Issue 155 June 2003
E-Field Evalulation Module

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Start • Danger, Will Robinson • A Z8-Based E-Field EVM • Can't Touch This • Raise your Shields • Sources and PDF

“Motor-rola” is world famous for its embedded automotive electronics. However, some of Motorola’s auto-oriented parts serve two masters and find their way into embedded applications that don’t burn gas or require regular oil changes. We all know Motorola as a supplier of RF products and the semiconductors behind them. Also, you would have to be isolated in the deepest of jungles or marooned on Gilligan’s Island from birth not to know about Motorola’s microcontroller and microprocessor product lines.

In the article that follows, I won’t be punching any significant holes in the earth’s magnetic field or crunching complex numbers on a 32-bit microcontroller. Instead, I’m going to take you into a world that has been restricted to science fiction until now. Remember Lost in Space, the television series? The very first thing Will Robinson’s father did was set up the “force field” around his family (Robot included) and the Jupiter spacecraft. Likewise, Captains Kirk and Picard deployed “shields” to protect them from hostile environments and unfriendly attacks. Fortunately, I don’t expect any extraterrestrials to pounce on the Florida Room, but I do have a “force field” device of my own: the Motorola MC33794 electric field imaging device (EFID).

MOTOROLA'S EFID

You can’t tell from its innocent looks in Photo 1, but it’s pretty obvious the MC33794 EFID was originally intended to accompany automobile passengers on road trips. You don’t find built-in ISO 9141 interfaces thrown into IC designs just in case. For those of you who don’t work in Detroit, ISO 9141 is the core physical interface for that little diagnostic box your mechanic plugs into your ailing ride. Another giveaway as to where this little device lives is the 12-VDC power pin and the 12-VDC indicator lamp interface pins.

(Click here to enlarge)

Photo 1—The MC33794 electric field imaging device (EFID) is housed in a “heat slug” 44-lead heatsink small outline package (HSOP) package. This is a top and bottom view. If you want to play with a MC33794 without soldering it down, you can get a through-hole test socket from WELLS-CTI.

The MC33794 also incorporates interface pins that accept and produce standard 5-V logic levels. To that end, the MC33794 contains an internal 5-V regulator capable of supplying 75 mA to an external load as well as tapping some of the incoming voltage to power its own internal circuitry. The presence of a 5-V regulator and TTL-compatible I/O means the EFID has internal circuitry that is capable of establishing communications with and supporting external devices such as microcontrollers. A further indication that the MC33794 is microcontroller-friendly is the inclusion of an active-low reset (RST) output and a watchdog input (WD IN).

The MC33794’s internal oscillator, which doesn’t require a crystal or resonator, supplies a clock output (CLK) that can be tied to the watchdog input if the watchdog function is not needed. The MC33794’s CLK output is a square wave representation of the internal oscillator’s sine wave signal. The analog outputs (LEVEL, VDDMON, PWR IN MON, and LAMP MON) suggest that if a microcontroller were added to the system, it would need to be equipped with an analog-to-digital converter subsystem. In case your MC33794 design needs additional external analog circuitry, the device also provides a regulated 8.5-VDC power source. With these obvious analog, TTL, and microcontroller-oriented interfaces, it’s easy to see that the MC33794 EFID doesn’t have to be restricted for use in automotive applications.

The MC33794 was designed to sense objects in its proximity using a low-level electric field. So, unlike the plasma and electrical fields found around the famous TV spacecraft, the MC33794 won’t zap you or produce an electric field that will stop you like a sheet of invisible glass.

The device’s electric field is derived from a low-harmonic content, 5-VPP, low-frequency RF sine wave that is generated by the oscillator circuitry within the MC33794. A single 39-kW resistor is used to tune the internal oscillator’s frequency to around 120 kHz. The signal produced by the internal sine wave generator is passed through an internal 22-kW resistor. The sine wave signal flows through the 22-kW resistor into an internal multiplexer that routes the signal onto one of 11 output pins, which are selected by the ABCD mux select pins. The ABCD mux selector inputs are TTL-compatible and allow only one electrode at a time to be accessed by the internal analog circuitry.

With the exception of the reference inputs, Ref A and Ref B, each unselected electrode is automatically grounded internally by the logic. The current flowing between the active electrode and any other grounded objects within the influence of the active electrode’s electric field including the grounded deselected electrodes generates a voltage drop across the internal 22-kW resistor that’s located at the output of the sine wave generator. Thus, an electric field is set up between the active electrode and any grounded object the generated electric field can envelope. Objects entering or exiting this electric field affect the capacitance of the electric field, thereby changing the current flowing through the 22-kW resistor. This results in a voltage drop across the 22-kW resistor, which, in turn, results in a voltage change at the LEVEL pin.

A receiver multiplexer that follows the selected electrode is connected to the output pins as well and routes the selected electrode signal into a detector on-board the MC33794. The detector converts the sensed sine wave signal from the active electric field to a DC level. The received DC level is then filtered, multiplied, and offset. All that is needed to enable the MC33794 to process the receiver’s DC signal is a single 10-nF filter capacitor tied to the LP CAP pin. The 10-nF value is coupled to an internal resistance; this provides adequate noise filtering while enabling the signal to settle well inside the detector’s response-time window. The processed DC signal is then passed out through the LEVEL pin for your processing pleasure.

Let’s talk about electrodes as they pertain to the MC33794. An electrode can be anything you desire it to be as long as it can participate in the MC33794’s electric field. Electrodes can be attached directly via wire or coax cable. If you have to use coaxial cable to attach your special electrode, the MC33794 is equipped with shield driver circuitry. The MC33794 drives a matching signal on the shield, which allows the shield voltage to closely follow the center conductor voltage. This reduces the effective capacitance of the coax line, which is necessary because the MC33794 recognizes  capacitance changes in the electric field as an indicator of an object’s proximity to an electrode. You don’t want your coax line introducing a significant capacitance to your electrodes. As an object gets closer to an electrode, the effective capacitance increases, and thus prompts a change in the electric field. By employing multiple electrodes, it is possible to get an idea about the size and shape of an object influencing the MC33794’s electric field depending on which electrodes indicate a change in their electric field.

The coax shields are not grounded. Instead, each electrode’s coax shield is tied to the MC33794’s shield driver pin. In addition to driving the coax shield to minimize cable capacitance, the shield driver also can be used to test the electrical integrity of the coax by toggling the shield disable signal (SHLD DIS) and checking for a change in the LEVEL output. If no change is noted, either the coax shields are compromised or the shield signal isn’t being presented to the coax shields.

Voltage at the LEVEL pin is inversely associated with the capacitance between an electrode and other objects in the selected electrode’s electric field. As capacitance increases, the associated voltage decreases, and vice versa. The 22-kW internal resistor and nominal 120-kHz oscillator frequency provide a close linear relationship between capacitance and voltage over a capacitance range of 10 to 100 pF.

Of course, many factors can affect the value measured at the LEVEL pin. Component values may change over time or be affected by environmental variables such as temperature and humidity. To compensate for this, the MC33794 relies on two reference inputs, Ref A and Ref B, that can be loaded with known capacitances. It is recommended that one reference capacitor be near the minimum capacitance at the electrode and the other fly close to the maximum capacitance to be expected at the electrode. Using these reference capacitances and their corresponding voltages (measured at the LEVEL pin using the ABCD-controlled output mux) provides a set of references that can be used to respond to sensor-system changes caused by time, humidity, and temperature. If your electrodes aren’t guaranteed to always have a high DC resistance to ground or a voltage source, then you can insert a 10-nF capacitor in series between the MC33794’s electrode pins and your electrode.

The lamp control and monitoring features of the MC33794 are pretty easy to comprehend, so I won’t cover them here.