My apologies. I think I was over-simplifying some assumptions for the pixel clock. Not knowing the exact standards, I think the "cvt" tool in Linux gives me the answers I need.
I've highlighted the resulting pixel clock for each. *For 1920x1080x60:* # 1920x1080 59.96 Hz (CVT 2.07M9) hsync: 67.16 kHz; pclk: 173.00 MHz Modeline "1920x1080_60.00" *173.00* 1920 2048 2248 2576 1080 1083 1088 1120 -hsync +vsync *For 1920x1080x30:* # 1920x1080 29.95 Hz (CVT) hsync: 33.01 kHz; pclk: 79.75 MHz Modeline "1920x1080_30.00" *79.75* 1920 1976 2168 2416 1080 1083 1088 1102 -hsync +vsync *For 1680x1050x60:* # 1680x1050 59.95 Hz (CVT 1.76MA) hsync: 65.29 kHz; pclk: 146.25 MHz Modeline "1680x1050_60.00" *146.25* 1680 1784 1960 2240 1050 1053 1059 1089 -hsync +vsync *For 1440x900x60:* # 1440x900 59.89 Hz (CVT 1.30MA) hsync: 55.93 kHz; pclk: 106.50 MHz Modeline "1440x900_60.00" *106.50* 1440 1528 1672 1904 900 903 909 934 -hsync +vsync So, assuming I'm reading this right, I think this clears up what I can and cannot do... Apologies for the confusion. On Tuesday, January 19, 2021 at 9:40:02 PM UTC-5 Matt Parnell wrote: > I've screwed up something in my math above...sorry. Reading up on pixel > clocks. > > On Tuesday, January 19, 2021 at 9:28:34 PM UTC-5 Matt Parnell wrote: > >> Robert, >> >> Thank you so much for the quick reply and clarification regarding >> enabling/disabling outputs. Makes sense. >> >> But I'm still not sure why I can't get 1680 x 1050. >> >> I believe the AM335x pixel clock is limited to 126.5 MHz. If I >> understand how this works, and the relationship is 1:1 (pixels per Hz; >> color is communicated via parallel bits), 1920 x 1080 x 30 Hz = 124.4 MHz >> (just below the limit). So, it's clear why we can't do 1080p at 60 Hz. >> >> That said, 1680 x 1050 x 60 Hz = 105.8 MHz. This *should* be below the >> limit, right? What am I missing? >> >> BTW - While I didn't have success with it in prior iterations, I just >> tried "video=HDMI-A-1:1920x1080@30e". I didn't get video, but I did get >> my monitor indicating "Input Signal Out of Range" - which tells me the BBB >> is doing its job, my monitor just can't handle 30 Hz (not unexpected). I'm >> not sure what's different this time around; I swear in the past it would >> just revert to 1280x1024 when I tried that setting, but maybe I'm not >> remembering some detail. >> >> On Tuesday, January 19, 2021 at 8:55:12 PM UTC-5 RobertCNelson wrote: >> >>> On Tue, Jan 19, 2021 at 7:16 PM Matt Parnell <[email protected]> >>> wrote: >>> > >>> > I've been working for some time on trying to achieve the highest >>> resolution possible for a BeagleBone Black system I'm setting up as a >>> dashboard running Chronograf. >>> > >>> > I've tried multiple operating systems and builds (including a brief >>> stint with FreeBSD), but what I'm using now is a Debian 10.7 console build >>> (4.19.94-ti-r57) downloaded from Robert Nelson's server >>> (BBB-blank-debian-10.7-console-armhf-2021-01-11-1gb.img.xz). FWIW, none of >>> the versions I've tried (including those with LXQT) seem to make much >>> difference with regard to the issue I'm facing... >>> > >>> > The BBB seems to really want to default to 1280x1024x60. I can't get >>> the resolution I want - which is 1680x1050 (I really want 1920x1080, but I >>> suspect my monitor won't support 24 or 30 Hz). >>> > >>> > I've been able to get 1440x900x60 by adding >>> "video=HDMI-A-1:1440x900@60e" to "cmdline" in uEnv.txt. Unfortunately, >>> setting this to 1680x1050x60e seems to cause the resolution to revert to >>> 1280x1024. BTW - what does the "e" suffix do? I've tried with and without, >>> and haven't figured out what its significance is. >>> > >>> > I don't see 1680x1050 in the list on >>> https://elinux.org/Beagleboard:BeagleBoneBlack_HDMI#Supported_Resolutions, >>> but from I've read it should work? >>> > >>> > I read somewhere that resolutions/bandwidth are limited via the audio >>> channel. So I've also tried disabling HDMI audio (I don't use it) by >>> un-commenting "disable_uboot_overlay_audio=1". >>> > >>> > My monitor, by the way, is an older HP Compaq LA2405x. I have >>> confirmed that it supports 1680x1050 at 60 Hz. It is connected to the BBB >>> via a mini HDMI to HDMI cable, then an HDMI to DVI cable. I'm nearly >>> certain that the monitor isn't the issue, as it will report on-screen if an >>> input resolution/frequency is out of range (and I'm not getting that). >>> > >>> > I've also tried setting the resolution for my Xorg session, but from >>> what I can tell, the driver (fbdev?) doesn't permit changing resolution. >>> When I run xrandr, it seems to indicate that my minimum and maximum >>> resolution is equal to the resolution I've already forced via uEnv.txt. >>> > >>> > Forgive me if I've missed important details or this is a bit >>> scattered. Most of this effort occurred over the weekend (before I had >>> access to this group), so I'm trying to re-visit the key steps I took >>> (ignoring many that appeared to have no impact). >>> >>> I don't remember where this is explained in the u-boot/kernel code. >>> But the BeagleBone Black's AM335x just doesn't have enough memory >>> bandwidth to fully support "large" screen sizes.. >>> >>> This is why 1080p maxes out at 30 fps, or 24 fps when audio is also >>> involved.. >>> >>> The 'e' is documented here: >>> >>> >>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/fb/modedb.rst?h=v5.11-rc4 >>> >>> >>> "DRM drivers also add options to enable or disable outputs: >>> >>> 'e' will force the display to be enabled, i.e. it will override the >>> detection >>> if a display is connected. 'D' will force the display to be enabled and >>> use >>> digital output. This is useful for outputs that have both analog and >>> digital >>> signals (e.g. HDMI and DVI-I). For other outputs it behaves like 'e'. If >>> 'd' >>> is specified the output is disabled."... >>> >>> Regards, >>> >>> -- >>> Robert Nelson >>> https://rcn-ee.com/ >>> >> -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/beagleboard/0c31fb3b-dfed-4abd-91a5-ab549c7254ean%40googlegroups.com.
