port pins

[piglet]

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

Many thanks for the tip! Currently all my projects are AVR and PIC but the
M0 could be next step up.

[Dan Purgert]

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.

I pick whichever I/O port is

- free
- close
- not an absolute pain layout wise.

On the PDIP-28 ATMegaX8's; this usually amounts to PortD, down the
left-hand side (assume Pin1 is upper-left). There's only VCC/GND to
contend with in the middle (7,8) and then potentially the oscillator on
PB7,0 (9,10).

If I'm using the internal oscillator, PortB is also viable (but it gets
a bit weird, since the layout is 6,7,0,1,2,3,4,5 (pins 9,10,14-19))

One could test and bit-bang each bit and port individually, and then
strobe the DAC, but that's inelegant.

Ow, yeah, that'd be painful.

[Phil Hobbs]

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

[piglet]

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.