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XPost: sci.electronics.design
On Thu, 12 Sep 2024 18:24:41 +0100, The Natural Philosopher
wrote:
>On 12/09/2024 17:43, Theo wrote:
>> In comp.sys.raspberry-pi john larkin 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.
RP2040 has I think 2 PIO blocks and each has a couple of hardware
programmable state machines, and those can be set up to drive pins.
I'm mostly concerned now with a PCB schematic design, picking the
appropriate and easy-to-route pins from the CPU to things.
My general question, for various processors, is how to associate bits
in integer variables with physical pins on the chip.
--- SoupGate-Win32 v1.05
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