QLpicoVGA - Video Interface QL-to-VGA with a Pi Pico

This project creates the firmware for a DIY video interface based on the Rapsberry Pi Pico controller. It allows to connect a Sinclair QL computer to a standard VGA monitor.

One difficulty when using an old Sinclair QL nowadays is finding monitor that works well. The QL video output is a bit non-standard, many displays do not show the full 512 horizontal pixel, but instead crop the screen in some way.

This small Video-to VGA converter project is specially made for the QL signal and translates the 512x256 pixel screen to a standard pseudo-XGA resolution (1024x768 with a single QL pixel translating to a block of 2x3 XGA pixels). The Gold Standard for such a translation is the "QL-VGA" FPGA-based device by MarcelK. This project is a µC based C/C++ implementation, utilizing the highly configurable I/O of the RP2040 and its DMA capabilities for signal input and output. Phase and frequency detection and adjustment is done by the CPU on the fly utilizing fixed-point arithmetics.

• The VGA signal generation uses MoeFH's simple but clever DMA-driven approach for the low-level part. Kudos to MoeFH!
• QL video signals are sampled at 32MS/s - with the QL Pixel clock running at 10MHz
• All timing parameters are specifically preset for the QL's video output, so the software will definitely not work well with any other video source, the project is not meant as a generic RGB-to-VGA converter.
• As a small diagnostic aid, the unavoidable "No Signal" bluescreen shows a simplified logic-analyzer type of graph for the input signals

Hardware

This project is based on a Raspberry Pi Pico board and only needs a few additional components (resistors) for voltage level translation:

Controller Module: Raspberry Pi Pico 

QL to Pico:

QL RGB                            Pi Pico
Connector                          Module

7 (QLRED)   ----[R 330 Ohm]----  GP3=Pin 5 ----[R 680 Ohm]---- GND 
6 (QLGREEN) ----[R 330 Ohm]----  GP5=Pin 7 ----[R 680 Ohm]---- GND
8 (QLBLUE)  ----[R 330 Ohm]----  GP7=Pin10 ----[R 680 Ohm]---- GND

4 (QLCSYNC) ----[R 100 Ohm]----  GP9=Pin12 ----[R 680 Ohm]---- GND

2 (QLGND)   -------------------  GND=Pin8

Pico to VGA:

Pi Pico                          VGA
Module                        Connector

GP11=Pin15 ----[R 330 Ohm]----  1 (RED)
GP13=Pin17 ----[R 330 Ohm]----  2 (GREEN)
GP15=Pin20 ----[R 330 Ohm]----  3 (BLUE)

GP16=Pin21 ----[R 100 Ohm]---- 13 (HSYNC)
GP17=Pin22 ----[R 100 Ohm]---- 14 (VSYNC)

GND=Pin23  ------------------- 6,7,8,10,5 (GND)

Note: Place all resistors close to the Pi Pico module (<5cm) for best signal integrity.

Power Supply Input

Raspberry Pi Pico needs a standard USB power suppy (needs <100mA, so no special requirements for the power supply). Some VGA monitors may even provide USB voltage via an integrated USB hub.

1st Prototype

(Ehem - yes I know but it works ;-)

Loading the Firmware into the Pi Pico

Pi Pico operates like a USB memory stick when pushing the BOOTSEL button during while plugging into the USB port of a computer. Put the firmware "video_if_ql_vga.uf2" onto the drive. LED should show short flashes with a period of about 2 seconds. Please also refer to the Pi Pico documentation regarding firmware programming.

https://github.com/holmatic/video_if_ql_vga/releases

PCB and 3D Printable Case

So here for info on creating your own PCB and case

Version Info

0.01 Initial version

0.05 Extend boot delay (startup fix for issue opbserved on some boards); minor adjustments to timing

Instruction v1.0