Hi,
On 28-01-15 10:13, Ian Campbell wrote:
On Sun, 2015-01-25 at 15:35 +0100, Hans de Goede wrote:
On both my A13-OLinuxIno and my A13-OLinuxIno-Micro, the vga output gives an unstable image when active low v or hsync is used, so when an external vga dac is used force v and hsync active high independent of what the modeline says.
Does this not just mean the modeline is wrong and should be fixed or an alternative provided which is correct for these systems?
I would rather not, the modeline uses is a vesa standard timing, so it certainly is correct.
The problem seems to be specific with the OLinuxIno A13 (normal & micro) boards. I've just looked up the schematics and they use an opendrain driver for the vga sync lines, and with sync pulses it is the logical high->low edge of the pulse which counts for the timing, which with an active low sync is being driven by the pull-up, and that simply seems to not drive it hard enough to get a stable image.
In the code comment you say "some dacs", what about ones which don't have this issue or, worse, ones which have the inverse issue and don't work with active high?
At the least I'd say this aught to be controllable via Kconfig.
I agree this should be a Kconfig and only turned on on the Olinuxino 13 boards. I'll do a v2.
Regards,
Hans