Reset Boot Test (Neo Geo AES)
The next step after confirming your power rails are good is to test if you are getting basic video out.
The best test for this allows us to bypass all of the following key items that are not required to even be present on the board or working:
- 69k CPU
- BIOS
- Z80 CPU
- Memory Card ICs
- Controller ICs
- Yamaha Sound & Headphone ICs
- Palette RAM (CXK5864BSP)
- Video RAM (CXK5814P) for LSPC
- Video RAM (CXK58257AM) for LSPC
- CMOS Mask ROM (TC531000) for LSPC
The below test if it works confirms the LSPC, NEO-G0 (or 74LS245 on early models) and CXA are all working.
I term this test the Reset Boot, because all you have to do is keep the reset button permanently held and turn on.
If you do this, after a few seconds you should see jumbled graphics on the screen or faded white to black, or faded red to black, similar to under-voltage screens. The output depends on the presence and state of the video and palette RAM. Even though they are not needed to be installed or working they will change what is display on screen.
The key here is we are looking only for an actual output on screen, whether it be black, faded gradients or garbled screen. A screen output is confirmation the LSPC and CXA encoder, power supply and AV cable are all good.
If you see nothing on screen, continue the tests below to confirm the LSPC is working and then the NEO-G0 (or 74LS245 on early models) and CXA are all working.
Make sure to test both composite and RGB for video output. Sometimes the RGB won't be working or wired up right, and other times the composite might not work but the RGB will.
There can be a fault on the board where the CSYNC pin outputs just the HSYNC pulses but no VSYNC, but composite will work. So it helps to test both.
Once you get basic video output to the screen, it is recommended then to install a Diagnostics BIOS to test the rest of the system.
If you do not yet get any video out, continue to the tests below.
As a basic check the LSPC is working, it is easy to just probe the D0-15 pins (almost always the first pins around pin 2 to 30, depending on revision).
Turn on the console with all components installed, and probe the pins 2 to 30 on the LSPC looking for basic pin activity.
If you see activity on most pins you can generally assume the LSPC is running.
If you have a bad Z80 CPU, 68k, Work RAM, BIOS or other things, you will not be able to check the LSPC activity on D0-15 pins as it requires those to work.
However, you can check all other pins of the LSPC even in reset without any processors, RAM or BIOS present.
The LSPC uses FV Video RAM of CXK5863P (28 pin) or CXK5814P (24 pin) SRAM.
The address and data pins connect to the LSPC on the Cx and Fx pins (or on early revisions labelled FV Ax and FV Dx).
Turn on the console and monitor for general activity on these pins. If you do not see activity, its likely your LSPC is dead or has shorts on it, or is missing power or clock signals.
If you hold RESET while powering on, the pins should show random data.
If you do not hold RESET, and you have no game in, it should show solid data (for blue or white screen depending on BIOS and state of console).
The LSPC uses SV Video RAM of UPD43256 or CXK58257AP SRAM.
The address and data pins connect to the LSPC on the Bx and Ex pins (or on early revisions labelled SV Ax and SV Dx).
Turn on the console and monitor for general activity on these pins. If you do not see activity, its likely your LSPC is dead or has shorts on it, or is missing power or clock signals.
If you hold RESET while powering on, the pins should show random data.
If you do not hold RESET, and you have no game in, it should show solid data (for blue or white screen depending on BIOS and state of console).
The SYNC output goes to the CXA1145 and is required along with the RGB pins in order for the receiver to recognise RGB video output.
Confirm you see 15.74kHz sync pulses on the SYNC pin (LSPC-A0 pin 126, LSPC-A2 pin 150). Also confirm it reaches the CXA1145 pin 10.
You should also see periodic low long pulses signifying the start of frames. You will need to capture on a trigger that detects over 10us long low pulses, and then check the timing between them also matches the correct timing.
TODO: Image of vsync part of VSYNC
Also confirm you see 3.579MHz pulses on the DIVI pin (LSPC-A2 pin 143). If you have an LSPC-A0 revision, instead check the CXA1145 pin 6 has that pulse (it is fed directly from the crystal on A0 revisions, not the LSPC).
If your LSPC is running and you see pulses on FV, SV and D lines, your next step is to check the NEO-B1 chip is working.
The NEO-B1 chip processes graphics and sets the Palette RAM address pins working in conjunction with the LSPC/NEO-G0 to set the RGB data that goes into the flip flops, through the resistor ladder and ultimately to the CXA1145.
If you have an active LSPC and NEO-B1 but still don't get any video out, check your CXA1145 video encoder with an oscilloscope.
Check you have valid Red / Green and Blue In pulses (around 1-2V pulses).
Check you have valid Red / Green and Blue Out pulses (around 1-2V pulses).
Check the XO Out and XO In have 3.579MHz pulses.
Check you have Composite Out signal.
Check you have CSYNC In and Out pulses (15.74kHz sync pulses, with longer low pulses indicating full frames for VSYNC).
If you have all valid signals from the CXA1145 chip, make sure the composite out and RGB out, and CSYNC if used, make it to the AV port pins.
If they don't check the resistors, capacitors, inductors and traces between the CXA and AV port.
If all of the above check-out, it might be worth testing your AV cable for continuity from pin to pin (end to end), or replacing it.
