This is a project for the Stellaris Launchpad LM4F120 evaluation board. It allows to use the GBS8200 RGB to VGA converter in simple line doubling mode, avoiding all the artifacts and problems caused by resampling and 50>60Hz conversion.
The output is progressive scan VGA at 50Hz and 31.5KHz.
Quite simple actually.
This was tested with GBS8200 V4.0 (datecode 2014.05.15) and EK-LM4F120XL dev. board.
First of all, you need to disable the built-in controller of the GBS8200. This is done by closing the P8 jumper.
Then, you need to conenct the Launchpad to the internal I2C bus on the P5 connector. Note that you may need to solder pins to this connector as it is usually not populated.
The wiring is as follows:
- PB2 > P5.SCL
- GND > P5.GND
- PB3 > P5.SDA
Leave P5.VCC unconnected. The Launchpad board needs to be powered separately (with 5v) on its USB port.
I used a standard GCC for ARM. You will need to install or compile one (usually, Mentor Graphics toolchain is used for people who don't manage to compile gcc themselves, but any version should do).
Once you have the compiler installed, just run 'make'.
Once you get the binaries, you can flash the Stellaris board using lm4tools. This needs the board to be connected using the appropriate USB port.
Also quite simple.
First connect the video inputs and outputs. You may need a SCART to VGA adapter for the input (or whatever connector you want to use). Plug the power supply to the GBS8200. Nothing happens at this point, this is expected. Connect power to the Launchpad. It automatically sets the chip up and starts running.
A serial terminal is available on the Launchpad debug USB port. It allows you to peek and poke the video controller registers for adjustments and experimentation. Later, more high-level commands will be added.
You can also reflash the Stellaris or press the reset button at any time. It will reprogram the scandoubler again, which is useful if something goes wrong or if you want to quickly test changes to the code.
This was confirmed working with the following input sources:
- Amstrad 6128Plus
- Amstrad CPC
- Thomson TO8
And the following display hardware:
- No name "TFT color monitor" with VGA input, MST703 based.
- ACER video projector at Forever partyplace.
- "Linetech" brand TFT TV, manufactured by Vestel.
The tolerance on off video timings is currently quite bad (far from what a standard TV would accept). This creates problems with some Amstrad games using hardware scrolling tricks (seen on Relentless for example), and also with ZX spectrum ULA-128 timings. Ways to improve this are being investigated.
- Fix issues with ZX Spectrum +2
- Allow adjusting the V/H position and other settings in an user friendly way
- USe the Launchpad LED to show current status by reading the status registers
- Port the whole thing to 8051 and let it run on the GBS built-in chip (and re-add all the bells and whistles like overlay support, etc).
- Better document what is done with the video controller and how it is configured.
Code for this implementation by PulkoMandy/SHR.
Original idea and experiments with Raspberry Pi by dooklink (https://github.com/dooklink/gbs-control)
Digispark Pro C code and register value list converter tool by mybook4 (http://shmups.system11.org/viewtopic.php?f=6&t=52172)
Thanks to mike/zeroteam, the secrets of ZX spectrum video timings have been revealed:
- ULA 48k: 312 lines, 64us/line, 19968us/frame, 50.080128Hz (Spectrum 48K)
- ULA 1 : 314 lines, 64us/line, 20096us/frame, 49.761144Hz (Didaktik)
- ULA-128: 311 lines, 64.281485us/line, 199991.5us/frame, 50.021153Hz (128k/+2/+3)
The 48k ULA is exactly identical to Amstrad CPC/Thomson timings and should not be a problem. However, for some reason the timings for the other variants are a bit off, and this seems to confuse the GBS converter (but also a lot of other devices). Symptom: the first frame (boot screen) will be displayed fine, but then the screen never gets updated.
A set of datasheets was assembled by dooklink. It contains details on most of the chips used on the GBS8200. http://shmups.system11.org/viewtopic.php?f=6&t=52172
