Assuming you are using an HDMI to VGA converter, try "tvservice -m DMT" to see what modes are available.
On one of my systems I get
Code: Select all
pi@raspi:~$ tvservice -m DMT
Group DMT has 9 modes:
mode 4: 640x480 @ 60Hz 4:3, clock:25MHz progressive
mode 6: 640x480 @ 75Hz 4:3, clock:31MHz progressive
mode 9: 800x600 @ 60Hz 4:3, clock:40MHz progressive
mode 11: 800x600 @ 75Hz 4:3, clock:49MHz progressive
mode 16: 1024x768 @ 60Hz 4:3, clock:65MHz progressive
mode 18: 1024x768 @ 75Hz 4:3, clock:78MHz progressive
mode 21: 1152x864 @ 75Hz 4:3, clock:108MHz progressive
(prefer) mode 35: 1280x1024 @ 60Hz 5:4, clock:108MHz progressive
mode 36: 1280x1024 @ 75Hz 5:4, clock:135MHz progressive
(Note, that particular monitor is DVI-D, but I have used its VGA input with an adapter as well)
So if I had to set it manually I'd put hdmi_mode=35.
Have a look at http://elinux.org/RPi_config.txt#Video
for a full list of modes and more info.