Hacking 2.4GHz Radio Control

Many modern radio control (RC) systems use frequency hopping to prevent interference. Unfortunately, hopping all over the 2.4GHz band can interfere with video or WiFi using the same frequency band. [Befinitiv] was trying to solve this problem when he realized that most of the systems used a TI CC2500 chip and a microcontroller. The microcontroller commands the chip via SPI and controls the frequency by writing into a frequency register.

Updating the microcontroller firmware was impractical. The firmware is encrypted, for one thing. In addition, the change would have to be reinserted on any future updates and repeated for every RC vendor. So [Befinitiv] took a different approach. He did a classic man in the middle attack by inserting an CPLD in between the controller and the CC2500.

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LED Matrix Failure and Vindication

If necessity is the mother of invention, what’s failure the mother of? Improvement? Anyway, [prpplague]’s second version of his roll-up 70×30 RGB LED display looks a lot better and more reliable than the first, and that’s precisely due to “failing”.

Sometimes you design the hardware around the software, and sometimes vice-versa. It’s all about the balance of pain. [prpplague] initially wired the strips together in a consistently left-to-right raster arrangement to make the coding easier, but this means long wires on the backside of the fabric returning from the right side back to the start again at the left. These long wires snagged on stuff, and pulled the solder connections apart.

600px-Dotstar-adapter-solder3The fix? Alternate rows of left-to-right with right-to-left to minimize wiring and make nice, robust connectors for the ends, and a much more elegant implementation at the expense of more complicated software to drive the device. (Alternating rows have to be flipped horizontally, so this means custom driver routines.)

The second gremlin was that the interfacing board that [prpplague] was using didn’t have enough current sourcing capability on the SPI lines, and he discovered that he couldn’t communicate reliably with the strings if the first pixel was more than 24″ of wire away from the board. Once the signal got to the first pixel, though, everything was fine. [prpplague] figured out that the RGB LEDs themselves had more drive capability than the SPI source.

The solution? Add a single pixel at the front of the chain to buffer the SPI lines and serve as a bonus status indicator. Cute.

We’d hardly call these “fails”, but rather “learning experiences”. Anyway, here’s two design “mistakes” that we won’t make when making a roll-up flexible pixel display. Thanks [prpplague].

Embed with Elliot: Multiplexing SPI Uses Few Pins

[Ralph Doncaster], aka Nerdralph, seems to be absolutely driven to see how few resources he can use on a microcontroller to get the job done. In this post on his blog, [Ralph] writes some custom bit-banged SPI code to cut the number of SPI lines necessary to drive an nRF24L01+ radio module from four down to two. That really helps if you’re using a micro with only six free pins, like an ATtiny85.

If you’re going to say, “why don’t you just buy a bigger microcontroller?”, you’re missing the point. This exercise strikes us as optimization for optimization’s sake and a dirty hack, both of which are points in its favor.  There are also a couple of techniques here for your mental toolbox. We thought it was interesting enough to look at in depth.

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Optimizing AVR LCD Libraries

A while ago, [Paul Stoffregen], the creator of the Teensy family of microcontrollers dug into the most popular Arduino library for driving TFT LCDs. The Teensy isn’t an Arduino – it’s much faster – but [Paul]’s library does everything more efficiently.

Even when using a standard Arduino, there are still speed and efficiency gains to be made when driving a TFT. [Xark] recently released his re-mix of the Adafruit GFX library and LCD drivers. It’s several times faster than the Adafruit library, so just in case you haven’t moved on the Teensy platform yet, this is the way to use one of these repurposed cell phone displays.

After reading about [Paul]’s experience with improving the TFT library for the Teensy, [Xark] grabbed an Arduino, an LCD, and an Open Workbench Logic Sniffer to see where the inefficiencies in the Adafruit library were. These displays are driven via SPI, where the clock signal goes low for every byte shifted out over the data line. With the Adafruit library, there was a lot of wasted time in between each clock signal, and with the right code the performance could be improved dramatically.

The writeup on how [Xark] improved the code for these displays is fantastic, and the results are impressive; he can fill a screen with pixels at about 13FPS, making games that don’t redraw too much of the screen at any one time a real possibility.

Simple Keypad Scanning With SPI And Some Hardware

16-button keypads have a clever method of encoding their data into 8 pins. Pins are mapped to four rows and four columns on the keypad. A user reads the keypad by bringing each row up to logic: HIGH, and reading the corresponding column values, (HIGH or LOW). Keypad scanning can be farmed out to a microcontroller with a simple finite-state machine and some button debouncing techniques. [Mario], [Glen], and [Paul] on the Netduino forums took an entirely different route: they’ve designed and implemented a Keypad Scanner using any microcontrollers SPI peripheral and a 74HC595 Shift register.

The trio’s solution is an elegant adventure into circuit design. With two diodes and a voltage divider, they devise a simple circuit that pulls the SPI MISO line LOW if a button in the corresponding circuit’s row is pushed closed. Copied four-fold, this circuit joins the rows and columns of the 74HC595 to the keypad matrix. To scan across the four columns, the microcontroller performs an SPI transfer of the key value: 0x01. To decode which button is pushed, the value received back from the SPI bus encodes which button was pushed out of the 16 possible buttons. Note: some cases for ambiguity as to “which button was pressed” do exist if multiple buttons are pushed at the same time, but for the general case where we’re punching in values one-by-one, this circuit works perfectly.

The team’s hack is a clever use of existing hardware to outsource a microcontroller’s software problem to hardware while leveraging the SPI peripheral to cleverly decrypt and retrieve data back from the keypad. Kudos to the team of three over at the Netduino Forums, and we’re always thrilled to see and idea grow from one person to the next. In case you want to take a step lower and build up the keypad itself, here’s a blast from the past that does just that.

High Speed SSD1306 Library

[Lewin] wrote in to tell us about a high speed library for Arduino Due that he helped develop which allows interfacing OLED displays that use the SSD1306 display controller, using DMA routines for faster display refresh time.

Typically, displays such as the Monochrome 1.3″ 128×64 OLED graphic display , are interfaced with an Arduino board via the SPI or I2C bus. The Adafruit_SSD1306 library written by [Limor Fried] makes it simple to use these displays with a variety of Arduinos, using either software or hardware SPI. With standard settings using hardware SPI, calls to display() take about 2ms on the Due.

[Lewin] wanted to make it faster, and the SAM3X8E on the Due seemed like it could deliver. He first did a search to find out if this was already done, but came up blank. He did find [Marek Buriak]’s library for ILI9341-based TFT screens. [Marek] used code from [William Greiman], who developed SD card libraries for the Arduino. [William] had taken advantage of the SAM3X8E’s DMA capabilities to enable faster SD card transfers, and [Marek] then adapted this code to allow faster writes to ILI9341-based screens. All [Lewin] had to do was to find the code that sent a buffer out over SPI using DMA in Marek’s code, and adapt that to the Adafruit library for the SSD1306.

There is a caveat though: using this library will likely cause trouble if you are also using SPI to interface to other hardware, since the regular SPI.h library will no longer work in tandem with [Lewin]’s library. He offers some tips on how to overcome these issues, and would welcome any feedback or testing to help improve the code. The speed improvement is substantial. Up to 4 times quicker using standard SPI clock, or 8 times if you increase SPI clock speed. The code is available on his Github repo.

Using I2C Addresses as Chip Selects

I2C has a seven-bit address space, and you’re thinking “when do I ever need more than 127 devices on a pair of wires?” So you order up some parts only to find that they have one, two, or three user-configurable address pins for any given device type. And you need a bunch more than four or eight capacitive sensor buttons on your project. What do you do?

If you’re reader [Marv G], you think outside the box and realize that you can change the addresses on the fly by toggling address pins high and low with your microcontroller. That is, you can use a single I2C address pin for each device as a chip select signal just like you would have with SPI.

That’s it, really. [Marv G] goes through all of the other possible options in his writeup, and they’re all unsavory: multiple I2C busses, a multiplexer, buying different sensors, or changing micros. None of these are as straightforward as just running some more wires and toggling these with your micro.

We’d even go so far as to suggest that you could fan these chip select lines out with a shift register or one of those 1-of-N decoder chips, depending on how many I2C devices you need to chip-selectify. (We’re thinking 74HC595 or 74HC154.)

Along the way, we found this nice list of the number of address pins for a bunch of common peripherals provided by [LadyAda], in case you don’t believe us about how ubiquitous this problem is. How many devices on that list have one (1!!) address pin?

At the end of his post, [Marv G] asks if anyone else has thought of this chip select trick before. We hadn’t. Here’s your chance to play the smart-ass in the comments.