IEEE 1451.4 Mixed Mode Interface (MMI) Smart Transmitter Digital Driver

Figure 2 shows a block diagram of a 3-wire voltage-powered sensor with a common power line. The signal line is dedicated to delivering the analog output voltage of the transmitter to the DAS. The diodes allow sequential access to the amplifier or TEDS memory by reversing the polarity of the power line. When the control switch is in the "analog" position, the positive supply from the DAS power supply powers the amplifier through the upper diode. When the control switch is in the "digital" position, the memory device is powered by the negative logic supply through the lower diode.

Figure 2. IEEE 1451.4 Class 1 MMI, shared power line.

Figure 3 adds another wire to create a 4-wire voltage powered sensor with a common return (usually a ground path) or shield. The sensor and TEDS memory have independent power supplies and can operate simultaneously. A switch to select analog and digital modes is still required to disable the digital function when the sensor is in use. This helps reduce the mutual interference noise between the analog signal and the digital TEDS data caused by the voltage drop in the common return. The diode and Rt are not required in this configuration. The resistor can be omitted and the diode can be replaced by a shorting line.

Figure 3. IEEE 1451.4 Class 1 MMI, shared return line.

TEDS Memory

The DS2430A 256-bit 1-Wire EEPROM is a typical TEDS memory chip. Since this chip has no VCC pin (i.e., it uses parasitic power), only two pins are required: IO and GND. The block diagram in Chapter 8.1.2 of the IEEE standard does not mention these pin names, but instead uses "+" to represent IO and "-" to represent GND. Figure 4 shows the digital portion of an IEEE 1451.4-compliant sensor, using the actual model number and pin names. The standard (Chapter 8.5, Family Code) does not specify a dedicated family code for TEDS memory. Therefore, it is allowed to use 2-pin 1-Wire memory chips other than the DS2430A. The general-purpose diode 1N4148 can be replaced by a Schottky diode, which has a forward bias voltage of approximately 0.3V. The Rt resistor value is not particularly critical, and the circuit was tested with 100kΩ.

Figure 4. Class 1 sensor, TEDS operating principle.

Building a Class 1 MMI Digital Driver Circuit

The operating signal level of a 1-Wire device is 3V to 5V (pull-up voltage) in the idle state and 0V in the active state. This voltage is the voltage between the IO terminal (positive terminal) and the GND terminal (negative terminal). Class 1 MMI connects the IO pin to 0V and modulates the negative voltage of the memory chip's GND pin (Figure 5). Compared to the nominal 1-Wire signal level, the MMI signal is inverted and shifted 5V to the negative direction.

Figure 5. Nominal 1-Wire and Class 1 MMI signal levels.

The memory chip cannot distinguish and does not care how the voltage at its terminals is generated. When responding, it just applies a short-circuit signal to its port for a specified time. Under "normal conditions", this short-circuit signal is observed as a voltage close to 0V at the IO port. For Class 1 MMI, the short circuit causes the voltage on the digital communication line to rise from -5V (idle) to the diode drop -VF (-0.7V).

MMI Driver Description

Figure 6 shows the MMI driver circuit. The circuit consists of a forward path (top, master to sensor, write) and a return path (bottom, sensor to master, read). An IEEE 1451.4-compliant sensor is connected to TP4 through an analog/digital switch. The return path is connected to the driver's 0V (GND). The signal levels at TP2 and TP6 correspond to the nominal 1-Wire levels (5V for idle state and 0V for active signal level). V+ corresponds to the operating voltage of the microcontroller, which can range from 3V to 5V. TP2 is connected to an open-drain output (write) of the microcontroller, and TP6 is connected to an input port.

Figure 6. Class 1 MMI digital driver with sensor.

Connecting to a Bidirectional 1-Wire Master

Connecting a bidirectional master requires the additional circuit shown in Figure 7. Due to the different propagation delays for the rising and falling edges of the level-shifting section, the MMI driver using a bidirectional 1-Wire master may be unstable when the operating voltage is too high. For this reason, the positive supply needs to be limited to approximately 3.3V. Therefore, the bidirectional master must be a 3V powered device, such as the DS2482. Using a 5V bidirectional master (such as the DS2480B) will cause the COM and NO voltages of the analog switch to exceed the V+ level, which does not meet the required operating conditions.

Figure 7. Additional circuitry for bidirectional 1-Wire master interface.

verify

The circuit in Figure 6 was tested with the additional circuit in Figure 7. The 1-Wire master was the DS9097U-S09, which is based on the DS2480B driver chip. The positive voltage (V+) was set to 3.4V to ensure stability. The 1-Wire master operates at 5V, which does not meet the voltage requirements of the MAX4561 analog switch (signal voltage must not exceed the supply voltage). This explains the interference on TP2, but has no other adverse effects on the circuit's function.

Reset/Online Detection Cycle

Figure 8 shows the signals at TP2 (top), TP4 (middle), and TP6 (bottom). Due to the presence of diodes in the sensor network, the 0V level is not completely reached when the slave presence pulse is asserted. The bottom waveform shows a clear presence pulse. The positive amplitude at TP6 corresponds to V+ 3.4V.

Figure 8. Reset/online detection

Read time slot

The nodes shown in Figure 9 are the same as before (TP2 = top, TP4 = middle, TP6 = bottom). The first time slot reads 1 and the second time slot reads 0.

Figure 9. Communication time slots

Summarize

The circuits described in this article can be used when the microcontroller is acting as a 1-Wire master and using separate ports for read and write operations. However, the application software that generates the time slots and reset/presence detect signals has strict timing requirements and may have to be programmed in assembly language. The additional circuitry of the bidirectional 1-Wire driver chip allows the application software to be developed in a high-level language.

Due to its asynchronous operation, the add-on circuit can cause a spike when the master stops pulling the 1-Wire line low. When reading a 0, the spike triggers the driver's active pullup, causing a conflict between the driver's pullup and the MAX4561's pulldown. Therefore, when using the DS2482 driver, the active pullup should be turned off. The spike is also the reason why the bidirectional 1-Wire driver add-on circuit cannot support 1-Wire slaves on the master side.