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Update README.md with how I control the CH446Qs
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Kevin Santo Cappuccio
GitHub
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@ -276,7 +276,7 @@ Index Name Number Nodes Bridges
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17 Net 17 17 9,8 {9-8}
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```
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## Pathfinding
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# Pathfinding
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This is the *really* tricky part. I probably wrote all this code about 4 times, trashed it and started over from scratch with only the lessons learned from the last attempt.
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Earlier versions would add connections one at a time, but you'd end up in weird states because it has no knowledge of what other paths it needs to make room for. So the final version here clears the connections from the last update, takes in all the connections to be made, and finds paths for the whole board every time you add a new wire. All the old connections usually follow the same paths as last time unless they need to be nudged over to make room for some other path, and the actual connection won't be interrupted at all.
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@ -615,7 +615,252 @@ path net node1 type chip0 x0 y0 node2 type chip1 x
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```
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# Part 3 - Controlling the Crosspoint Switches
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## What crosspoint switches crave
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Okay, so now we have all our paths filled out with what chips need to have which X and Y inputs connected to make the magic happen.
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The CH446Qs are basically clones of the MT8816, except for one important difference, they accept serial addressing. The datasheet is kinda vague about how, but it turns out it's just a kinda weird version of SPI.
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Basically, all the chips see the same data signal, and whichever one sees a pulse on the STB when the last bit comes in will connect or disconnect the selected X and Y inputs. The state of the DAT line when the Strobe comes in determines whether it's connecting or disconnecting. That stretched out clock line shows that it doesn't care about real time, which comes in handy.
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## PIO State Machine
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So I have to do something that's kinda like SPI but not quite, this looks like a job for the RP2040 PIO State Machines.
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Even knowing assembly, the learning curve for writing a PIO program is steep. The documentation is really hit-or-miss, the examples are uncommented and written like they're playing code golf, insanely terse. Like these people do realize you can name variables after what they do, right? And these are the Official examples in the datasheet. Anyway after a few days of staring at what looks like gibberish, it starts to click.
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I copied the SPI.pio example and edited it from there. Let me try to explain some of the things I learned to hopefully make it easier for you to write a PIO program in the future.
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I'm just compiling this with the online pioasm compiler and then pasting the compiled code into spi.pio.h
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https://wokwi.com/tools/pioasm
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Here's where we are:
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;this is basically spi but it sets a system IRQ on the last bit to allow the chip select pulse to happen
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```
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.program spi_ch446_multi_cs
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.side_set 1
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.wrap_target
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bitloop:
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out pins, 1 side 0x0 [2]
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nop side 0x1 [2]
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jmp x-- bitloop side 0x1
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out pins, 1 side 0x1
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mov x, y side 0x1
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irq 0 side 0x1
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wait 0 irq 0 rel side 0x1
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jmp !osre bitloop side 0x0
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public entry_point: ; Must set X,Y to n-2 before starting!
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pull ifempty side 0x0 [1] ; Block with CSn high (minimum 2 cycles)
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nop side 0x0 [1]; CSn front porch
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.wrap
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```
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What wasn't explained well is what the hell a sideset pin is. Basically you do your normal-ish assembly code on the left, and then each operation also affects the sideset pin on the right. It's kind of a hack to allow you to control 2 pins in a single clock cycle. In this case, the sideset pin is attached to the CLK, and pins, 1 is DAT.
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So, whats going on is that in the regular code, I'm sending a byte to the sm register with this line
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pio_sm_put(pio, sm, chAddress);
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(the last bit of chAddress is set to 1 or 0 depending if I want to connect or disconnect)
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and that pull ifempty will pull in a byte to the working register and send it out one bit at a time while toggling the clock. When it's out of data to send, it triggers a system interrupt request that can be seen outside of the PIO state machine and I deal with it in an ISR in CH446Q.cpp
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At this point, here's where we are in the timing diagram:
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Now we need to select the correct CS line to make the right chip make the connection
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```
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void isrFromPio(void)
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{
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switch (chipSelect)
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{
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case CHIP_A:
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{
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digitalWriteFast(CS_A, HIGH);
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break;
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}
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case CHIP_B:
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{
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digitalWriteFast(CS_B, HIGH);
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break;
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}
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case CHIP_C:
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{
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digitalWriteFast(CS_C, HIGH);
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break;
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}
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case CHIP_D:
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{
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digitalWriteFast(CS_D, HIGH);
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break;
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}
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case CHIP_E:
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{
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digitalWriteFast(CS_E, HIGH);
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break;
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}
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case CHIP_F:
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{
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digitalWriteFast(CS_F, HIGH);
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break;
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}
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case CHIP_G:
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{
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digitalWriteFast(CS_G, HIGH);
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break;
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}
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case CHIP_H:
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{
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digitalWriteFast(CS_H, HIGH);
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break;
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}
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case CHIP_I:
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{
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digitalWriteFast(CS_I, HIGH);
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break;
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}
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case CHIP_J:
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{
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digitalWriteFast(CS_J, HIGH);
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break;
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}
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case CHIP_K:
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{
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digitalWriteFast(CS_K, HIGH);
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break;
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}
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case CHIP_L:
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{
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digitalWriteFast(CS_L, HIGH);
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break;
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}
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}
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```
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```
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delayMicroseconds(1);
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```
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```
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digitalWriteFast(CS_A, LOW);
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digitalWriteFast(CS_B, LOW);
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digitalWriteFast(CS_C, LOW);
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digitalWriteFast(CS_D, LOW);
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digitalWriteFast(CS_E, LOW);
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digitalWriteFast(CS_F, LOW);
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digitalWriteFast(CS_G, LOW);
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digitalWriteFast(CS_H, LOW);
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digitalWriteFast(CS_I, LOW);
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digitalWriteFast(CS_J, LOW);
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digitalWriteFast(CS_K, LOW);
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digitalWriteFast(CS_L, LOW);
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irq_flags = pio0_hw->irq;
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pio_interrupt_clear(pio, PIO0_IRQ_0);
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hw_clear_bits(&pio0_hw->irq, irq_flags);//clears the IRQ
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}
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```
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The reason I had to do it in an external interrupt instead of in the PIO code is because there's a limit to how many pins can be attached to a single state machine, 8. And this is just way easier to do.
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## The C
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This all runs on the second core just so it can stay somewhat timing sensitive while not worrying about what's going on elsewhere. How this process is triggered is that when the pathfinding algorithm is finished running in core 0, it sets
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```
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volatile int sendAllPathsCore2 = 1; // this signals the core 2 to send all the paths to the CH446Q
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```
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Then in loop1, is just constantly checks if that's a 1 and will send the paths and set it back to 0. Just a reminder that the cores on an RP2040 share global variables, because it's very useful.
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Here's what the SendAllPaths() functions look like.
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```
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void sendAllPaths(void) // should we sort them by chip? for now, no
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{
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for (int i = 0; i < numberOfPaths; i++)
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{
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sendPath(i, 1);
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}
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}
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void sendPath(int i, int setOrClear)
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{
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uint32_t chAddress = 0;
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int chipToConnect = 0;
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int chYdata = 0;
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int chXdata = 0;
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for (int chip = 0; chip < 4; chip++)
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{
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if (path[i].chip[chip] != -1)
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{
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chipSelect = path[i].chip[chip];
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chipToConnect = path[i].chip[chip];
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chYdata = path[i].y[chip];
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chXdata = path[i].x[chip];
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chYdata = chYdata << 5;
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chYdata = chYdata & 0b11100000;
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chXdata = chXdata << 1;
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chXdata = chXdata & 0b00011110;
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chAddress = chYdata | chXdata;
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if (setOrClear == 1)
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{
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chAddress = chAddress | 0b00000001; // this last bit determines whether we set or unset the path
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}
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chAddress = chAddress << 24;
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// delayMicroseconds(50);
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delayMicroseconds(30);
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pio_sm_put(pio, sm, chAddress);
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delayMicroseconds(40);
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//}
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}
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}
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}
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```
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The whole process to connect the whole board takes a couple milliseconds at most. But you can shave down these delays if you have some reason to go faster.
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## To be continued in part 4 - LEDs
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