GBS-82XX experiments part 2

GBS-8200 and GBS-8220 experiments part 2

Work has continued, experimenting with these low cost but potentially highly capable video adaptor boards.

Fixing the random speckles on the display

I believe I have found the cause of the occasional white speckles seen on the display. The default setting for the GBS-82XX board is to clock the 166 MHZ speed grade SDRAM at 162 MHz. It appears to be worse if the Hynix HY57V643220DT-6 is fitted compared to the EtronTech EM638325TS-6G device, which appears to be fine.

I measured the SDRAM clock at 162 MHz and recorded this:

Whilst the signal is a bit noisy, it does not violate the +/-2V overshoot/undershoot limits of the SDRAM devices used.

The simplest fix was to reduce the SDRAM clock speed to 129.6 MHz, with a single I2C write. This has fixed the issue. Halving the SDRAM speed to 81 MHz caused distortion on the video, the next speed increment of 108 MHz was sufficient for 1360×768 pixel output. A proper fix would be to adjust the timing of the DQM strobes with regard to the data bus as on SDRAM the DQM strobes clock the data out of the SDRAM. If this is adjusted, you also need to consider the timing of the SDRAM clock to the control bus (RAS, CAS, CKE, CS, BS0/1 and WE) and the Data bus. This is not too difficult if you have the PCB artworks as you can measure the PCB track lengths and adjust the timing by 7ps/mm. With the GBS-82XX board, it would be tricky and fraught with false starts. We could of course measure it and adjust accordingly, or reduce the clock speed by 20% and have more timing margin (an extra nanosecond) on the clock.

Now that the speckling problem appears to be fixed, I am looking at some general video quality issues. When using a 50 Hz PAL screen-mode, scan-converted to 60 Hz, there are some noise bands, particularly noticeable on a grey Workbench background. It is hoped that some digital filtering/sampling will help alleviate this.

Synchronising the GBS-82XX

I have experimented with different ways to synchronise the GBS-82XX device. As those of you that have experimented with the board now, sometimes it can be a bit ‘hit and miss’ with the video source (games console or computer).

When using the GBS-82XX with the Amiga, this is the cable I use:

The 680 ohm resistor is essential. It reduces the CMOS Composite sync signal from the Amiga to < 3.6V. The TVIA-5725, the device under the big heatsink, accepts a maximum voltage of +3.6V and a minimum voltage of -0.3V. The Amiga’s video sync, measured on the A1200, looks like this:
The TVIA-5725 accepts a 3.3V TTL signal, via a Schmitt input, this changes logic levels slightly so a logic 1 is 2.4V to 3.3V,  and a logic 0 is 0 to 1.0V. The 680 Ohm resistor works as the GBS-82XX follows the Vesa VSIS specification, which requires synchronisation inputs to have a 2K impedance to ground. My 680 ohm resistor (+47 ohms in the Amiga), creates a potential divider, reducing the signal level to a safer level. This is cheap and easy to implement and does not degrade the video sync significantly.

A number of people use the venerable LM1881 video sync separator, any device or circuit that uses this, should have the 680 ohm resistor added as shown above. The LM1881 outputs 4-5V sync levels that are not compatible with the GBS-82XX/TVIA-5725. Alternative devices are the EL1883 or the LMH1980 but they are not available in hobbyist friendly DIP packaging.

I have tested the LMH1980 with the Amiga, I used the composite video output of the A1200 to create LVTTL (3.3V) HSYNC, VSYNC and CSYNC. The GBS-8200 I have synchronised with the CSYNC perfectly, the same as using a resistor. It did not work reliably when I supplied separate HSYNC and VSYNC, I had a very wavy display.

A final note, the GBS-8200 did synchronise to the Composite video output of the A1200 but it is not recommended. The 2V (approx) video contains colour information that is not filtered out and would in all eventualities, cause problems at a later date.

Cleaning up the power supplies

I have been reading numerous web-forums on GBS-8200/GBS-8220 problems to look for common trends and solutions. One topic that crops up a lot is related to the power supplies. I have already proven, in part one, that a 5V 2A supply is not required.

During my testing, I noticed an occasional, high frequency, burst of noise, on the +3.3V supply. I traced it back through the circuit to the power input. Changing the timebase of the oscilloscope, I spotted something important, it happened at 20ms/50Hz intervals. In the UK and Europe, the AC mains operates at 50Hz so when you see a 50Hz noise pulse, you know where it came from. Currently my GBS-8200 is powered from my bench power supply, built 20 years ago, with 3 linear regulators and still on the original electrolytic capacitors.

Also connected to the +5V output was my 5V to 3.3V TTL buffer board. Two of the eight inputs were in use, the other six were floating. I made a mistake here. You should never leave TTL inputs unconnected, they will pick up noise and oscillate, in this instance, they picked up 50Hz mains noise. Quickly dis-connecting the buffer board, removed the noise. The 3.3V (switchmode) and 1.8V (linear) regulator supplies now have about 20-30mV of noise, perfectly acceptable.

To ensure I do not have any further conducted noise issues I added a clip on ferrite bead:

You can see the buffer board in the top right of the photo. Whilst this ferrite was a little large, it did cut the noise out. If you are using the DC power jack (the black plug) either pick a PSU that has a ferrite fitted or measure the cable diameter and purchase a clip-on ferrite from a local supplier or ebay. I spent two hours trying to work out where the noise was coming from, reading datasheets and measuring the board.

To date I have changed a single capacitor on the board. I briefly touched on this in my first post but after additional testing, I am happy to confirm it needs changing.
Here it is:

The 1.8V regulator is used for the core supply of the TVIA-5725. with a ceramic capacitor, I was able to cause the power supply to glitch by switching my overhead inepection light on/off or my soldering iron transformer. Since changing it to a 16V, 22uF, tantalum bead capacitor, this has not happened. The original capacitor, shown under Kapton tape, had an ESR of 0.02 ohm, the recommended range for the LM1117 (similar to the AMS1117 used here) is 0.3 to 22 ohms! The part I fitted had an ESR of around 2 ohms.

The software settings solution?

Some people ask what the final software settings solution will be?

My preferred option is to use an Arduino Nano like this:

(Image from http://arduino.cc/en/Main/arduinoBoardNano)

This would be used to read and write the I2C commands to the TVIA-5725 device which provides the video scaling functions, among others. It is readily available and clones can be cheaply procured, finally it can be easily updated using the Arduino environment.

My aim is to make the design data readily available, for free. This will include the video settings. I’ve seen too many scammers on ebay selling ‘Amiga modified’ equipment for extortionate prices, I will not have this solution exploited.

Another option, still using the Arduino approach is a module that piggybacks on the 8051 microcontroller clone on the GBS board. This would allow access to the onboard switches and no wiring. The downside is it would be more expensive.

Until the final settings are known, the end solution is fluid.

Until the next update.