That is Half 2 of an exploration into the surprising quirks of programming the Raspberry Pi Pico PIO with Micropython. If you happen to missed Part 1, we uncovered 4 Wats that problem assumptions about register depend, instruction slots, the conduct of pull noblock, and good but low cost {hardware}.
Now, we proceed our journey towards crafting a theremin-like musical instrument — a mission that reveals a few of the quirks and perplexities of PIO programming. Put together to problem your understanding of constants in a manner that brings to thoughts a Shakespearean tragedy.
Wat 5: Inconstant constants
On the planet of PIO programming, constants must be dependable, steadfast, and, nicely, fixed. However what in the event that they’re not? This brings us to a puzzling Wat about how the set instruction in PIO works—or doesn’t—when dealing with bigger constants.
Very like Juliet doubting Romeo’s fidelity, you would possibly end up questioning if PIO constants will, as she says, “show likewise variable.”
The issue: Constants will not be as large as they appear
Think about you’re programming an ultrasonic vary finder and must depend down from 500 whereas ready for the Echo sign to drop from excessive to low. To arrange this wait time in PIO, you would possibly naïvely attempt to load the fixed worth straight utilizing set:
; In Rust, make certain 'config.shift_in.course = ShiftDirection::Left;'
set y, 15 ; Load higher 5 bits (0b01111)
mov isr, y ; Switch to ISR (clears ISR)
set y, 20 ; Load decrease 5 bits (0b10100)
in y, 5 ; Shift in decrease bits to type 500 in ISR
mov y, isr ; Switch again to yApart: Don’t attempt to perceive the loopy jmp operations right here. We’ll talk about these subsequent in Wat 6.
However right here’s the tragic twist: the set instruction in PIO is restricted to constants between 0 and 31. Furthermore, the star-crossed set instruction doesn’t report an error. As an alternative, it silently corrupts the entire PIO instruction. This produces a nonsense consequence.
Workarounds for inconstant constants
To deal with this limitation, take into account the next approaches:
- Learn Values and Retailer Them in a Register: We noticed this strategy in Wat 1. You possibly can load your fixed within the
osrregister, then switch it to y. For instance:
# Learn the max echo wait into OSR.
pull ; similar as pull block
mov y, osr ; Load max echo wait into Y- Shift and Mix Smaller Values: Utilizing the isr register and the in instruction, you possibly can construct up a relentless of any dimension. This, nonetheless, consumes time and operations out of your 32-operation finances (see Part 1, Wat 2).
; In Rust, make certain 'config.shift_in.course = ShiftDirection::Left;'
set y, 15 ; Load higher 5 bits (0b01111)
mov isr, y ; Switch to ISR (clears ISR)
set y, 20 ; Load decrease 5 bits (0b10100)
in y, 5 ; Shift in decrease bits to type 500 in ISR
mov y, isr ; Switch again to y- Gradual Down the Timing: Scale back the frequency of the state machine to stretch delays over extra system clock cycles. For instance, reducing the state machine pace from 125 MHz to 343 kHz reduces the timeout fixed
182,216to500. - Use Additional Delays and (Nested) Loops: All directions help an non-compulsory delay, permitting you so as to add as much as 31 further cycles. (To generate even longer delays, use loops — and even nested loops.)
; Generate 10μs set off pulse (4 cycles at 343_000Hz)
set pins, 1 [3] ; Set set off pin to excessive, add delay of three
set pins, 0 ; Set set off pin to low voltage- Use the “Subtraction Trick” to Generate the Most 32-bit Integer: In Wat 7, we’ll discover a solution to generate
4,294,967,295(the utmost unsigned 32-bit integer) by way of subtraction.
Very like Juliet cautioning in opposition to swearing by the inconstant moon, we’ve found that PIO constants will not be at all times as steadfast as they appear. But, simply as their story takes surprising turns, so too does ours, shifting from the inconstancy of constants to the uneven nature of conditionals. Within the subsequent Wat, we’ll discover how PIO’s dealing with of conditional jumps can depart you questioning its loyalty to logic.
Wat 6: Conditionals via the looking-glass
In most programming environments, logical conditionals really feel balanced: you possibly can take a look at if a pin is excessive or low, or test registers for equality or inequality. In PIO, this symmetry breaks down. You possibly can leap on pin excessive, however not pin low, and on x!=y, however not x==y. The foundations are whimsical — like Humpty Dumpty in By means of the Wanting-Glass: “Once I outline a conditional, it means simply what I select it to imply — neither extra nor much less.”
These quirks pressure us to rewrite our code to suit the lopsided logic, making a gulf between how we want the code might be written and the way we should write it.
The issue: Lopsided conditionals in motion
Think about a easy situation: utilizing a variety finder, you need to depend down from a most wait time (y) till the ultrasonic echo pin goes low. Intuitively, you would possibly write the logic like this:
measure_echo_loop:
jmp !pin measurement_complete ; If echo voltage is low, measurement is full
jmp y-- measure_echo_loop ; Proceed counting down except timeoutAnd when processing the measurement, if we solely want to output values that differ from the earlier worth, we might write:
measurement_complete:
jmp x==y cooldown ; If measurement is identical, skip to chill down
mov isr, y ; Retailer measurement in ISR
push ; Output ISR
mov x, y ; Save the measurement in XSadly, PIO doesn’t allow you to take a look at !pin or x==y straight. You should restructure your logic to accommodate the obtainable conditionals, reminiscent of pin and x!=y.
The answer: The best way it have to be
Given PIO’s limitations, we adapt our logic with a two-step strategy that ensures the specified conduct regardless of the lacking conditionals:
- Bounce on the alternative conditional to skip two directions ahead.
- Subsequent, use an unconditional leap to achieve the specified goal.
This workaround provides one further leap (affecting the instruction restrict), however the further label is cost-free.
Right here is the rewritten code for counting down till the pin goes low:
measure_echo_loop:
jmp pin echo_active ; if echo voltage is excessive proceed depend down
jmp measurement_complete ; if echo voltage is low, measurement is full
echo_active:
jmp y-- measure_echo_loop ; Proceed counting down except timeoutAnd right here is the code for processing the measurement such that it’s going to solely output differing values:
measurement_complete:
jmp x!=y send_result ; if measurement is totally different, then ship it.
jmp cooldown ; If measurement is identical, do not ship.
send_result:
mov isr, y ; Retailer measurement in ISR
push ; Output ISR
mov x, y ; Save the measurement in XClasses from Humpty Dumpty’s conditionals
In By means of the Wanting-Glass, Alice learns to navigate Humpty Dumpty’s peculiar world — simply as you’ll study to navigate PIO’s Wonderland of lopsided circumstances.
However as quickly as you grasp one quirk, one other reveals itself. Within the subsequent Wat, we’ll uncover a shocking conduct of jmp that, if it had been an athlete, would shatter world information.
In Part 1’s Wat 1 and Wat 3, we noticed how jmp x-- or jmp y-- is usually used to loop a set variety of instances by decrementing a register till it reaches 0. Easy sufficient, proper? However what occurs when y is 0 and we run the next instruction?
jmp y-- measure_echo_loopIf you happen to guessed that it does not leap to measure_echo_loop and as an alternative falls via to the following instruction, you’re completely appropriate. However for full credit score, reply this: What worth does y have after the instruction?
The reply: 4,294,967,295. Why? As a result of y is decremented after it’s examined for zero. Wat!?
Apart: If this doesn’t shock you, you possible have expertise with C or C++ which distinguish between pre-increment (e.g.,
++x) and post-increment (e.g., x++) operations. The conduct ofjmp y--is equal to a post-decrement, the place the worth is examined earlier than being decremented.
This worth, 4,294,967,295, is the utmost for a 32-bit unsigned integer. It’s as if a track-and-field lengthy jumper launches off the takeoff board however, as an alternative of touchdown within the sandpit, overshoots and finally ends up on one other continent.
Apart: As foreshadowed in Wat 5, we will use this conduct deliberately to set a register to the worth 4,294,967,295.
Now that we’ve discovered easy methods to stick the touchdown with jmp, let’s see if we will keep away from getting caught by the pins that PIO reads and units.
In Dr. Seuss’s Too Many Daves, Mrs. McCave had 23 sons, all named Dave, resulting in infinite confusion at any time when she known as out their title. In PIO programming, pin and pins can discuss with utterly totally different ranges of pins relying on the context. It’s laborious to know which Dave or Daves you’re speaking to.
The issue: Pin ranges and subranges
In PIO, each pin and pins directions rely upon pin ranges outlined in Rust, outdoors of PIO. Nonetheless, particular person directions typically function on a subrange of these pin ranges. The conduct varies relying on the command: the subrange might be the primary n pins of the vary, all of the pins, or only a particular pin given by an index. To make clear PIO’s conduct, I created the next desk:
This desk reveals how PIO interprets the phrases pin and pins in numerous directions, together with their related contexts and configurations.
Instance: Distance program for the vary finder
Right here’s a PIO program for measuring the gap to an object utilizing Set off and Echo pins. The important thing options of this program are:
- Steady Operation: The vary finder runs in a loop as quick as attainable.
- Most Vary Restrict: Measurements are capped at a given distance, with a return worth of
4,294,967,295if no object is detected. - Filtered Outputs: Solely measurements that differ from their speedy predecessor are despatched, lowering the output charge.
Look over this system and see that though it’s working with two pins — Set off and Echo — all through this system we solely see pin and pins.
.program distance
; X is the final worth despatched. Initialize it to
; u32::MAX which suggests 'echo timeout'
; (Set X to u32::MAX by subtracting 1 from 0)
set x, 0
subtraction_trick:
jmp x-- subtraction_trick
; Learn the max echo wait into OSR
pull ; similar as pull block
; Important loop
.wrap_target
; Generate 10μs set off pulse (4 cycles at 343_000Hz)
set pins, 0b1 [3] ; Set set off pin to excessive, add delay of three
set pins, 0b0 ; Set set off pin to low voltage
; When the set off goes excessive, begin counting down till it goes low
wait 1 pin 0 ; Anticipate echo pin to be excessive voltage
mov y, osr ; Load max echo wait into Y
measure_echo_loop:
jmp pin echo_active ; if echo voltage is excessive proceed depend down
jmp measurement_complete ; if echo voltage is low, measurement is full
echo_active:
jmp y-- measure_echo_loop ; Proceed counting down except timeout
; Y tells the place the echo countdown stopped. It
; can be u32::MAX if the echo timed out.
measurement_complete:
jmp x!=y send_result ; if measurement is totally different, then despatched it.
jmp cooldown ; If measurement is identical, do not ship.
send_result:
mov isr, y ; Retailer measurement in ISR
push ; Output ISR
mov x, y ; Save the measurement in X
; Quiet down interval earlier than subsequent measurement
cooldown:
wait 0 pin 0 ; Anticipate echo pin to be low
.wrap ; Restart the measurement loopConfiguring Pins
To make sure the PIO program behaves as supposed:
set pins, 0b1ought to management the Set off pin.wait 1 pin 0ought to monitor the Echo pin.jmp pin echo_activemust also monitor the Echo pin.
Right here’s how one can configure this in Rust (adopted by an evidence):
let mut distance_state_machine = pio1.sm0;
let trigger_pio = pio1.widespread.make_pio_pin({hardware}.set off);
let echo_pio = pio1.widespread.make_pio_pin({hardware}.echo);
distance_state_machine.set_pin_dirs(Path::Out, &[&trigger_pio]);
distance_state_machine.set_pin_dirs(Path::In, &[&echo_pio]);
distance_state_machine.set_config(&{
let mut config = Config::default();
config.set_set_pins(&[&trigger_pio]); // For set instruction
config.set_in_pins(&[&echo_pio]); // For wait instruction
config.set_jmp_pin(&echo_pio); // For jmp instruction
let program_with_defines = pio_file!("examples/distance.pio");
let program = pio1.widespread.load_program(&program_with_defines.program);
config.use_program(&program, &[]); // No side-set pins
config
});The keys listed here are the set_set_pins, set_in_pins, and set_jmp_pin strategies on the Config struct.
set_in_pins: Specifies the pins for enter operations, reminiscent of wait(1, pin, …). The “in” pins have to be consecutive.set_set_pins: Configures the pin for set operations, like set(pins, 1). The “set” pins should even be consecutive.set_jmp_pin: Defines the one pin utilized in conditional jumps, reminiscent ofjmp(pin, ...).
As described within the desk, different non-compulsory inputs embrace:
set_out_pins: Units the consecutive pins for output operations, reminiscent of out(pins, …).use_program: Units a) the loaded program and b) consecutive pins for sideset operations. Sideset operations permit simultaneous pin toggling throughout different directions.
Configuring A number of Pins
Though not required for this program, you possibly can configure a variety of pins in PIO by offering a slice of consecutive pins. For instance, suppose we had two ultrasonic vary finders:
let trigger_a_pio = pio1.widespread.make_pio_pin({hardware}.trigger_a);
let trigger_b_pio = pio1.widespread.make_pio_pin({hardware}.trigger_b);
config.set_set_pins(&[&trigger_a_pio, &trigger_b_pio]);A single instruction can then management each pins:
set pins, 0b11 [3] # Units each set off pins (17, 18) excessive, provides delay
set pins, 0b00 # Units each set off pins lowThis strategy helps you to effectively apply bit patterns to a number of pins concurrently, streamlining management for purposes involving a number of outputs.
Apart: The Phrase “Set” in Programming
In programming, the phrase “set” is notoriously overloaded with a number of meanings. Within the context of PIO, “set” refers to one thing to which you’ll be able to assign a worth — reminiscent of a pin’s state. It does not imply a set of issues, because it typically does in different programming contexts. When PIO refers to a set, it normally makes use of the time period “vary” as an alternative. This distinction is essential for avoiding confusion as you’re employed with PIO.
Classes from Mrs. McCave
In Too Many Daves, Mrs. McCave lamented not giving her 23 Daves extra distinct names. You possibly can keep away from her mistake by clearly documenting your pins with significant names — like Set off and Echo — in your feedback.
However should you suppose dealing with these pin ranges is hard, debugging a PIO program provides a completely new layer of problem. Within the subsequent Wat, we’ll dive into the kludgy debugging strategies obtainable. Let’s see simply how far we will push them.
I prefer to debug with interactive breakpoints in VS Code. I additionally do print debugging, the place you insert momentary information statements to see what the code is doing and the values of variables. Utilizing the Raspberry Pi Debug Probe and probe-rs, I can do each of those with common Rust code on the Pico.
With PIO programming, nonetheless, I can do neither.
The fallback is push-to-print debugging. In PIO, you briefly output integer values of curiosity. Then, in Rust, you employ information! to print these values for inspection.
For instance, within the following PIO program, we briefly add directions to push the worth of x for debugging. We additionally embrace set and out to push a relentless worth, reminiscent of 7, which have to be between 0 and 31 inclusive.
.program distance
; X is the final worth despatched. Initialize it to
; u32::MAX which suggests 'echo timeout'
; (Set X to u32::MAX by subtracting 1 from 0)
set x, 0
subtraction_trick:
jmp x-- subtraction_trick
; DEBUG: See the worth of x
mov isr, x
push
; Learn the max echo wait into OSR
pull ; similar as pull block
; DEBUG: Ship fixed worth
set y, 7 ; Push '7' in order that we all know we have reached this level
mov isr, y
push
; ...Again in Rust, you possibly can learn and print these values to assist perceive what’s occurring within the PIO code (full code and project):
// ...
distance_state_machine.set_enable(true);
distance_state_machine.tx().wait_push(MAX_LOOPS).await;
loop {
let end_loops = distance_state_machine.rx().wait_pull().await;
information!("end_loops: {}", end_loops);
}
// ...
Outputs:
INFO Hiya, debug!
└─ distance_debug::inner_main::{async_fn#0} @ examplesdistance_debug.rs:27
INFO end_loops: 4294967295
└─ distance_debug::inner_main::{async_fn#0} @ examplesdistance_debug.rs:57
INFO end_loops: 7
└─ distance_debug::inner_main::{async_fn#0} @ examplesdistance_debug.rs:57When push-to-print debugging isn’t sufficient, you possibly can flip to {hardware} instruments. I purchased my first oscilloscope (a FNIRSI DSO152, for $37). With it, I used to be capable of verify the Echo sign was working. The Set off sign, nonetheless, was too quick for this cheap oscilloscope to seize clearly.
Utilizing these strategies — particularly push-to-print debugging — you possibly can hint the movement of your PIO program, even with out a conventional debugger.
Apart: In C/C++ (and probably Rust), you will get nearer to a full debugging expertise for PIO, for instance, through the use of the piodebug project.
That concludes the 9 Wats, however let’s carry the whole lot collectively in a bonus Wat.
Now that every one the elements are prepared, it’s time to mix them right into a working theremin-like musical instrument. We want a Rust monitor program. This program begins each PIO state machines — one for measuring distance and the opposite for producing tones. It then waits for a brand new distance measurement, maps that distance to a tone, and sends the corresponding tone frequency to the tone-playing state machine. If the gap is out of vary, it stops the tone.
Rust’s Place: On the coronary heart of this method is a operate that maps distances (from 0 to 50 cm) to tones (roughly B2 to F5). This operate is straightforward to write down in Rust, leveraging Rust’s floating-point math and exponential operations. Implementing this in PIO could be nearly unattainable attributable to its restricted instruction set and lack of floating-point help.
Right here’s the core monitor program to run the theremin (full file and project):
sound_state_machine.set_enable(true);
distance_state_machine.set_enable(true);
distance_state_machine.tx().wait_push(MAX_LOOPS).await;
loop {
let end_loops = distance_state_machine.rx().wait_pull().await;
match loop_difference_to_distance_cm(end_loops) {
None => {
information!("Distance: out of vary");
sound_state_machine.tx().wait_push(0).await;
}
Some(distance_cm) => {
let tone_frequency = distance_to_tone_frequency(distance_cm);
let half_period = sound_state_machine_frequency / tone_frequency as u32 / 2;
information!("Distance: {} cm, tone: {} Hz", distance_cm, tone_frequency);
sound_state_machine.tx().push(half_period); // non-blocking push
Timer::after(Length::from_millis(50)).await;
}
}
}Utilizing two PIO state machines alongside a Rust monitor program helps you to actually run three applications directly. This setup is handy by itself and is important when strict timing or very high-frequency I/O operations are required.
Apart: Alternatively, Rust Embassy’s async duties allow you to implement cooperative multitasking straight on a single primary processor. You code in Rust relatively than a combination of Rust and PIO. Though Embassy duties don’t actually run in parallel, they swap shortly sufficient to deal with purposes like a theremin. Right here’s a snippet from theremin_no_pio.rs displaying an analogous core loop:
loop {
match distance.measure().await {
None => {
information!("Distance: out of vary");
sound.relaxation().await;
}
Some(distance_cm) => {
let tone_frequency = distance_to_tone_frequency(distance_cm);
information!("Distance: {} cm, tone: {} Hz", distance_cm, tone_frequency);
sound.play(tone_frequency).await;
Timer::after(Length::from_millis(50)).await;
}
}
}See our recent article on Rust Embassy programming for extra particulars.
Now that we’ve assembled all of the elements, let’s watch the video once more of me “taking part in” the musical instrument. On the monitor display, you possibly can see the debugging prints displaying the gap measurements and the corresponding tones. This visible connection highlights how the system responds in actual time.
Conclusion
PIO programming on the Raspberry Pi Pico is a fascinating mix of simplicity and complexity, providing unparalleled {hardware} management whereas demanding a shift in mindset for builders accustomed to higher-level programming. By means of the 9 Wats we’ve explored, PIO has each stunned us with its limitations and impressed us with its uncooked effectivity.
Whereas we’ve lined important floor — managing state machines, pin assignments, timing intricacies, and debugging — there’s nonetheless far more you possibly can study as wanted: DMA, IRQ, side-set pins, variations between PIO on the Pico 1 and Pico 2, autopush and autopull, FIFO be part of, and extra.
Really useful Sources
At its core, PIO’s quirks replicate a design philosophy that prioritizes low-level {hardware} management with minimal overhead. By embracing these traits, PIO won’t solely meet your mission’s calls for but in addition open doorways to new prospects in embedded programs programming.
Please follow Carl on Towards Data Science and on @carlkadie.bsky.social. I write on scientific programming in Rust and Python, machine studying, and statistics. I have a tendency to write down about one article per thirty days.
