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   XPost: sci.electronics.design   
      
   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/   
   ello_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.   
      
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