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I just updated my Pi image so that I could use the Raspberry Pi B+ and there was a subtle change regarding the default resolution that the Pi uses when it is not plugged into any monitor at boot time but hotplug is enabled. With my image for the old Bs, if the Pi was booted and not plugged in via HDMI it would choose 720p (CEA) resolution. With the B+ image it chooses 640x480 (DMT). Both images are just customizations of a stock Raspbian image and there's about a 9 months of difference in the firmware versions.

Ideally, I would like the Pi to boot to a good resolution for TVs but I don't want to hardcode to a specific resolution as I'd like it to read the EDID data and choose an appropriate one if it is booted when plugged in via HDMI. I thought this was exactly what hdmi_force_hotplug=1 was supposed to do, but apparently I'm wrong. I tried to set the framebuffer_width and _height but that didn't seem to actually adjust the output resolution over the wire.

This is cross posted to the raspberrypi forums. If it gets answered in either forum, I'll gladly post a summary in the other forum. For all the gory config details, see the raspberrypi forum post but here's a summary:

## /boot/config.txt same on both B and B+
gpu_mem=128
hdmi_force_hotplug=1
config_hdmi_boost=4
disable_overscan=1
display_rotate=0

# Firmware version B+
Nov 13 2014 17:26:15
Copyright (c) 2012 Broadcom
version 4b43a6c1fb32bd23b26f999eef78312d71df9b7a (clean) (release)

# Firmware version B
Jan 6 2014 21:19:57
Copyright (c) 2012 Broadcom
version b00bb3ae73bd2799df0e938b7a5f17f45303fb53 (clean) (release)
  • Something must have changed in the driver defaults over time. Not sure where they are even. What happens when you turn the monitor / tv off and on after you hot plugged it? – Piotr Kula Nov 27 '14 at 23:10
  • Turning the monitor (well, TV in my case) on and off has no effect as far as changing the resolution goes. Every time it comes up in 640x480. – davidfischer Nov 29 '14 at 0:51
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I ended up filing a bug against the firmware and discovered that the resolution change was by design.

I haven't finalized a solution, but for my use I think what I'll do is detect a really low resolution and have a delayed reboot until a suitable monitor can be found.

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