4 Replies Latest reply on Jan 14, 2019 12:53 PM by user_246598725

    32-bit parallel non-contiguous GPIO write technique


      Hello all,

      In this post, I am showing the solution I worked out utilizing the PSoC 5LP UDB control registers and forced UDB placement to enable parallel 32-bit writes to GPIO pins which are not contiguous with a single write instruction. This can be used for writing 8 to 32 pins in parallel, but I am showing a 32-bit example. I found this to be extremely helpful and wanted to share it with the community (the project is attached).


      I needed a way to use the PSoC's CPU to write out 32-bits on the GPIO in parallel at a rate of about 16MHz. The trick to accomplishing this, and it should not be done unless you understand what data you are potentially overwriting in memory, is to force the UDB placement of control registers into UDBs located next to each other in memory. Writing the GPIO through the UDBs enables more pins to be accessed at one time than you could do using the CPU. The "map" I used to determine which UDBs to force the control registers to be in was provided by reddit user eric_j here. The post disputes the accuracy of the map, but for the UDBs I chose, it was accurate. The UDB placement I used can be seen in the image below:



      These placements correspond to the control registers in the following schematic:


      What this placement does is leverage the fact that the 8-bit control registers for these UDBs are contiguous in the PSoC memory.

      These are the address of the "[component name]_Sync_ctrl_reg__CONTROL_REG" pointed to in the cyfitter.h file.

           Bits_0_to_7       address = 0x4000647A

           Bits_8_to_15     address = 0x4000647B

           Bits_16_to_23   address = 0x4000647C

           Bits_24_to_31   address = 0x4000647D


      By writing an unsigned 32-bit value to the base address of the first 8-bit register will overflow the additional 24-bits into the next 3 8-bit registers resulting in a parallel 32-bit write to the connected GPIO pins. A similar configuration can be used to read when using status registers. Remember all the possible issues that come with abusing pointers and overwriting memory. Below is the code required to do this.


      Note: I have had write outputs higher than 20MHz when using optimized code where in-line literals are written and the for-loop is unrolled.


      I am no expert in the field of embedded systems and PSoCs, so there may better ways to do this, but this was the only solution I could come up with and worked amazingly well. I would love to hear any thoughts, criticisms, improvements, or applications that you can think of for this!



      - Brian

        • 1. Re: 32-bit parallel non-contiguous GPIO write technique



          Thx a lot for sharing, will certainely be useful to the community, and I'm one who's gonna implement it in one of my open projects.



          • 2. Re: 32-bit parallel non-contiguous GPIO write technique

            Hi Brian,


            this is great, it would also enable to work with DMA. Do you know a way to use the directives in a component, where the directives ensure that the control registers are placed contigous, but not into specific control registers?


            • 3. Re: 32-bit parallel non-contiguous GPIO write technique


              Interesting question. From what I understand of the directives provided by PSoC Creator (see pg. 336-340 of the user manual), there doesn't seem to be a way to dynamically ensure the registers get placed in contiguous memory. I tried using the placement_group attribute in a control file (see pg. 330-334 of the user manual) using control file pattern matching to ensure grouping but had no luck. It would place the first two registers contiguously, and the second two contiguously, but never all four contiguously.


              Something I have considered doing is creating my own custom 32-bit control/status register component using the UDB editor or basic Verilog script, then coupling that with a general directive to group its signals, but I'm unsure of how stable my self-created component would be in comparison to the PSoC provided components coupled with static placement directives.


              If you or someone else finds a way, I'd be excited to hear it!




              • 4. Re: 32-bit parallel non-contiguous GPIO write technique

                Hello Brian,


                can you provide your attempt with grouping by control file? I want to try a few things, but since I've never worked with control files yet it might be better to start from your implementation and try to modify.

                For example, I'd try to make three groups: one which has registers 0 + 1, one having 2 + 3, and a third which has 1 + 2. The first two groups would result in what your implementation already does, and the third group might be the "missing part". I don't know if the fitter would accept this or if it's totally nonsense But that's something I'd try.


                Or is it simply adding the four control registers in a component and a control file with the three grouping statements, using the control register names as "string"?