MatHertels WebLog: April 2013

30 April 2013

RGB pixels from 1998

Last year I saw these fluorescent lamps from the Omega company that were used to display huge “bildboard” signs.

No, blue LEDs have not been evented yet.

14 April 2013

Hints on using NRF24L01+ with Arduino

I like to share some liks to the ressources on the internet that I found helpful for implementing projects using the Ultra low power 2.4GHz RF Transceiver NRF24L01+ with the Arduino platform.

The chip specifications

The NRF24L01 (no + at the end) was replaced by the NRF24L01+ (or sometimes called NRF24L01P). Be sure to get the new chip when buying an breakout board.

The nRF24L01+ is drop-in compatible with nRF24L01 and on-air compatible with nRF2401A, nRF2402, nRF24E1 and nRF24E2. Intermodulation and wideband blocking values in nRF24L01+ are much improved in comparison to the nRF24L01 and the addition of internal filtering to nRF24L01+ has improved the margins for meeting RF regulatory standards.

The old chip: http://www.nordicsemi.com/eng/Products/2.4GHz-RF/nRF24L01

The new chip: http://www.nordicsemi.com/eng/Products/2.4GHz-RF/nRF24L01P

You should read the nRF24L01+DataSheet, even if it is long and with many details – but that’s what you will need: http://www.nordicsemi.com/eng/content/download/2726/34069/file/nRF24L01P_Product_Specification_1_0.pdf

About ShockBurst^TM

This is a nice to read whitepaper with a description of the ShockBurst^TM transfer features of the chip:

http://www.semiconductorstore.com/pdf/NewSite/nordic/WP_nRF240x_ShockBurst.pdf

Arduino compatible Libraries for NRF24L01

There is no official “Arduino” library that is maintained by the core Arduino team so you have to look for the available libraries on the internet.

The web site http://www.tinkerer.eu/ from Stefan Engelke in 2009 provides a native NRF20L01 library, the original MiRF library (not Arduino specific) and a native SPI library See http://www.tinkerer.eu/AVRLib/nRF24L01, and http://www.tinkerer.eu/AVRLib/SPI . I mention this library because it uses interrupts that most Arduino samples don’t.

The MiRF library was ported to the Arduino platform and is using the Arduino SPI library and is described on the arduino playground article: See http://playground.arduino.cc/InterfacingWithHardware/Nrf24L01

J. Coliz (a.k.a. maniacbug) implemented a ARDUINO specific library often called the RF24 library. Its available at https://github.com/maniacbug/RF24/ and was used as the base for implementing other libraries (see below) the last commit to this projects was last on 2012/06/23.

Greg Copeland forked (copied over) the library from maniacbug and added some useful stuff including a scanner sample application. He is currently (2013.04.09) supporting his work. See https://github.com/gcopeland/RF24.

Arco van Geest (a.k.a. gnulnulf) also forked the library from maniacbug is currently adding support for the Raspberry Pi. See https://github.com/gnulnulf/RF24

Stanley Seow forked the version gnulnulf https://github.com/stanleyseow/RF24 also working on upport for the Raspberry Pi.

Critics

There are too much libraries out there for my opinion and those starting from J. Coliz are using the GNU General Public License (not LGPL) and therefore are not compatible with the Arduino license policy.

J. Coliz also used the stdio.h printf() functionality ??? causing more usage of the program memory

For my personal projects I used the library from Greg Copeland for my first steps to discover the functionality of the chip.

Samples and Projects

Poor Man's 2.4 GHz Scanner:
http://arduino.cc/forum/index.php/topic,54795.0.html

Home Automation – Designing a remote module:
http://geekboy.it/projects/home_automation/home-automation-design

Sample without using a library:
http://blog.iteadstudio.com/nrf24l01-wireless-module-with-arduino/

A sample using the MiRF library:
http://www.bajdi.com/playing-with-nrf24l01-modules/

Step-by-Step introduction using the RF24 library:
http://arduino-info.wikispaces.com/Nrf24L01-2.4GHz-HowTo

Network Layer for RF24 Radios:
http://maniacbug.github.io/RF24Network/index.html

A very detailed tutorial in German:
http://www.mikrocontroller.net/articles/NRF24L01_Tutorial

Other Stuff

These links are not related to Arduino itself but when reading you may also find some interesting information.

This web site contains several tutorials from 2011 that still apply: http://blog.diyembedded.com/
http://nrqm.ca/nrf24l01/
http://wiibrew.org/wiki/Logitech_USB_steering_wheel

...

07 April 2013

Update to the OneButton Arduino library

I wrote this small Arduino library more than 2 years ago for some of my projects to use a single input button for multiple purposes. This library detects clicks, doubleclicks and long press situations without a lot of coding on the sketch side.

This enables you to reuse the same hardware button and input pin for several functions and lowers the hardware invests.

I just updated it including a new sample that shows how to setup the library and bind a "machine" that can blink the LED slow or fast.

See Arduino OneButton Library on my side.

06 April 2013

Using the SPI bus with Ethernet, SD-card and NRF24L01 and other slave devices together

I currently work on an project using the popular NRF24L01 module where I came across the question how to avoid conflicts of new designs and existing Arduino boards.
The problem when using the W5100 based Ethernet shield including the SD Card adapter and the NRF24L01 Modules together with a Arduino UNO board is that all of them are using the same SPI bus for the data transfer to the chips. Here is an overview of how to make them work all together.
Here the master is always the same ATMEGA328 chip on the Arduino board and the 3 chips are slave devices.

The SPI bus concept

A brief explanation of the SPI bus can be found on wikipedia. See http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus
The SPI bus needs 4 signals to work:
SCK or SCLK (serial clock) The clock signal that sets the tempo for any data transfer. This signal is generated by the master chip.
On the Arduino Uno this signal is available on pin 13 and ICSP 3.
MISO (master input, slave output) This signal line transfers the data from a slave device to the master chip.
On the Arduino Uno this signal is available on pin 12 and ICSP 1.
MOSI (master output, slave input) This signal line transfers the data from the master to the slave devices.
On the Arduino Uno this signal is available on pin 11 and ICSP 4.
SS or CSN (Slave Select) This signal makes a slave active by selecting it with a low signal.
On the Arduino Uno this signal has to be discussed…
Remark: The signal lines SCK, MOSI and MISO are also available on the ICSP header, see below.
The signal lines SCK, MOSI and MISO can permanently remain connected to all participating chips. However the Slave Select signal has to be different for every device.

Advantages in using the ICSP header

The SPI signals on an Arduino Mega board are not available on the pins 10, 11, 12 and 13 because this board uses another processor. But because programming an ATMEGA chip through the ICSP is internally also using the SPI interface, the signals are available on the ICSP header too. With the Arduino Ethernet Shield Version 06 the design has changed so that the SPI signals are not taken from the pins 11, 12 and 13 but from the pins ISP 4, 1 and 3. If you use an Arduino UNO or 2009 however these pins are connected on the main board. When designing new shields for Standard and MEGA boards with SPI usage I recommended using the ICSP header like the Ethernet Shield does.

Pin 10

The Arduino pin 10, also labeled SS, also known as PB2 for ATMEGA328, is used by ATMEL to indicate whether the processor is used as an SPI master or slaver device. That pin 10 must remain an output or the SPI hardware will go into 'slave' mode.
Using this pin 10 for example on the Ethernet Shield is fine, but if you use other SPI slave devices, keep in mind to no use this pin for input purpose – better don’t use it to avoid conflicts.

Using multiple SPI bus slave devices

When using multiple slave devices on the same SPI bus there is no principle problem with the lines SCK, MOSI and MISO. Because every slave device needs its own slave select signal there are conflicts using the available Arduino pins. When you stack multiple ARDUINO shields on a single Arduino board you may also have conflicts with other pins the shield uses so you have to take care of other conflicts too. Here are some common pin usages:

Arduino Shield	Salve device	Arduino Pin
Ethernet version 01	W5100 chip Slave Select	10
Ethernet version 06	W5100 chip Slave Select	10
Ethernet version 06	SD card Slave Select	4

Using a NRF24L01 radio module

In different samples where the NRF24L01 radio is using the Arduino pin 9 for SS and using pin 8 for the Chip Enable signal but also 8 and 7 is used in same samples. There is no standard definition right now and therefore you have to tell the NRF24L01 library what signals to use for Slave Select and Chip Enable.
I personally recommend using 7 for Chip Enable and 8 for Slave Select because the pin 9 is one of the possible PWM output pins. When designing new Shields these pins assignments should be changeable in any way, just to be friendly to other shields… and having the chance to settle conflicts.