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ESDK Gets Remote Sensor Capabilities

ESDK Cabled Range Extender (CRE) board

ESDK Cabled Range Extender (CRE) boards allow for remote mounting of sensor modules.

In this article, we’ll be taking a look at the new ESDK Cabled Range extender board features and the design process.

The CRE-A Board

ESDK Cabled Range Extender board CRE-A

The ESDK CRE-A board features a connector to plug into the existing ESDK ecosystem, an eight-pin RJ45 connector that allows for the use of a standard Ethernet cable for extension of the sensor chain, and a 12V boost converter to reduce voltage losses owing to cable resistance.

CRE-A features a Texas Instruments boost converter controller

On board the CRE-A is a Texas Instruments LMR62014 “SIMPLE SWITCHER” boost converter controller that requires only a small handful of external components, including one inductor and diode, and a handful of passives. We specifically picked this regulator as it is supported by TI Webench Designer, which massively reduces the legwork required to produce a working design.

Parameters were picked so that the boost converter would provide 12V at up to 350mA, switching at 1.6MHz, helping to keep the switcher footprint smaller whilst also offering a greater transient response. The output is then fused down to 200mA to be on the safe side, and to avoid overloading the regulator.

Graphs showing operation under various conditions

A simulation is also run as part of the Webench design process with a number of plots produced that illustrate the operation of the converter under various output conditions, including plots of efficiency at various output currents.

Webench also produces a sample PCB layout, which we roughly based our layout around whilst remembering to adhere to best practices set out in the datasheet and app notes for the switcher.

CRE boards feature the NXP PCA9615

Both CRE boards feature the NXP PCA9615 differential I2C bus buffer to help ensure that the I2C bus can be transmitted over long distances without corruption. The bus buffer requires a minimum number of external components, and a suitable bias resistor arrangement to minimise reflections from the cable.

The PCA9615 supports two voltage rails, one to supply the system interface and one to drive the cable. To improve noise immunity the cable side is driven with 5V rather than 3.3V — the level shifting between the two voltages is provided as part of the differential conversion.

Routing of the signals related to the I2C interface was less critical than the boost converter, but we opted to provide ample separation between the two to reduce the chance of any issues.

The CRE-B Board

CRE-B Board

The ESDK CRE-B board is very similar to the CRE-A, apart from the boost converter which has been replaced with a buck converter that produces a 5V output and a linear regulator to then regulate the 5V rail to 3.3V.

The ESDK CRE-B board has a buck converter

Again, a Texas Instruments switching controller was picked — this time the LMR12010 which comes in a compact SOT-23 package. This part is supported by Webench, which we used to do the design and simulation.

The output parameters were set at 5V with a maximum current of 300mA and a switching frequency of 1.6MHz, again to keep component sizes down and improve transient response — important as the CO2 sensor can sink quite considerable pulses of current. Fusing is provided by a polyfuse that protects both the 5V and 3.3V rails.

On the buck converter, a zener diode is utilised to reduce the voltage to the “BOOST” pin — in the LM12010 datasheet the maximum voltage is 5.5V, and we’d be seeing somewhere closer to 12V. This is a solution suggested by TI to safely run the IC.

Graphs of CRE-B produced under differing operation

Again, plots are produced when the simulation is run. All the plots confirmed that the regulator would behave, and with a reasonable efficiency of up to 88%. The PCB layout was copied into our design and slightly modified to accommodate the two layer board stack-up.

As before, the same PCA9615 bus buffer was used to provide the differential-single ended conversion. On this board, two pull-up resistors need to be added to the normal I2C interface as the driver does not include any.

TVS diodes were added to all the buses on the board, including the two differential I2C lines and then the single-ended lines heading to the sensor chain. This provides additional reinforcement to help protect the components on the board from ESD damage.

Mod Wires Galore

Initial bring-up of the CRE boards was (mostly) a success, with the power supplies working and outputting correct voltages for both the 5-12V boost converter, and the 12-5V & 3.3V buck converter and regulator.

Problems began to arise when we moved on to testing the I2C drivers, in that the bus was being pulled up on both ends but no data was being transmitted down the cable when running an I2C bus scan.

Some probing with an oscilloscope on the CRE-A end showed the outputs of the PCA9615 were stuck at the voltage levels set by the bias resistor divider — eventually leading to us checking our schematics to ensure everything was connected as it should be, which is where the moment of realisation hit.

We had inadvertently swapped the SDA line positive and negative outputs to the wrong side of the bias network, which seemed to completely stop the chip operating — even SCL would not toggle with a clock on the input. This error was present on both CRE-A and CRE-B boards, so a quick modification would be needed for both.

Wire modifications made to the boards

As we didn’t have a fibreglass pencil to hand we used the back side of a scalpel blade to scrape away the solder mask, revealing the bare copper traces below. Cuts were then made to disconnect the traces at a convenient point.

A generous application of flux and solder to tin the traces, then two short sections of magnet wire from a wiring pencil were soldered to the tinned traces and jumpered to the correct points on the board.

We then connected the boards with a length of Ethernet cable and powered them back up — much to our relief I2C data was successfully passed between the two. We then modified the schematics and board layout, and pushed the changes to GitHub.

To Finish

The CRE board set should enable all manner of new use cases for the ESDK, by enabling sensors to be located at least up to 100m away from the main unit, with boosted and regulated power, plus robust differential communications, to the remote end. An obvious application is making outdoor air quality sensing possible using the existing kit, together with the CRE boards and an appropriate outdoor enclosure.

Design files for the CRE boards are available on GitHub.

Engineer of mechanical and electronic things by day, and a designer of rather amusing, rather terrible electric "vehicles" by night.
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