Power play#
Fri, 20 Jul 2018 12:54:10 +0000
Last week in NixWRT we
made kexec with uImages work despite unexpected complexity; this week
we realised it's not even going to be as simple as we thought it would
be after last week's work. Because u-boot is often delivered in
lobotomised form on end-user devices we are using the kernel
CONFIG_CMDLINE_OVERRIDE options to specify the command line and
ignore whatever weird defaults the bootloader wishes to provide, but
this then bites us when we want to boot the same kernel with kexec and
augment the overridden command line. Argh.
But I haven't really jumped into doing anything about that yet,
because I got sufficently annoyed with having to walk up and down
stairs to reset the device every time I crashed it (testing kexec)
that I decided to take some time out to add remote power switching to
it. Which is what you see to your right (if you are reading this in
HTML with CSS enabled: if you are an RSS subscriber or using Reader
mode or ... I dunno, it's probably somewhere around here). This is an
Arduino Yun running a sketch that turns pin 8 on or off when it gets a
1 (or y) or a 0 (or n) on its serial port, attached via a voltage divider to the
base of a 2N2222 transistor, whose collector is attached to the coil
of a small relay, whose switch is interposed in the path of the 5V
wire of a microUSB cable. Result: I can turn my GL-MT300N off and on
by running something like (echo n && sleep 1 && echo y) > /dev/ttyACM0
from the computer that the Arduino is plugged into.
The Arduino Yun, including as it does an entire embedded MIPS system , is definitely overkill to drive a single GPIO output, but it was lying on my desk and not currently doing much: it's not as though I bought it for this use.