Build an IEEE 1451.4 Class 1 MMI smart transducer digital driver circuit (Part 2 of 2)
Abstract: The IEEE 1451.4 mixed-mode interface (MMI) is the connection for both analog signals and digital transducer electronic data sheets (TEDS) between a transducer and a network-capable application processor (NCAP) or data acquisition system (DAS). The IEEE 1451.4 standard defines two classes of MMI. In Class 1, the TEDS shares one wire with the analog function, using negative voltage for communication. Class 2 provides the TEDS with its own pair of wires and uses positive voltage for communication.
Consequently, Class 2 is directly compatible to 1-Wire® drivers (masters). Due to the negative communication voltage, Class 1 requires a more complex driver circuit. This document explains how to build an IEEE 1451.4 Class 1 MMI to access the TEDS.
(Part 1, linked here, looked at the objectives and elements of the standard, IEEE 1451.4 mixed-mode interfaces (MMIs), the TEDS memory, and constructing a Class 1 MMI digital driver circuit.)
MMI Driver Description
Figure 6 shows the circuit of a MMI driver. The circuit consists of a forward path (top, master to sensor, write) and a return path (bottom, sensor to master, read). The IEEE 1451.4-compliant sensor is connected through the analog/digital switch to TP4. The return line is connected to 0V (GND) of the driver.
The signal levels at TP2 and TP6 correspond to normal 1-Wire levels (idle 5V, active 0V). V+ corresponds to the operating voltage of the microcontroller and could be in the range of 3V to 5V. TP2 is to be connected to the open-drain output (write) port of the microcontroller; TP6 connects to an input port.
Figure 6. Class 1 MMI digital driver with sensor attached.
Connecting a Bidirectional 1-Wire Master
Connecting a bidirectional master requires the additional circuit shown in Figure 7. Since the propagation of rising and falling edges in the level conversion section is not equal, the MMI driver with bidirectional 1-Wire master becomes unstable when the positive operating voltage is too high.
For this reason, the positive supply needs to be limited to approximately 3.3V. The bidirectional master, therefore, must be a 3V type, such as the DS2482. Using a 5V bidirectional master (e.g., DS2480B) causes the voltage at the COM and NO pin of the analog switch to exceed the V+ level, which violates the permissible operating conditions.
Verification
The circuit of Figure 6 was tested with the add-on of Figure 7. The 1-Wire master was a DS9097U-S09, which is based on the DS2480B driver chip. To ensure stability, the positive supply (V+) was set to 3.4V. The 1-Wire master operated at 5V, violating the MAX4561 analog switch’s requirement that no voltage be higher than the supply voltage. This explains the artifacts on the TP2 signal, but has otherwise no adverse effect on the function of the circuit.
Reset/Presence Detect Cycle
Figure 8 shows the signal at TP2 (top trace), TP4 (center trace), and TP6 (bottom trace). Because of the diode in the sensor, the 0V level is not fully reached when the slave asserts its presence pulse. The bottom trace shows a clean presence pulse. The positive amplitude at TP6 corresponds to the 3.4V for V+.
Figure 8. Reset/PD sequence.
Read Time Slots
Figure 9 shows the same nodes as before (TP2 = top trace, TP4 = center trace, and TP6 = bottom trace). The first slot reads a 1, the second slot reads a 0.
Figure 9. Communication time slots.
Conclusion
The circuit presented here works well for a microcontroller as 1-Wire master using separate ports for read and write. The part of the application software that generates the time slots and the reset-/presence-detect cycle, however, has strict timing requirements which may necessitate that it be written in assembly language. The add-on circuit for bidirectional 1-Wire driver chips allows application software development using a high-level language.
Due to its asynchronous operation, the add-on circuit causes a glitch when the master stops pulling the 1-Wire line low. When reading a zero, the glitch can trigger the active pullup of the driver, thereby causing a conflict between the driver’s pullup and the MAX4561’s pulldown. Therefore, when used with a DS2482 driver, the active pullup should be switched off. The glitch is also the reason why the add-on circuit for bidirectional 1-Wire driver does not tolerate 1-Wire slaves at the master’s side.
Notes:
- 1-Wire is a registered trademark of Maxim Integrated Products, Inc.
- IEEE is a registered service mark of the Institute of Electrical and Electronics Engineers, Inc.
About the author
Bernhard Linke, a principal member of the technical staff, came to Maxim in 2001, following Maxim’s acquisition of Dallas Semiconductor, which Bernhard joined in 1993. Before Dallas Semiconductor, he worked for Astek Elektronik Vertriebs GmbH, a distributor in Kaltenkirchen, Germany, and in various positions at Valvo Röhren- und Halbleiterwerke der Philips GmbH in Hamburg, Germany. In 1979 he received a Diplom-Ingenieur degree in Allgemeine Elektrotechnik from the Rheinisch-Westfälische Technische Hochschule in Aachen, Germany.
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