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From: The Natural Philosopher <tnp@invalid.invalid>
Newsgroups: sci.electronics.design,comp.sys.raspberry-pi
Subject: Re: uP port mapping
Date: Thu, 12 Sep 2024 18:24:41 +0100
Organization: A little, after lunch
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On 12/09/2024 17:43, Theo wrote:
> In comp.sys.raspberry-pi 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.
> 
> A lot of MPUs have 'ports', ie you don't have individual GPIOs, but P0.0 to
> P0.7, P1.0-P1.7, etc.  You can write to a whole 8 bit port in a single
> operation.  That way you know to wire up P0.0 to the LSB of your DAC and
> P0.7 to the MSB, or maybe use P1.0-1.3 for the upper 4 bits.
> 
> Slightly surprisingly I don't see that on the RP2040 datasheet, it looks
> like you can only set single bits at a time.  It seems you have to use the
> PIOs to get parallel transfers.  Maybe you can also do something with DMA?
> 
> There's a 'PIO and DMA (a logic analyser)" in 3.2.3 of the C/C++ SDK doc:
> https://datasheets.raspberrypi.com/pico/raspberry-pi-pico-c-sdk.pdf
> 
> which says:
> 
> "Pin Groups (Mapping)
> We mentioned earlier that there are four pin groups to configure, to connect a state machine’s internal
> data buses to the GPIOs it manipulates. A state machine accesses all pins within a group at once, and
> pin groups can overlap. So far we have seen the out, side-set and in pin groups. The fourth is set.
> The out group is the pins affected by shifting out data from the OSR, using out pins or out pindirs, up to
> 32 bits at a time. The set group is used with set pins and set pindirs instructions, up to 5 bits at a time,
> with data that is encoded directly in the instruction. It’s useful for toggling control signals. The side-set
> group is similar to the set group, but runs simultaneously with another instruction. Note: mov pin uses
> the in or out group, depending on direction."
> 
> and 3.2.5 in the RP2040 datasheet says:
> 
> "3.2.5. Pin Mapping
> PIO controls the output level and direction of up to 32 GPIOs, and can observe their input levels. On every system clock
> cycle, each state machine may do none, one, or both of the following:
> • Change the level or direction of some GPIOs via an OUT or SET instruction, or read some GPIOs via an IN instruction
> • Change the level or direction of some GPIOs via a side-set operation
> Each of these operations is on one of four contiguous ranges of GPIOs, with the base and count of each range
> configured via each state machine’s PINCTRL register. There is a range for each of OUT, SET, IN and side-set operations.
> Each range can cover any of the GPIOs accessible to a given PIO block (on RP2040 this is the 30 user GPIOs), and the
> ranges can overlap.
> For each individual GPIO output (level and direction separately), PIO considers all 8 writes that may have occurred on
> that cycle, and applies the write from the highest-numbered state machine. If the same state machine performs a SET
> /OUT and a side-set on the same GPIO simultaneously, the side-set is used. If no state machine writes to this GPIO
> output, its value does not change from the previous cycle.
> Generally each state machine’s outputs are mapped to a distinct group of GPIOs, implementing some peripheral
> interface."
> 
> so I suppose you want all of your pins consecutively in the same pin group.
> 
> It looks like this example does parallel output:
> https://github.com/raspberrypi/pico-examples/blob/master/gpio/hello_7segment/hello_7segment.c
> via gpio_set_mask() (set to high all the bits in the mask) and gpio_clr_mask()
> but I wouldn't say for sure that it's atomic.
> 
> Theo

I read something that indicates that groups of pins have their own tiny 
processor! You dont talk to the pins, you talk to the processor instead.

-- 
Climate Change: Socialism wearing a lab coat.