Raspberry Pi - Smart Home Automation with Linux and Raspberry Pi - page 275

When considering the low-level hardware implications, be aware that the full documentation for the Broadcom
BCM2835 chip (which does all the main work) is unavailable without an NDA. Most people will be happy using it at
a higher level, even programming the GPIO, or working from the kernel source. But if you're hoping to be educated
on the internal workings of a SoC, or wanting to investigate the drivers, then you're out of luck and should look to the
fully open options out there. 3
From an interfacing point of view, the lack of VGA port is the one with which most people will be concerned.
Although the Raspberry Pi foundation considers VGA to be an antiquated technology, the reality is that there are
many good value monitors available that don't support HDMI and so requiring you to buy a separate converter.
This problem is expounded when looking for small monitors, as there are very few of the smaller incarnations of
HDMI available and (as yet) no definitive compatibility list. (HDMI on Raspberry Pi seems to have a worse issue of
compatibility than HDMI on desktop PCs.) Consequently, you may be forced into using the composite video to make
use of a small display, at which point you're using an even older technology than VGA, and worse visual quality to
boot. Other visual hook-ups include the Kindle (needs a jailbreak and a terminal emulator, as covered at
http://www.ponnuki.net/2012/09/kindleberry-pi ), iPad (just use a standard terminal emulator), or even an
existing digital photo frame ( http://www.cjb.im/2012/06/raspberry-pi-wireless-display-using.html ).

N If your HDMI device is not correctly detected you will need to edit the /boot/config.txt file to include
parameters to explicitly define it. This appears as two lines, such as hdmi_group=1 and hdmi_drive=2 with the parameters
indicating the type of HDMI (CEA or DMT) and the resolution. Both are described at
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=26&t=5851 .

Caution

The biggest single problem with the Raspberry Pi appears to be the implementation of the USB stack, with issues
in both hardware and software. Although the situation is improving, anything that needs USB to be of a “mission-
critical” nature might need to be redesigned. This, unfortunately, includes Ethernet as that is provided through USB.
(This is also why you can't have gigabit Ethernet on the Raspberry Pi-you're limited by the speed of the basic USB port.)
At the time of writing, the USB issues haven't been entirely solved, but the crux is thus. The hardware component
inside the Broadcom chip, which handles the USB port, isn't particularly good and the driver (dwc_otg) that attempts
to tame it has some issues whereby if a NAK packet is received, the processor gets interrupted. At this time (but only
some of the time, it seems) the hardware portion of the USB stack resends an IN packet . . . that generates another NAK
packet… that causes another interrupt on the processor. And so on. This very quickly leads to an interrupt cascade
problem that can render the Raspberry Pi unusable for 20 seconds or so. For an interesting discussion on this topic,
and why the Raspberry Pi has 8000 interrupts per second when idle, the thread is still open at
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=28&t=7866&start=111 .
One fix to this problem is to disable the USB entirely:

echo 1 > /sys/devices/platform/bcm2708_usb/bussuspend

Naturally, this prevents the keyboard from working. And with Ethernet being controlled through USB also, this means
that you can't remotely control it to make it communicate with the outside world. It is possible to connect a keyboard or
similar device through the GPIO, but such circuitry is outside the scope for this topic. There is also some online discussion
on whether the Raspberry Pi (Model A) avoids this problem, or whether it hasn't been witnessed as often. A thread, more
detailed than the official one, can be found at http://www.element14.com/community/thread/18568 .

! PARTIALLISTOFTHESEALTERNATIVESCANBEFOUNDAT http://www.bigboardlist.com