PewPew Pi

Gaming on the Raspberry Pi Pico

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I'm still working on improving the PewPew series of devices. #PewPew M4 is pretty close to my ideal, but it would be nice to have something with a little bit more luxury: larger screen, volume control, more RAM memory. The Raspberry Pi Pico module seems to be a good vehicle for this: it has enough pins to use parallel interface to drive a bigger screen fast, and it has a lot of memory for such a tiny chip.

It won't be as cheap, of course — the screen alone is three times more expensive — but I think I can still keep the price reasonable.

  • On Hold

    deʃhipu06/11/2021 at 21:15 0 comments

    Since my experiments show that using the parallel interface for the display doesn't make it fast enough to handle the bigger display, I don't see much point in continuing this project. The larger display with parallel interface was really the most important innovation compared to my other projects.

    I might resurrect it later on, if I find a way to speed things up, or maybe use a different microcontroller again. For now, I'm putting it on the back burner.

  • Parallel Bus

    deʃhipu03/25/2021 at 22:24 0 comments

    The displays arrived a few days ago, but I really didn't have the energy to get them working until today. Yesterday I found my Universal Display Breakout Board™ and soldered the display to it. Today I connected a Feather RP2040 to it, and tried to get it working. Turns out there are a couple of tricks to it.

    First of all, the IM0-3 pins are not broken out, so there is no way to switch the interface mode, and it comes in the 16-bit parallel mode by default. But wait a minute, the Aliexpress page said it was compatible with 8-bit mode, so how? I asked the seller, but before I got the answer, I noticed this little note on the screenshots of the datasheet at the product page:

    That, I assume, explains, in perfectly clear Chinese, how to switch the iterface modes. It even tells you which pins to use in each mode! I mean, probably. I don't read Chinese. But let's look closer at the display:

    Ah-ha! The aforementioned R16 and R8, and only one of them is populated. And that is a 0Ω resistor, if I'm not mistaken. So I only need to move it from R16 to R8 and that should switch the mode. Easy. Later the seller confirmed this, I assume this is direct translation of the above note:

    When R16 connect resistance, it is 16-bit interface, DB15-DB0 are useful (default 16-bit)
    When R8 connect resistor, it is 8-bit interface, DB15-DB8 are useful

    With that out of the way, there is one more trick. Have you noticed how it tells you that DB15-DB8 should be used in 8-bit mode? But the datasheet for ILI9341 says DB0-DB7... I wonder why they specified those pins in reverse order, hmm... Yes, of course the pins are reversed. What is labeled as DB15 is actually DB0, and so on. I feel really smart for figuring it out at first try, before I connected the wires.

    But no worries, I immediately felt less smart for making stupid off-by-one errors connecting the wires, so that's balanced.

    The final trick relates to the backlight LEDs. The pinout has two pins marked as LED-A, and five pins marked as LED-1 to LED-5. So logically, the two pins are common anodes (doubled so that you can put more current through the pins), and each of the five LED pins is the cathode of one of the five backlight LEDs. So in the final design, we will need a MOSFET on the anodes, and five resistors on the cathodes. Well, closer examination of the traces tells a different story. Both LED-A pins merge into a single trace — that is somewhat expected, even though that trace is no wider than others. But also all the cathodes merge into a single thin trace, and then split again near the display where the backlight is connected! That is some bad practice there, but at least that saves my four resistors, I suppose.

    With everything connected, a little bit of code:

    import displayio
    import board
        b"\x01\x80\x80"  # Software reset then delay 0x80 (128ms)
        b"\xc0\x01\x23"  # Power control VRH[5:0]
        b"\xc1\x01\x10"  # Power control SAP[2:0];BT[3:0]
        b"\xc5\x02\x3e\x28"  # VCM control
        b"\xc7\x01\x86"  # VCM control2
        b"\x36\x01\x38"  # Memory Access Control
        b"\x37\x01\x00"  # Vertical scroll zero
        b"\x3a\x01\x55"  # COLMOD: Pixel Format Set
        b"\xb1\x02\x00\x18"  # Frame Rate Control (In Normal Mode/Full Colors)
        b"\xb6\x03\x08\x82\x27"  # Display Function Control
        b"\xF2\x01\x00"  # 3Gamma Function Disable
        b"\x26\x01\x01"  # Gamma curve selected
        b"\xe0\x0f\x0F\x31\x2B\x0C\x0E\x08\x4E\xF1\x37\x07\x10\x03\x0E\x09\x00"  # Set Gamma
        b"\xe1\x0f\x00\x0E\x14\x03\x11\x07\x31\xC1\x48\x08\x0F\x0C\x31\x36\x0F"  # Set Gamma
        b"\x11\x80\x78"  # Exit Sleep then delay 0x78 (120ms)
        b"\x29\x80\x78"  # Display on then delay 0x78 (120ms)
    bus = displayio.ParallelBus(
    display = displayio.Display(bus,...
    Read more »

  • Display

    deʃhipu03/05/2021 at 13:19 0 comments

    The display can be considered the centerpiece of this device. It's the largest and most expensive component. Pretty much everything else is decided by its choice — the size, the shape, the connections, etc. So let's look closely what we have to choose from.

    I'm a huge lover of pixel art, and I want the games for this console to have large, visible pixels. That means, on one hand, that we want a display with a large diagonal, but also, at the same time, we want low resolution, so that the displayed pixels are physically large. Low resolution is also important for two other reasons: the less data we have to send to the display, the faster we can do it and the more time the microcontroller has for other calculations, and it's also much easier to learn about game development at low resolution, when you can actually see what is happening and can count pixels between things with your naked eye.

    This is why for #PewPew M4 I have settled on the 1.8" ST7735 display — with the resolution of 160x128, it's the largest pixels you can have, and the $4-ish price is also very nice. Unfortunately, 1.8" is still a bit small. So what can we do?

    What if we doubled the size of the pixels, and used a display with larger resolution? One immediate problem is that we would need to send four times more data per frame, on average. But what if we also switched from a 1-wire SPI to 8-wire parallel protocol? That's eight times faster, so it should still give us two times speedup!

    So what are our options for a doubled resolution? If we want our display to be cheap and easy to source, then we have basically two options: ILI9341 and ST7789V, which are both pretty much equivalent. They have 320x240 resolution, which is almost double of 160x128, except we lose those 16 pixels vertically. I think I can live with that. A little bit of browsing and looking around lets us conclude, that the largest ILI9341 display you can find is 3.2" diagonal — that is almost twice the size of the ST7735! As usual, there are variants with different interfaces, but considering we need parallel interface, I think the best for us is the Z320IT002 — a 3.2" display with 37 pins.

    Now, that we know the dimensions of the display, we can say a little bit more about the device we want to build. It needs to be at least 8.5cm wide, in order for the functional part of the display to be centered on it. I could place the buttons on the sides, for a Switch-like landscape mode, or under the screen, for a more classic Gameboy-like portrait layout — both should be wide enough to hold the device comfortably. However, if I went for the landscape mode, then the device would be wider than 10cm, which pushes us into the area of expensive PCBs. Anything that fits in 10x10cm is relatively cheap these days (including laser-cutting of the acrylic case), but going over that makes the price grow four times larger. I could make it use several smaller PCBs, but that's again increasing the price, as now you have to pay for two or more PCBs instead of just one. So let's keep it within the 10x10cm limit. What about the height? If we look for a "standard" size that is close to the 8.5cm width, we will find the ISO B7 format, also known as ISO/IEC 7810 ID-3, at 12.5x8.8cm. But that takes us outside of our 10x10cm comfort zone. So why not just keep the minimal required 8.5cm width, and make the height the same? Turns our we should be able to fit everything in that form factor nicely, as long as the Raspberry Pi Pico goes on the back of the device.

    By the way, that display is also available with a touch screen, but I think I'm going to ignore that option — it makes it a bit more expensive, makes the whole assembly thicker, and it would require me to rethink the whole case — I could no longer have transparent acrylic in front of the screen! Also, I think it would unnecessarily complicate the games, and shift the focus away from what I want to teach with this.

  • Challenges

    deʃhipu03/05/2021 at 11:16 0 comments

    You might think that there is absolutely nothing challenging in simply connecting a screen and a bunch of buttons to a microcontroller development board, and you would be right. I have done it so many times by now, that it should be simple even for me. However, as the saying goes, the devil is in the details.

    First of all, this is the first time I'm using the Raspberry Pi Pico development board, and it's not like other development boards out there. On one hand, it has a build-in buck-boost converter, which makes it nice to use with the batteries, on the other hand it doesn't have the power switching circuitry that disables battery when USB is connected — so there is an extra bit of electronics with a MOSFET needed to be added. I'm worried about this, because I have always had bad experiences with power circuits like that.

    Next is the audio output. On my previous consoles, there simply was a DAC pin, where you connected the speaker or the amplifier and speaker, and you were done. Here the audio output is PWM-based, and I will need a low-pass filter to smooth it a bit before feeding to an amplifier. Plus, I want to have a volume control, but the only small sliding potentiometer I found is around 9kΩ, which is a bit low to just make it a simple voltage divider. Some experimenting will be needed, and again, analog circuits are scary.

    Then there is the display backlight. It uses four internal LEDs, and I could do what I did previously, just connect them straight to the battery with a current-limiting resistor. But it would be nice to have backlight control, and even a light sensor, to adjust the backlight brightness to the environment. More scary transistors and analog circuits to worry about.

    Finally, and that is going to actually take the most work, there is the question of design. Selecting the best (both in terms of functionality and price) components, and finding an arrangement of them that is both functional and aesthetically pleasing. Designing the shape of the device, its case and its controls. I have already done some work on this, much much more remains.

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Makerfabs wrote 03/18/2021 at 07:09 point

i am also making something on Pico...

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