MCE Blaster

A simple standalone MCA/CGA/EGA to VGA adapter based on a raspberry pi Pico.

Videos

MCE Blaster video part 2: https://www.youtube.com/watch?v=Neg1WR7Hz5s
MCE Blaster video part 1: https://www.youtube.com/watch?v=kgDOiGoxKvE

What it does

The Pico reads the TTL input video signal, writes the pixels to a buffer and then generates the VGA signal using the pixels from the buffer.

Features

No need for a VGA upscaler or a 15KHz VGA monitor. Works with standard VGA modes.
Pixel-perfect output:
- VGA 640x400 for CGA/EGA 320x200,640x200 and 640x350 inputs
- VGA 800x600 for MDA 720x350
Pixel clock tuning for each mode
Auto-adjustment functionality that centers the image on screen
EGA brown color correction
On-screen messages
Two PCB variants, one SMD and one Through-Hole.

Installing the firmware to the Pico

Download firmware (MCEBlaster.uf2): https://github.com/scrapcomputing/MCEBlaster/releases
Unplug the Pico
Press and hold the small "BOOTSEL" button on the Pico
While holding the BOOTSEL button, connect the Pico to your PC with a micro-USB cable
The Pico should show up as a mass-storage device
Copy the MCEBlaster.uf2 firmware to the drive associated with the Pico
Safely eject the mass-storage device

Usage

Adjusting the pixel clock

Push the PIXEL CLOCK button once, this will enter the pixel adjust mode.
Pushing PIXEL CLOCK again will increment the pixel clock by 10KHz.
Medium-pushing (about 1 second push) PIXEL CLOCK will increment by 1KHz
Pushing AUTO ADJUST will decrement by 10KHz.
Medium-pushing (about 1 second push) AUTO ADJUST will decrement by 1KHz.
Exit pixel clock adjust mode by not pushing any buttons for a few seconds. This won't save the settings to flash.
Save settings to flash memory by long-push (keep pushed until you see message) PIXEL CLOCK.

Centering the image

Push the AUTO ADJUST button. This works best when the image shown is full from border to border.
Long-push the AUTO ADJUST button turns on continuous auto-centering mode. This may be useful occasionally, but won't work well if the image is mostly blank as it may keep re-adjusting the image.

How it works

Overview

The first core reads the TTL video data (through the level-shifter) from a PIO state-machine and places the pixels onto a buffer in memory. The second core works completely independently from the first core and reads the pixel from the buffer and feeds them, along with the necessary Horizontal and Vertical Sync signals to a PIO state machine that writes to GPIO outputs. The output is converted to analogue with the help of a simple R-2R DAC consisting of two resistors. Feel free to check out the video part 1 for a visual overview.

PIO state machines

The PIO state machines are used not only receiving the TTL video data and writing the VGA video data, but also for other helper functions including:

Detecting no input signal: This checks if the horizontal sync signal is active.
Detecting the image offset: This counts the black border pixel until a non-black pixel is found across all lines. This count is used for centering the image automatically.
The EGA PIO state machine also does the Brown pixel fix because the overhead of doing that in a CPU core was too high.

Sampling clock

The TTL video input is read by a PIO state-machine (one version for each video mode). But getting it to sample the pixels at the right time requires us adjust the sampling period in a precise way to match the TTL video card's internal pixel clock. Using the PIO clock divider does not give us a good-enough precision, because the fractional part of the divider is an 8-bit number, meaning that we get precision of ~0.004ns, but what we really need is about 0.001ns. So what we do is that we have multiple implementations of the state-machine code, each with a different sampling period and we look for the one that has the lowest division error using the PIO's 8-bit divider. This process is explained in more detail in the video part 2.

SMD Version

THT (Through-Hole) Version

Schematic

Bill Of Materials

There are two versions of the PCB available:

The SMD version, and
The Through-hole version (with the _THT file suffix)

So please select the parts according to the PCB version you are planning to build.

Download gerbers: https://github.com/scrapcomputing/MCEBlaster/releases

Reference	Quantity	Value	Description
C1	1	Capacitor SMD 0.1uF 1206 (or disk ceramic for Through-Hole)	Decoupling capacitor for level-shifter IC
D1	1 (optional)	Diode Through-hole (e.g., Schottky 1N5817 or silicon 1N4001)	For powering the MCE Blaster from the PC (instead of the Pico's micro-USB)
J2	1 (optional)	1x02 through-hole Male PinHeader 2.54mm	For alternative external power
J1	1	DB9 Male Horizontal	For connecting to TTL video card
J3	1	DB15 Female HighDensity Connector	For connectint to VGA monitor
R3 R4 R6	3	422 Ohm 1206 SMD (or Through-hole) resistor 1% (alternatively 470 Ohm)	For VGA signal DAC
R2 R5 R7	3	845 Ohm 1206 SMD (or Through-hole) resistor 1% (alternatively 1K Ohm)	For VGA signal DAC
SW1,SW2	1	6mm Through-hole push button	Auto-adjust and pixel-clock buttons
U1	1	74LVC245 SO-20 SMD 12.8x7.5mm aka SOIC (or DIP-20 + optional socket for Through-hole)	Level-shifter IC
U2	1	RaspberryPi Pico	Pi Pico
N/A (for Pico)	2	1x20 female through-hole pin-header 2.54mm pitch	PCB Pico headers
N/A (for Pico)	2	1x20 male through-hole pin-header 2.54mm pitch	Headers for the Pico

scrapcomputing/MCEBlaster