Displays We Love Hacking: SPI And I2C

I’ve talked about HD44780 displays before – they’ve been a mainstay of microcontroller projects for literal decades. In the modern hobbyist world, there’s an elephant in the room – the sheer variety of I2C and SPI displays you can buy. They’re all so different, some are LCD and some are OLED, some have a touchscreen layer and some don’t, some come on breakouts and some are a bare panel. No matter which one you pick, there are things you deserve to know.

These displays are exceptionally microcontroller-friendly, they require hardly any GPIOs, or none extra if you already use I2C. They’re also unbelievably cheap, and so tiny that you can comfortably add one even if you’re hurting for space. Sure, they require more RAM and a more sophisticated software library than HD44780, but with modern microcontrollers, this is no problem at all. As a result, you will see them in almost every project under the sun.

What do you need for those? What are the requirements to operate one? What kind of tricks can you use with them? Let’s go through the main aspects.

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A Simple Guide To Bit Banged I2C On The 6502

We covered [Anders Nielsen]’s 65duino project a short while ago, and now he’s back with an update video showing some more details of bit-banging I2C using plain old 6502 assembly language.

Obviously, with such a simple system, there is no dedicated I2C interface hardware, so the programmer must take care of all the details of the I2C protocol in software, bit-banging it out to the peripheral and reading back the response one bit at a time.

The first detail to concern us will be the I2C addresses of the devices being connected to the bus and how low-level bit manipulation is used to turn the 7-bit I2C address into the byte being bit-banged. As [Anders] shows, setting a bit is simply a logical-OR operation, and resetting a bit is a simple logical-AND operation using the inversion (or one’s complement) bit to reset to form a bitmask. As many will already know, this process is necessary to code for a read or a write I2C operation. A further detail is that I2C uses an open-collector connection scheme, which means that no device on the bus may drive the bus to logical high; instead, they must release the drive by going to the high impedance state, and an external pull-up resistor will pull the bus high. The 6532 RIOT chip (used for I/O on the 65unio) does not have tristate control but instead uses a data direction register (DDR) to allow a pin to be an input. This will do the job just fine, albeit with slightly odd-looking code, until you know what’s going on.

From there, it’s a straightforward matter to write subroutines that generate the I2C start, stop, and NACK conditions that are required to write to the SSD1306-based OLED to get it to do something we can observe. From these basic roots, through higher-level subroutines, a complete OLED library in assembly can be constructed. We shall sit tight and await where [Anders] goes next with this!

We see I2C-connected things all the time, like this neat ATtiny85-based I2C peripheral, and whilst we’re talking about the SSD1306 OLED display controller, here’s a hack that shows just how much you can push your luck with the I2C spec and get some crazy frame rates.

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An 128x64 OLED display with a weird image on it, showing a mouse cursor, date and time in the bottom right corner, and a whole lot of presumably dithered dots

Making Your Own Technically-HDMI OLED Monitor

One day, [mitxela] got bored and decided to build his own HDMI monitor – the unconventional way. HDMI has a few high-speed differential pairs, but it also has an I2C interface used for detecting the monitor’s resolution and issuing commands like brightness control. In fact, I2C is the backbone for a lot of side channels like these – it’s also one of our preferred interfaces for connecting to cool sensors, and in this case, an OLED display!

[mitxela] describes his journey from start to end, with all the pitfalls and detours. Going through the pinout with a broken hence sacrificial HDMI cable in hand, he figured out how to probe the I2C lines with Linux command-line tools and used those to verify that the display was recognized on the HDMI-exposed I2C bus. Then, he turned to Python and wrote a short library for the display using the smbus bindings – and, after stumbling upon an FPS limitation caused by SMBus standard restrictions, rewrote his code to directly talk to the I2C device node, raising FPS from 2 to 5-10.

From there, question arose – what’s the best software route to take? He tried making a custom X modeline on the HDMI port the display was technically attached to, but that didn’t work out. In the end, he successfully employed the Linux capability called “virtual monitors”, and found out about an interesting peculiarity – there was no mouse cursor to be seen. Turns out, they’re typically hardware-accelerated and overlaid by our GPUs, but in [mitxela]’s case, the GPU was not involved, so he added cursor support to the picture forwarding code, too.

With partial refresh, the display could be redrawn even faster, but that’s where [mitxela] decided he’s reached a satisfactory conclusion to this journey. The write-up is a great read, and if videos are more your forte, he also made a video about it all – embedded below.

We first covered the ability to get I2C from display ports 14 years ago, and every now and then, this fun under-explored opportunity has been popping up in hackers’ projects. We’ve even seen ready-to-go breakouts for getting I2C out of VGA ports quickly. And if you go a bit further, with your I2C hacking skills, you can even strip HDCP!

We thank [sellicott] and [leo60228] for sharing this with us!

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Miniature Space Invaders game

Arduino And An OLED Make This Space Invaders Cabinet Tiny

For as simple as it appears now, Space Invaders was one machine from the Golden Age of video games that always seemed to have a long line waiting for a chance to lose a couple of quarters. And by way of celebrating the seminal game’s influence, [Nick Cranch] has executed what might just be the world’s smallest Space Invaders replica.

It appears that this started mainly as an exercise in what’s possible with what’s on hand, which included a couple of quite small OLED displays. For the build photos it looks like there’s an Arduino Nano running the show; [Nick] relates that the chosen hardware proved challenging, and that he had to hack the driver library to make it work. Once he got a working game, [Nick] didn’t rest on his laurels. Rather, he went the extra mile and built a miniature cabinet to house everything in.

The video quality below may be poor, but it’s more than enough to see how much work he put into detailing the cabinet. The graphics of the original US release of the game cabinet are accurately represented, right down to the art on the front glass. The cabinet itself is made from 1.5 mm plywood which he cut by hand. It even looks like he recreated the original scheme of cellophane overlays on the monochrome screen to add a little color to the game. Nice touch!

We really appreciate the attention to detail here, with our only quibble being no schematics or code being posted. Hopefully, we’ll see those later, but for now, this looks like a fun project and a nice trip down memory lane. But if you think it’s too small, no worries — we’ve got a much, much bigger version of the game too.

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