Entry 20: Redesigns 'R Us

A project log for Aiie! - an embedded Apple //e emulator

A Teensy 3.6 running as an Apple //e

Jorj BauerJorj Bauer 07/09/2020 at 14:270 Comments

Welcome to Redesigns 'R Us, where we come up with new ways to do old things!

Over the last few years, my Aiie has mostly been sitting collecting dust. Sure, I spent some time working on WOZ disk format (which I love), and I've got a half dozen private branches of the code repo where I've been working on various features - but there are two major obstacles that I've talked about before that have kept me from really pursuing any of them:

  1. There's only so much RAM. When I'm working on code that's timing-critical, and then have to read a new track from the virtual floppy, the interaction with the MicroSD card is really a killer. I'd rather cache floppy images in RAM and forget about it - but there just isn't enough RAM to do that.
  2. There's only so much CPU. In some of my branches, I've been working on peripherals that have complicated timing interactions with the 65C02 - which lead to increasingly complex hoops I've been jumping through to try to keep them working. In the end this became too complex and I had to put it down for a while.

But now there's the Teensy 4.1! It's a nice bump, from 180 MHz to 600 MHz; and it has pads for an additional 16MB of PSRAM. Those sound enticing - but come at a cost; there are fewer pins available.

The Teensy 3.6 had a boatload of pins available via pads on the underside of the board. Aiie v1 used many of those (lazily, via the Tall Dog breakout board). But with something like 17 fewer pins (if I've counted rightly) on the Teensy 4.1, I've got a problem.

So, redesign decision 1: how do I squeeze the same hardware in to a smaller footprint?

Well, back in Entry 17, I faced the same general question when I experimented with adding external SRAM. My choice then is the same as it is now - swap out the display. I originally picked a 16-bit parallel display because I wanted to throw data at it very quickly. And it did, at first, until I wound up complicating the codebase which dropped the framerate to a sad 6FPS. (This is on my list of "things that bug me about Aiie v1" - as it became more Apple //e-correct, it became much less responsive.)

So the 16-bit nature of the display isn't the problem. It's the code (primarily) and the available CPU (to a lesser extent). The display I picked then is the same one I'm picking now - the ILI9341, an SPI-driven display. In theory I can run it via DMA which will reduce the CPU overhead too. My only beef here is that the version of the ILI9341 that's in the Teensy store is a 2.8" display, where I picked a 3.2" display for Aiie v1 - but there are 3.2" versions of the ILI9341 available, and I have one of them, so I'm pretty satisfied on that front.

And with that much information, it's time to try it out! I've still got the original Aiie prototype board sitting around, and it doesn't seem too daunting to rewire it for this. First step, remove all the stuff I don't need, like that nRF24L01 serial interface that I wound up replacing with an ESP-01 in the final v1 circuit and all of those extra pins broken out from the bottom of the Teensy 3.6...

... not too hard.

There's also this rat's nest on the backside that has to go.

And then I need to figure out how I'm powering it. Lately I've been liking these MakerFocus battery charger / boost modules - they're obviously intended as the core of a battery booster pack, and fairly elegantly handle the charging of the battery, boost to 5v, and display of the battery's state. Single presses of the button turn it on, and a double-press turns it off. So adding one of those and a 3.3v linear regulator to safely drive the display...

Re-add the rat's nest of wiring underneath...

use some velcro to tape the battery in place...

and what do you know, if we hand-wave through the little bit of code that needed adjusting, we wind up with

36 frames per second on mostly-unoptimized code. Oh yeah, I like this.

The new code is in my Github repo, in the 'teensy41' branch. If you look at the timestamps you'll find it's got me very excited - I've been working on this as much as possible between other tasks, and it has me distracted enough that I took a day off of work to try to get some of this out of my head today.

Post on Hackaday: check. Next up? Board layout, so I can get a fabrication house started on the board manufacturing for Aiie v2...