Testing DS18B20 temperature sensing on emonTx v3

2013-05-25 09:34:00 +00002013-05-25 09:34:00 +0000 Trystan Lea
two minutes reading time

temperature
emonTx

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Hello, so I thought Id try something new on this blog, I'm going to try writing short blog posts about progress and general work on things day to day, these are not intended to mark significant developments or milestones which many of the other blog posts have tended to be, something more like a log book that will hopefully give insight into the development process.

Testing DS18B20 temperature sensing on emonTx v3

Glyn and I with help from Robert are continuing with testing emontx V3, there have been a few niggling issues that we're working on but its pretty much there. This morning I tested the DS18B20 temperature sensor connection, it all worked fine, one of the nice things of the new emontx is that there are screw terminals for connecting up the temperature sensor which makes connecting up the encapsulated DS18B20 temperature sensors much easier.

DS18B20 connected to emontx v3

Another new addition for temperature sensing is the ability to switch the sensor power pin on and off for use when powering the emontx of batteries (this avoids the hack introduced in the low power temperature node that uses the emontx v2.2 pcb). Anyway the circuit was originally designed with analog 5 (used as digital pin 19) being used for data and digital 5 used for power. While reading about using analog inputs as digital pins I came across this note on the arduino site:

The Atmega datasheet also cautions against switching analog pins in
close temporal proximity to making A/D readings (analogRead) on other
analog pins. This can cause electrical noise and introduce jitter in
the analog system. It may be desirable, after manipulating analog pins
(in digital mode), to add a short delay before using analogRead() to
read other analog pins.

It turns out one can just switch the atmega pins used around so analog 5 (digital 19) is used for power and digital 5 is used for the data which will be higher frequency switching and so good to bring that off the analog pin. As power switching is only needed in battery operation one could make a temperature measurement after the analog read section just before putting the emontx to sleep which should not cause interference.

Update (15/06/13): On future emonTx V3 PCB revisions 3.x ADC 5 (Dig 19) and Dig5 have been swapped round on the PCB to fix this issue. Dig 5 is now DS18B20 one-wire signal and ADC5 (Dig19) is now DS18B20 power.

If no DS18B20 temperature sensor is connected the ports can be used for other functions. ADC 5 can be used as a analogue input/output and Dig 5 (with R27/R24 4.7K pull up removed) can be used as a general Digital I/O with PWM capabilities.

The DS18B20 data 4.7K pull-up resistor on Dig 5 and the Dig 2 IRQ 10K pull-down resistor have been designed to take a thru-hole resistor if the SMT resistor was removed to make it possible to change the value of these resistors. To engage in discussion regarding this post, please post on our Community Forum.