many functions useful in an embedded design. These added functions enable a smaller PCB footprint with a minimum of added ICs, leading to a quicker design time. Additional functions like a voltage regulator, diagnostics, POR, and WDT make it a "System’s Chip." The SmarTMOS IC process is ideally suited for implementing power handling functions along with high-voltage capability and logic circuits.

Question: Why does the MC33794 come in both a 54-lead SOICW-EP and a 44-lead heatsink HSOP package?

Answer: The choice of packages is driven by the heat dissipation the subject packages and the user’s ability to handle fine-pitch packages. The heat generated by the on-board regulators and other internal circuitry limits the maximum power dissipation of the part. Attaching the package’s backside exposed pad to copper cladding on the PCB to act as a heat spreader will allow the maximum available current to be drawn.

Question: What affects the MC33794 e-field sensing capability?

Answer: The size of the electrode, its spacing, and the presence/proximity of system and board grounds.

Question: Do leakage currents from V_CC affect the electric field sensitivity?

Answer: Yes. For uses where leakage current can happen because of electrode layout or other application position-dependent use, it’s recommended that the electrodes E1 to E9 be coupled to the part via a capacitor.

Question: Is there a preferred spacing between sense electrodes?

Answer: No, each application presents its own sensing electrode mounting challenges.

Question: Is there a unique serial port protocol that must be used?

Answer: No, the included ISO 9141 port provides a physical translation of the Rx and Tx serial signals to those specified by the ISO standard if that feature is used.

Question: What is an ISO 9141?

Answer: ISO 9141 is an in-vehicle automotive serial communications protocol used by many automakers for on-board diagnostic (OBD) uses.

Question: How do I do accommodate the "NC" pins of the part?

Answer: It is recommended NOT to ground the "NC" pins.

Question: Do I need a microcontroller to use this part?

Answer: Depending on the particular design, an MCU may not be needed. To fully utilize the capabilities of the MC33794 would suggest some form of program control done by an MCU with an on-board analog-to-voltage converter. The choice of the MCU is the designer’s.

Question: Should I tie the power-on reset (RST) pin of my MCU to the RST pin on the MC33794?

Answer: It is suggested if the MC33794 is used to power an MCU. It’s important that the MCU voltage be stable before the MCU program initialization. Using the MC33794’s RST signal guarantees that the MCU voltage is stable.

Question: Isn’t the watchdog timer redundant if my MCU has one?

Answer: For MCUs with watchdog timers, the MC33794 WDT can be defeated, used as a backup, or used for some other user-defined purpose. For safety-critical systems, a second WDT (independent of the MCU’s) provides an added level of confidence that the MCU application will operate correctly.

Question: Why is a 5-V regulator included?

Answer: The on-chip regulator provides the sensor with the necessary self-contained 5-V supply. It also offers the designer a means to power external circuitry. This approach eases the design complexity and PCB size concerns.

Question: Why is there no 3.3-V regulator?

Answer: A 5-V regulator was chosen because it remains a popular operating voltage for a wide range of MCUs and components, and because it’s the chosen internal voltage for the MC33794 design. A 5-V supply rail has the advantage of outputting a larger detected electric field voltage for digitization by an off-board A/D converter.

Question: How much 5-V current can the MC33794’s on-chip regulator deliver?

Answer: The 5-V regulator’s capability is limited by the maximum sustained power dissipation of the package. Soldering the MC33794 exposed backside pad to copper cladding on the mounting side of a PCB for heatsinking purposes allows the maximum available current to be drawn—limited by the temperature rise. Do not exceed the part’s thermal rating.

Question: Can I power circuitry from the V_DD pin?

Answer: No, the V_DD pin is on the exposed side of the MC33794’s voltage regulator, which supplies the analog portions of the design. Only a power supply bypass filter capacitor should attached to this pin.

Question: The oscillator for the sensor is tunable. Why?

Answer: In some applications, multiple MC33794s may be used. It may be desirable to tune the nominal 120-kHz oscillator of the second (or third MC33794) away from the other part(s) to prevent harmonics or interference that may affect other electronics.

Question: How are the sense electrodes selected?

Answer: The MC33794 has four control lines (A, B, C, and D) that represent the BCD representation of the sensing electrodes (E1 to E9), along with the selection of the Ref_A and Ref_B pins, among other selections. By toggling the A, B, C, and D selection lines, the user is able to select the sensing electrodes whose electrical potential is to be detected. Note: only one electrode is selected at a time for processing, although the order of electrode selection and the amount of gain for that electrode is up to the user.

Question: What is the output of the MC33794 I will see or use?

Answer: The output of the MC33794 is the LEVEL or SIGNAL pin. The user sees the output of the internal low-pass RC filter with capacitance set by the capacitance to the LPF pin filtered. The processed signal is presented at the LEVEL pin. Alternatively, the user may decide to look at the gained, but detected and unfiltered, output signal at the SIGNAL pin. In either case, the detected voltage it that sensed at one of the eight electrodes, which are selected for processing by setting the sense electrode’s BCD value at the A, B, C, or D inputs. At this point, the user’s application may digitize the value or use it in real time to compare to a threshold and control something if the value meets the threshold criteria.

Question: Why is there a lamp monitor?

Answer: The lamp monitor circuit is available to detect a 12-V lamp-fault condition. An example is a burned out indicator lamp that draws no current when the Lamp Control command is asserted to turn on a bulb if one is used. Non-automotive applications may not use this feature as it was intended.

Question: Why is there a Lamp Sense?

Answer: For vehicular applications, a lit indicator lamp is the key indicator of a system malfunction. If a lamp is burned out or not seated in place, then its use as a warning is lost. Reporting a defective service lamp prior to a real safety warning is a key automotive need. Lamp Sense provides a scaled voltage that is in the range of an MCU’s voltage with an A/D converter.

Question: Can I use the Lamp_Out to drive other circuits?

Answer: The Lamp_Out is the low-side power driver with a typical 1.75-W on resistance. It’s capable of sinking 400 mA plus of current with the load connected to a 12-V supply. This capability could be used for a relay driver, lamp driver, or any other application that is suited to low-side drivers and operates from a 12-V or lower voltage source.

Question: Can I run the MC33794 with voltages other than 12 V?

Answer: Not at this time. The MC33794 generates a fixed 5 V for internal/external use and 8.5 V for internal use off a nominal 12-V supply; going lower reduces the operating stability of the part’s internal 8.5-V regulator. Applying voltages greater than 12 V will increase the electrical stress on the MC33794’s voltage regulators and degrade the part’s thermal performance. Transient operation over 12 V is considered normal.

Question: Why does the MC33974 have an unfiltered signal and level output?

Answer: The part’s raw signal output is provided for users who may have an MCU/DSP signal-processing algorithm in mind and don’t want to use on-chip filtering hardware or signal preprocessing.

The level signal is passed through an on-chip low-pass RC filter. The C value is selected by adding a capacitor to the LP CAP pin. The nominal R value is set at 50 kW for roll-off and response-time calculations.

Question: What is the TEST pin, and what should I do with it?

Answer: The factory uses the TEST pin for production testing. In normal operation, this pin is permanently tied to ground.

Question: How fast can I move to and detect on a selected electrode?

Answer: Electrode selection via the A, B, C, and D select lines and e-field detection can be quick, but an application may be limited by real world factors:

Whether the object being sensed is slow moving or rapid

Whether the electrode needs to be sampled once, or multiple electrodes need to be resampled in a cyclic pattern

The speed of the signal-processing algorithm or decision-making process based on the detected e-field. Note: There are nine electrodes per MC33794, but only one electrode is selected at a time for detection.

The response time of the on-chip LPF (or external filter)

The speed of the A/D converter or the effect of hysteresis in a simple comparator-detection circuit

The intended accuracy of the measurements

Question: Can multiple MC33794s be used to increase the number of sensing electrodes and increase the coverage area in an application?

Answer: Yes, multiple MC33794s can be used in a single application. Users may encounter limitations in the physical placement of the electrode(s) and ground. Each application will offer unique mounting challenges for electrode size, spacing, and cabling.

Question: I need more help or have a unique question. Where can I get help?

Answer:Please review the MC33974 datasheet's "Application Section" for more detailed information. Technical support is always available on the 'Net by going to http://www.motorola.com/. Click on the "Semiconductor" tab and then "Contact Us" tab and finally on "Technical Helpline". First-time users will need to register. Help can also be obtained by calling 1-800–521-6274 or 480-768-2130.