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Sunday, November 09, 2014
Using the SDcard, RTC and SRAM on WiNode and Nanode RF
This is an update of a post to the Nanode Website from October 2012.
Part 1. Using the SD Card and RTC to make a 4 channel datalogger.
Nanode and Winode are something of a departure from the usual Arduino hardware, and when functioning as a pair, form a unique extension of the Arduino platform to include ethernet and low power wireless connectivity.
Nanode and Winode are both based on the ubiquitous ATmega328 microcontroller, but include the following common functional blocks.
1. An RFM12B low power wireless transceiver module.
2. A MCP79410 Real Time Clock. This provides RTC, two alarms, 64 bytes of battery backed RAM and 1kB of EEPROM.
3. microSD socket – accepts up to 2GB microSD cards for long term datalogging.
4. 32kB battery backed SRAM. Can be used for downloading sketches or buffering larger quantities of data from the web.
Using freely available libraries it is possible to utilise these hardware blocks, and quickly put together applications on the Nanode or Winode.
The RFM12 is connected to the SPI bus and is selected by Digital 10
The MCP97410 RTC is an I2C device using An4 and An5. It’s multipurpose output connects to Digital 3 – or Interrupt 1
The SD card is also connected to the SPI bus and is selected by Digital 4
The 23K256 SRAM is connected to the SPI bus and is selected by Digital 8 (Winode) and Digital 9 (Nanode).
In this post I will look at the operation of the SD card and the Real Time Clock.
The SD card is extremely useful for long term datalogging applications whilst the RTC, as well as providing clock and calendar functions for real time control, also provides two alarms or a square wave output which can be used to wake up the ATmega from sleep at regular intervals. The RTC also contains 64 bytes of non-volatile SRAM and 1K of EEPROM.
The SD card Library is included with the Arduino IDE. For my examples I am using Arduino IDE 1.0.1. This is an easy to use library, and if you are a newcomer I suggest Jeremy Blum’s excellent video tutorial. This has 3 examples, how to write to the SD card, read and write and how to datalog several sensors to the SD card.
For both the Nanode and Winode, the SD card is selected with Digital 4. You will need to modify the code to use the correct chip select line.
The MCP94710 is a powerful little chip – not just a RTC, but includes 2 independent alarms, 64 bytes of battery backed SRAM and 1024 bytes of E2. If you buy the MCP94712 version, it also includes a uniquely assigned 64 bit MAC address. These devices are available from Farnell for about £1.00. It is connected to the ATmega via the I2C bus, and so needs the Wire library to support its functionality. It has a multipurpose output which is connected to the Digital 3 (interrupt 1) input of the ATmega. This can output a squarewave to generate regular interrupts, or be triggered on either of the Alarm conditions. Please refer to the Microchip MCP9471X data sheet for full details of the internal registers.
I decided to write some code to exercise the main features of this RTC and also make use of the microSDcard for datalogging.
Before using these devices we have to include the SD card and Wire (I2C) libraries – so remember to add the #include statements at the top of the code.
The sketch initialises the RTC and the alarms, and then writes a 64 character text message to the battery backed SRAM.
The main loop then proceeds to log the analogue input from ADC0 to the SDcard every 5 seconds. The alarm has been configured to trigger every minute, and print out the 64 byte message to the serial terminal window, as well as illuminating the LED.
The code for this demo is available here on Github Gist. It compiles to 18054 bytes on the ATmega328.
This second sketch on Github Gist logs the four analogue input readings to the SD card, once per second. It compiles to 19166 bytes on the ATmega328. The sketch sends out the current time, the value of the four analogue input channels – and once per minute, acting on an alarm it outputs the message “The Quick Brown Foxy Leapt over the Lazy Dog whilst he snoozed”.
I found it important to declare and reserve space for the various strings:
//Create Data string for storing to SD card using the CSV Format
dataString = stringId + “, ” + stringZero + “, ” + stringOne + “, ” + stringTwo + “, ” + stringThree; // Concatenate the strings to write to the SD card
An early attempt using a different approach resulted in a memory leak which caused the ’328 to crash after about 5 writes to the SD card:
This code example will be further developed to make a real time logging, 4 channel temperature controller, as part of a central heating controller project.
The next post will combine the SDcard and RTC code example with simple 2-way wireless packet communication – allowing the temperature measurements to be send to a Nanode Gateway or other RF compatible device. November 2014 Update. It has been a couple of years since I wrote any code for WiNode. I got back from California and had to find myself a job, so WiNode went on the back burner for a while. It was always the intention to make the WiNode part of an integrated system based around the JeeNode RFM12B wireless communications library. This library is popular amongst the hobbyist community and is supported by several open source devices including the emonTx, emonGLCD and Nanode Gateway produce by OpenEnergyMonitor. The first step was to include the JeeNodes library with the code example - such that packets could be received from other RFM12B equipped nodes. I have put together a fairly simple example of this, plus the ability to read temperature values from local thermistors connected to the ADC channels 0- 3. This code is also available as a Github Gist
I am currently revisiting WiNode, with the intention of designing a new version aimed specifically at central heating control applications.
This will retain the RTC and SDcard of the original, but include a mains power supply based around a small encapsulated pcb transformer and bridge rectifier, and a pair of 8A rated mains relays.
The intention is that this variant will be compatible with existing central heating controllers, providing control over hot water and central heating functions, remotely via wireless, from either a display such as the emonGLCD or via a web Gateway such as the Nanode RF.
In fact any RFM12B or RFM69 based module should be able to interact with WiNode. This opens out applications for Raspberry Pi users, and also other applications that require mains loads to be switched - such as solar hot water system pumps.
I will cover more detail of the new design in a future post.