hi so today I'd like to discuss what
some of these signals mean when they
have an underscore L at the end of them
on a schematic so learning how to read a
schematic is not something that you can
you can just ask and get an answer to if
I ask you how do I read English you're
you're probably going to look at me
funny because there's just I don't
really even have to explain how many
things are wrong with that question in
its basic state the same way that I
don't even understand where to start
with how do you read a schematic what
are what is wrong with that question so
what I've done with a lot of the basic
Electronics videos I've tried to break
down what the components do and break
down what the circuits do in just really
basic language and I'm going to try to
do that here with with something else so
here we're going to delve what that
means so let's just move over to the
screen capture here and let's go a
really basic signal like PM bat lorl
this is a very basic signal that has an
underscore L at the end and because of
the way it's written out it should be
very easy to tell what this stands for
and what this means so this says system
Power Manager in the system power
management page and as I say a lot of
the time you're going to have to deduce
what this means by you know buying
vowels buying consonants so PM power
management I'm going to assume because
we're on the system power management
section of this chip that PM means power
management now bat low we're talking
about a laptop here so I'm going to
assume that that means battery low now
underscore L this is the important one
underscore l means that this signal will
be asserted when it is low in voltage
what do I mean by asserted when it's low
in voltage this signal exists to tell
this chip that the battery is low right
that's what the signal is there for so
underscore l means that if the signal is
0 volts then the battery is low if the
signal is 3.4 Vols then the signal is
not present so this confuses a lot of
people because people will measure 3.4
volts on PM bat low and they'll go oh my
God the PM bat low signal is present
that means that the battery is low um
but the battery is fully charged so I'm
really confused as to what's going on
with the system the way this works
remember again when you see underscore L
that means that this signal is asserted
when it is low so this chip over here
the system power management chip if it
sees zero volts at the bat low signal
because of this underscore L that stands
for low it's going to assume that the
battery is low if the underscore L if
it's Z volts so as long as you have an
underscore L that means at 0 volts the
signal is asserted so you know with the
with the newer retina MacBooks and the
newer MacBook airs you know how when you
decide that you are going to uh try to
turn it on but you have like 0.5%
battery you just get that little that
little battery symbol that blinks with
the little red at the end of it and it
doesn't turn on that means that the PM
bat low signal is zero volts if you
apply three volts there it will actually
try to boot even if the battery only has
you know um 1% volt uh 1% capacity left
so that's what that is for now we can
find a couple of other examples over
here of that signal so let's say PM
power button low so that means that when
that signal is low it thinks that you're
pressing the power button uh another one
let's say platform reset L right
platform reset l so let's say that you
have a let's say you have a desktop
computer when you hit the reset button
right when you hit the reset button
you're asserting the reset signal so
this is very similar it's not the exact
same thing but kind of similar so when
you hit the reset button over there what
it's doing is it's asserting the reset
signal which means that this would then
go low so if you notice that you let's
say you touch the reset button on your
desktop computer and you hold it down
you'll notice that it doesn't
continuously reset if you hold that
reset button down on many desktop
computers the desktop will simply never
turn on and boot up or do anything it'll
turn on but it's not going to boot it's
not going to actually it's not going to
post it's just stuck the same is true
here if I keep this platform reset
signal low the computer computer itself
is never going to post it's never going
to do anything it's going to be stuck so
this here says that the platform resect
signal will be asserted when it's low so
if I see that this signal is stuck at
zero volts when everything else is
coming up that means that something's
happening in the system to keep the to
keep the reset button pressed at all
times which is going to keep my system
from working uh hopefully this has been
informative and helped you in some way
hopefully now your understanding of the
underscore L helps you to troubleshoot
some of the issues that you were having
that you weren't able to troubleshoot
before if you have any questions leave
them in the comments I will try to
answer them so long as you're not asking
me a long-winded question on how to fix
your specific board thank you and have a
nice day