SPI: Let Go and Use the Force

Take a leap the next time you use SPI and don’t poll for the busy flag. “What, are you crazy? That’s the whole point of the busy flag! It’s a quick check to make sure you don’t kill a byte waiting to be shifted out!” Sure, we thought the same thing, but the other side of the coin is that it takes time to check the busy flag, and that’s time he could be transmitting data. [bigjosh2] calculates that his technique saves 20% of those wasted cycles in this particular case. And he’s “using the force” only because he’s a Jedi master able to rely on the cycle count of a chunk of assembly code.

He’s working with an AVR processor, and pumping out bits to drive the vintage LED display pictured above. The ancient chips don’t have buffered SPI so he has to blank the display while shifting new data in to prevent it from glitching. Because the display blank during the SPI transmission, the slower it goes, the dimmer the lights.

He attacks the problem with synchronous code. It takes 2 cycles for the hardware SPI to send each bit, so he twiddles his thumbs (that’s exactly what he wrote in his code comments) for 16 cycles before reloading the SPI register with his next value. This leaves it up to faith in the silicon that the shifting will always take the same number of cycles, but the nice thing about hardware is that it’s deterministic. He ends up killing a few cycles in order to save time by not polling the busy flag.

Still need a crash course in what SPI actually does? [Bil Herd] has you covered with this SPI communication demo.

Pi Zero Ethernet The Hard Way

[Alex@Raspi.tv] had the misfortune of blowing the USB hub and Ethernet port on a Raspberry Pi B+. He thought about using a cheap SPI to Ethernet board to rescue it, and while he bought the board, he never got around to interfacing it to the broken Pi. However, when he saw the Raspberry Pi Zero arrive and noticed that everyone wanted to connect it to the network, he remembered the SPI board, rescued it from his junk box, and a few hours later had Ethernet via Raspberry Pi GPIO working.

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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.