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PostPosted: Wed Dec 13, 2017 11:47 am 

Joined: Fri Feb 24, 2012 12:09 pm
Posts: 540
The AMT640A is a relative common video controller chip found in cheap mini TFT screens with composite input.
Those screens are often sold as "car rear view monitor" (or by similar names), although they are essentially fully-fledged portable TVs (minus tuner/speaker, and without advanced inputs like S-video or RGB).
The chip contains a 8031/8051 microprocessor, with firmware stored on an external SPI-bus FLASH memory chip.
Theoretically, it could be used as miniture monitor for gaming consoles. Unfortunately, it's working rather disappointing in practice:
  • Display is having some vertical-interlace-style flickering (even on non-interlaced pictures).
  • With 320x240 pix screen, resampling is smeared, possibly a bit uglier than required...
  • For NTSC, one could theoretically leave blank lines (with black borders)
  • For PAL, one could theoretically crop lines (eg. on homecomputers with 200 lines)
  • PAL60 produces wrong colors.
  • PAL60 forgets to update some scanlines (and draws wrong/older lines).
  • Backlight is at max brightness and cannot be changed.
  • Reportedly, signals from C64 aren't recognized (not tested myself yet).
Some of that issues might be firmware related. So I've spent 2-3 weeks on disassembling it: (AMT630A firmware disassembly, for 320x240 pixel 3.5" screen with 4:3 ratio). The disassembly is working as source code and can be re-assembled via no$x51. All code addresses are represented by lablels, so inserting custom stuff in code memory should work (whilst RAM/XRAM addresses are still hardcode, so better don't insert new stuff there; there are unused locations that could be used though).

I would be also interested in dumps from other firmware versions...
  • firmware for screens other than 320x240 pixels
  • firmware for 16:9 wide screens
  • firmware with infrared IR input support
  • firmware with volume control support
  • AMT630 firmware (to see if it's similar/compatible to AMT630A)
  • other older/newer firmware revisions
  • etc.
That stuff would help on finding which I/O ports have which functions (eg. for changing the screen resolution/resampling).

More notes on the hardware's component list and on SPI FLASH dumping are at the bottom of the disassembly. The AMT630A spec sheet doesn't contain any info about I/O ports, but the disassembly should explain the coarse functionality of the ADC, PWM, OSD registers, as well as SPI busses for FLASH (and LCD). For the composite to LCD video conversion, there are a lot of fixed initialization values - which, one could try to change them & see what happens...

PostPosted: Wed Dec 13, 2017 1:34 pm 

Joined: Sun Apr 13, 2008 11:12 am
Posts: 6641
Location: Seattle
A random source claims it is a clone of the MST703, for which I was able to find a datasheet that includes register definitions... in case that helps.

I've also been able to find a reference schematic on baidu ("AMT630_CAR_V3.4") , but they won't let me download it without logging in :/

PostPosted: Wed Dec 13, 2017 1:58 pm 

Joined: Fri Feb 24, 2012 12:09 pm
Posts: 540
Yes, I know that claim about AMT630 being a MST703 clone.... if the AMT630 is working similar to AMT630A then it's nonsense: The AMT630A registers at FB00h-FFFFh don't match up with the MST703 datasheet at all.

Apropos registers. The AMT630A spec sheet claims that it has some "standard" stuff like timer2 and watchdog. I know some 8031 variants with watchdogs at either SFR register FFh, or A6h. But the AMT630A seems to have the watchdog controlled via SFR registers B9h, BAh, BBh, BCh, BDh, and BEh. Are that really standard registers used in other 8031 variants, too?

PostPosted: Wed Dec 13, 2017 5:01 pm 
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Joined: Wed Feb 13, 2008 9:10 am
Posts: 606
Location: Estonia, Rapla city (50 and 60Hz compatible :P)
The jittery vertical lines are this sort of thing :

If you're lucky there's a small pot that allows to change the bias point and make both sides equal and reduce the flicker effect (it will not however completely fix it).

There are also some other controllers out there which require external MCU to do the work, and some do have their registers all available in a datasheet. I have one such somewhere and one day when I'm no longer lazy I'll write up a new MCU that enables RGB input (or very least YC) and adjusts geometry and timings etc. to suit particular machines.


PostPosted: Thu Dec 14, 2017 3:29 am 

Joined: Tue Oct 06, 2015 10:16 am
Posts: 627
If you manage to make it work, I'd get one, and probably many others too. Lots of sales for the chinese :P

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