Lars Bengtsson

A microcontroller-based lock-in amplifier for sub-milliohm resistance measurements

This paper presents a novel approach to the design of a digital ohmmeter with a resolution of <60 μΩ based on a general-purpose microcontroller and a high-impedance instrumentation amplifier only. The design uses two digital I/O-pins to alternate the current through the sample resistor and combined with a proper firmware routine, the design is a lock-in detector that discriminates any signal that is out of phase/frequency with the reference signal. This makes it possible to selectively detect the μV drop across sample resistors down to 55.6 μΩ using only the current that can be supplied by the digital output pins of a microcontroller. This is achieved without the need for an external reference signal generator and does not rely on the computing processing power of a digital signal processor.

Analysis of Direct Sensor-to-Embedded Systems Interfacing: A Comparison of Targets' Performance

This paper is concerned with the direct interfacing of resistive sensors to different embedded targets. The author uses the idea of direct sensor-to-microcontroller technique where analog sensors are interfaced directly to inherently digital controllers and we compare the performance of this technique when applied to a typical microcontroller PIC18, a CPLD and an FPGA. Experimental results show that 5 V systems, like the PIC18 controller, have an advantage over 3.3 V systems in terms of better precision performance, while the CPLD outperforms both the microcontroller and the FPGA in terms of accuracy. The accuracy depends mainly on the output impedance of the systems I/O ports and the precision depends mainly on trigger level noise. The PIC18 controller also has the best performance in terms of linearity and sensitivity. A lot of works have been published concerning direct interfacing to microcontrollers, but little attention has been paid to alternative targets like CPLD and FPGA. This work will benchmark these different kinds of targets and prove that the direct interfacing technique can also be applied to CPLDs and FPGAs.

Implementation of high-resolution time-to-digital converter in 8-bit microcontrollers

This paper will demonstrate how a time-to-digital converter (TDC) with sub-nanosecond resolution can be implemented into an 8-bit microcontroller using so called direct methods. This means that a TDC is created using only five bidirectional digital input-output-pins of a microcontroller and a few passive components (two resistors, a capacitor, and a diode). We will demonstrate how a TDC for the range 1-10 μs is implemented with 0.17 ns resolution. This work will also show how to linearize the output by combining look-up tables and interpolation.

Single-Chip Implementation of Level-Crossing ADC for ECG Sampling

This work demonstrates for the first time the implementation of a level-crossing analog-to-digital converter (LC-ADC) in a single, commercially available IC (that costs less than

New design ideas for TDR-based liquid level detectors

2). The implementation utilizes adaptive threshold levels in order to prevent overload distortions for fast-changing signals. The entire design is based on a 20-pin PIC16F1769 microcontroller from Microchip and no external components are required. In fact, the only external circuitry required is a single jumper wire. This is due to the fact that the new generation of microcontrollers have integrated core-independent hardware, analog as well as digital. This design takes full advantage of the core-independent logic and analog blocks in a PIC16F17xx circuit to implement the LC-ADC technique that so far has required multiple-circuit designs or ASIC implementation. The design is demonstrated on a standard electrocardiogram (ECG) signal.

Teaching transforms: a vector approach

This work suggests some new solutions to TDR-based liquid level detectors. A 3D-printed waveguide cylinder has been designed in order to increase the sensitivity of the level measurement. By wrapping the probing waveguide-pair around a cylinder, an inclination angle ϕ is introduced which increases the time-of-flight by a factor of 1/sinϕ and hence the level resolution is increased (by the same factor). This work also illustrates how nanosecond pulses (for TDR systems) can be generated by applying a technique usually found in FIMs (Field Ion Microscopes). A time resolution of 2 ns/mm (of liquid level) is reported and a generic nanosecond pulse generator is demonstrated.

Implementation of Vernier TDCs in 8-bit microcontrollers

This article treats the problem of introducing transform theory (Fourier, Laplace, z) to undergraduate students and we suggest a vector approach which means that signals (functions of time) should be treated as vectors from the beginning and that transforms are introduced as a scalar product; the transform should be presented as a tool to analyse the signal exactly in the same way as the dot product is used to analyse an ‘arrow’ vector in a Cartesian space. Hence, the transform becomes a tool to find the signals magnitude in the directions of the basis vectors.

Electromagnetic Interferences in Electrical Measurement Systems Demystified

This paper discusses the aspects of implementing a TDC with Vernier resolution in a microcontroller system. Results will show that the proposed solution have a potential time resolution corresponding exactly to the theoretically expected resolution (equal to the time difference in the Vernier clocks period), but also that there is an inherent sample-to-sample uncertainty due to the fact that microcontrollers cannot compare two (running) timer registers in hardware. The moment of coincidence of the Vernier clocks must be detected in software and that will generate an uncertainty in the coincidence detection that depends on the microcontroller architecture. In the design example proposed, a time resolution of 2 ns is achieved using a PIC microcontroller clocked with a 20 MHz. However, the proposed method is general and resolution is limited only to the frequency matching of the two Vernier clocks.

Generation and measurement of pulses and delays with RISC-controllers

This paper is concerned with the practical aspects of protecting an electrical measurement system against different kinds of interfering cross-talk. We categorise different kinds of cross-talk, describe their origin and how to decouple them in order to protect the system. This presentation is a primer for students or application engineers concerned with the practical operation of an electrical measurement system. We explain cross-talk phenomena with a minimum of electromagnetic theory, focusing more on understanding the origin of cross-talk in terms of basic electric and magnetic circuits rather than advanced electromagnetic wave equations, and how to decouple them. Compared to a typical textbook presentation on the subject, this work categorises all kinds of electromagnetic interferences using a minimum of mathematical equations, clearly explaining their origin and illustrating how each one is decoupled.

Lookup Table Optimization for Sensor Linearization in Small Embedded Systems

An instrument for the generation of pulses and delays has been ndeveloped for applications in laserspectroscopy experiments. The pulses from a nchopped laserbeam are counted, and when a preset value is reached, two signals nare generated: one delayed trigger pulse with constant length and one delayed n‘shutter pulse’ (referred to as the ‘shutter window’ below) which opens a light shutter nand allows one light pulse to enter the experimental region and excite the sample. nThe delays of the trigger pulse and the shutter window and the width of the shutter nwindow can all be set independently to any value from 0 up to 50 ms with 0.05 ms nsteps. A separate microcontroller measures the width of the shutter window with an naccuracy of 1 nmicro second ns. The pulse delays and the width of the shutter window are set nwith thumbwheel potentiometers connected as voltage dividers. A microcontroller nreads the set values with a four-channel, 12-bit AD converter. This technique nsaves a lot of hardware wiring as well as software writing when compared to the nalternative of using separate, BCD-coded thumbwheels for each timesetting. nIn total, the instrument consists of two independent RISC-controllers: one nPIC16C55 for counting chopper pulses and generating pulses and delays, and one nPIC16C74 for measuring (and displaying) the width of the shutter window. The nwidth of the shutter window is measured with 1 nmicro second resolution by taking full nadvantage of two different peripherial I/O devices in the PIC16C74: the 16-bit ‘input ncapture’ module and the external interrupt facility.