[libre-riscv-dev] [Bug 50] nmigen pinmux

[bugzilla-daemon at libre-riscv.org]

http://bugs.libre-riscv.org/show_bug.cgi?id=50

--- Comment #1 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---


On Fri, Apr 26, 2019 at 11:25 AM Rishabh Jain <rishucoding at gmail.com> wrote:
>
> hi luke,
>
> i am having difficulty in understanding nmigen Record (hdl/rec.py)
> i am able to  follow some part of hdl/rec.py like Direction, fields,
> __init__ in class Layout checking
> for valid field, are the elements of field valid, avoiding duplication of
> names.
> but, i am not able to understand the full picture :(

 in some ways, you don't actually need to read all the code, just know
 the function names and read the docstrings.  except (*sigh*) Record
 and Layout don't *have* any docstrings... *sigh*...


> can you give me some pointers over what is layout, shape?

 Layout is just a list of signal-width plus signal-Direction.
 (or signal-width plus signed/unsigned plus Direction).

 shape is a tuple of signal-width plus if the signal is signed or unsigned.


> from our diagram of GPIO  (https://libre-riscv.org/shakti/m_class/pinmux/)
> the i/o pad is a PHYSICAL PIN of processor which connects it to
> peripherals, memory, etc, right?
> so, we  mux (select one of many) inputs of processor coming to i/o pad from
> various peripherals and mux outputs of
> processor coming to i/o pad going to peripherals.
> I am not able to relate FAN_OUT  and FAN_IN with this diagram :(

FAN_OUT and FAN_IN are actually Muxers, nothing to do with Record/Layout...
there is an example somewhere of a multi-selection Mux...

https://github.com/m-labs/nmigen/blob/master/examples/pmux.py

ah, it's not quite the same.  actually, FAN_IN and FAN_OUT would be:

 with m.Switch(mux):
    with m.Case(0):
       m.d.comb += self.o.eq(self.a)
    with m.Case(1):
       m.d.comb += self.o.eq(self.b)

that would be one of them and the other would be:

 with m.Switch(mux):
    with m.Case(0):
       m.d.comb += self.a.eq(self.o)
    with m.Case(1):
       m.d.comb += self.b.eq(self.o)

you'll have to work out which is which :)


actually it turns out that doing this is not necessary, you can use an Array:

signals = [self.a, self.b, self.c, self.d]
array = Array(signals)

m.d.comb += self.o.eq(array[self.muxselector])

or for the other one:

m.d.comb += array[self.muxselector].eq(self.o)


so instead of a massive list of Switch/Case statements, just use Array.

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