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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: port pins
Date: Thu, 12 Sep 2024 20:08:33 -0000 (UTC)
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piglet <erichpwagner@hotmail.com> wrote:
> john larkin <jl@650pot.com> wrote:
>> Something I've wondered about:
>> 
>> Suppose we have a c program running on some little uP, and it has some
>> integer variable value, 8 or 12 bits or something, and wants to drive
>> a parallel DAC off-chip.
>> 
>> The msb...lsb bits of the variable obviously have to get to the right
>> pins of the DAC.
>> 
>> So, in general, how does one pick the physical i/o port pins on the
>> uP, to get the order right? The PCB layout is easiest if we just wire
>> the DAC to the handiest port pins.
>> 
>> One could test and bit-bang each bit and port individually, and then
>> strobe the DAC, but that's inelegant.
>> 
>> In the RP2040 chip, one can apparently write to a register in a
>> PIO/state machine block, where each bit of the register can be
>> assigned to drive a physical port pin. I think there are some
>> constraints on the selected pins.
>> 
>> 
>> 
> 
> I didn’t know that now exists - but sounds super useful. Would have saved
> much headache if it had been around years ago.
> 

We sometimes use the LPC804, which has a gigantic pin mux—you can
effectively put any function on any pin. IIRC reset and isp are a bit
special, but all the gpio and built-in peripheral functions can be
configured any way you like. 

Cheers 

Phil Hobbs 

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics