That is Half 2 of an exploration into the sudden quirks of programming the Raspberry Pi Pico PIO with Micropython. In case you missed Half 1, we uncovered 4 Wats that problem assumptions about register rely, 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 challenge that reveals a number of the quirks and perplexities of PIO programming. Put together to problem your understanding of constants in a method that brings to thoughts a Shakespearean tragedy.
Wat 5: Inconstant constants
On the planet of PIO programming, constants must be dependable, steadfast, and, properly, 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 may end up questioning if PIO constants will, as she says, “show likewise variable.”
The issue: Constants will not be as huge as they appear
Think about you’re programming an ultrasonic vary finder and must rely down from 500 whereas ready for the Echo sign to drop from excessive to low. To arrange this wait time in PIO, you may naïvely attempt to load the fixed worth immediately utilizing set:
; In Rust, make sure ‘config.shift_in.path = 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
Apart: 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 proscribed to constants between 0 and 31. Furthermore, the star-crossed set instruction doesn’t report an error. As an alternative, it silently corrupts all the PIO instruction. This produces a nonsense end result.
Workarounds for inconstant constants
To deal with this limitation, contemplate the next approaches:
Learn Values and Retailer Them in a Register: We noticed this strategy in Wat 1. You’ll be able to load your fixed within the osr register, then switch it to y. For instance:
# Learn the max echo wait into OSR.
pull ; identical 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 continuing of any dimension. This, nevertheless, consumes time and operations out of your 32-operation finances (see Half 1, Wat 2).
; In Rust, make sure ‘config.shift_in.path = 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: Cut back the frequency of the state machine to stretch delays over extra system clock cycles. For instance, reducing the state machine velocity from 125 MHz to 343 kHz reduces the timeout fixed 182,216 to 500.
Use Further Delays and (Nested) Loops: All directions help an non-obligatory 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 all the time as steadfast as they appear. But, simply as their story takes sudden 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 go away you questioning its loyalty to logic.
Wat 6: Conditionals by way of 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’ll be able to bounce 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: “After 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 could possibly be written and the way we should write it.
The issue: Lopsided conditionals in motion
Contemplate a easy state of affairs: utilizing a spread finder, you wish to rely down from a most wait time (y) till the ultrasonic echo pin goes low. Intuitively, you may 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 until timeout
And when processing the measurement, if we solely want to output values that differ from the earlier worth, we’d 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 X
Sadly, PIO doesn’t allow you to take a look at !pin or x==y immediately. You have to restructure your logic to accommodate the accessible 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:
Leap on the other conditional to skip two directions ahead.
Subsequent, use an unconditional bounce to achieve the specified goal.
This workaround provides one further bounce (affecting the instruction restrict), however the extra 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 rely down
jmp measurement_complete ; if echo voltage is low, measurement is full
echo_active:
jmp y– measure_echo_loop ; Proceed counting down until timeout
And 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 X
Classes 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 situations.
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 data.
In Half 1’s Wat 1 and Wat 3, we noticed how jmp x– or jmp y– is usually used to loop a hard and fast variety of instances by decrementing a register till it reaches 0. Simple sufficient, proper? However what occurs when y is 0 and we run the next instruction?
jmp y– measure_echo_loop
In case you guessed that it doesn’t bounce to measure_echo_loop and as an alternative falls by way of to the subsequent 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 probably have expertise with C or C++ which distinguish between pre-increment (e.g., ++x) and post-increment (e.g., x++) operations. The conduct of jmp 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 tips on how 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 limitless confusion at any time when she referred to 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 exhausting to know which Dave or Daves you’re speaking to.
The issue: Pin ranges and subranges
In PIO, each pin and pins directions rely on pin ranges outlined in Rust, outdoors of PIO. Nevertheless, particular person directions typically function on a subrange of these pin ranges. The conduct varies relying on the command: the subrange could possibly 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 exhibits how PIO interprets the phrases pin and pins in several directions, together with their related contexts and configurations.
Instance: Distance program for the vary finder
Right here’s a PIO program for measuring the space 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 potential.
Most Vary Restrict: Measurements are capped at a given distance, with a return worth of 4,294,967,295 if no object is detected.
Filtered Outputs: Solely measurements that differ from their quick predecessor are despatched, lowering the output fee.
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 ; identical as pull block
; Predominant 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 rely down
jmp measurement_complete ; if echo voltage is low, measurement is full
echo_active:
jmp y– measure_echo_loop ; Proceed counting down until 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
; Settle down interval earlier than subsequent measurement
cooldown:
wait 0 pin 0 ; Anticipate echo pin to be low
.wrap ; Restart the measurement loop
Configuring Pins
To make sure the PIO program behaves as meant:
set pins, 0b1 ought to management the Set off pin.
wait 1 pin 0 ought to monitor the Echo pin.
jmp pin echo_active also needs to 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.frequent.make_pio_pin({hardware}.set off);
let echo_pio = pio1.frequent.make_pio_pin({hardware}.echo);
distance_state_machine.set_pin_dirs(Route::Out, &[&trigger_pio]);
distance_state_machine.set_pin_dirs(Route::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.frequent.load_program(&program_with_defines.program);
config.use_program(&program, &[]); // No side-set pins
config
});
The keys listed here are the <sturdy>set_set_pins</sturdy>, <sturdy>set_in_pins</sturdy>, 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 of jmp(pin, …).
As described within the desk, different non-obligatory inputs embody:
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 spread 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.frequent.make_pio_pin({hardware}.trigger_a);
let trigger_b_pio = pio1.frequent.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 low
This strategy permits 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 price — reminiscent of a pin’s state. It doesn’t imply a group of issues, because it typically does in different programming contexts. When PIO refers to a group, 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’ll be able to 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 difficult, debugging a PIO program provides a completely new layer of problem. Within the subsequent Wat, we’ll dive into the kludgy debugging strategies accessible. 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 non permanent 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, nevertheless, I can do neither.
The fallback is push-to-print debugging. In PIO, you briefly output integer values of curiosity. Then, in Rust, you utilize 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 embody set and out to push a continuing 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 ; identical 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 taking place within the PIO code (full code and challenge):
// …
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 Whats up, 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:57
When 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, nevertheless, was too quick for this cheap oscilloscope to seize clearly.
Utilizing these strategies — particularly push-to-print debugging — you possibly can hint the stream of your PIO program, even with out a conventional debugger.
Apart: In C/C++ (and doubtlessly Rust), you may get nearer to a full debugging expertise for PIO, for instance, through the use of the piodebug challenge.
That concludes the 9 Wats, however let’s deliver the whole lot collectively in a bonus Wat.
Now that each one the elements are prepared, it’s time to mix them right into a working theremin-like musical instrument. We’d like 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 space is out of vary, it stops the tone.
Rust’s Place: On the coronary heart of this technique is a operate that maps distances (from 0 to 50 cm) to tones (roughly B2 to F5). This operate is easy to put in writing in Rust, leveraging Rust’s floating-point math and exponential operations. Implementing this in PIO can be nearly unimaginable on account of its restricted instruction set and lack of floating-point help.
Right here’s the core monitor program to run the theremin (full file and challenge):
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(Period::from_millis(50)).await;
}
}
}
Utilizing two PIO state machines alongside a Rust monitor program permits you to actually run three applications without delay. This setup is handy by itself and is crucial 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 immediately on a single principal processor. You code in Rust moderately than a combination of Rust and PIO. Though Embassy duties don’t actually run in parallel, they change shortly sufficient to deal with purposes like a theremin. Right here’s a snippet from theremin_no_pio.rs exhibiting 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(Period::from_millis(50)).await;
}
}
}
See our current 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 space 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 coated important floor — managing state machines, pin assignments, timing intricacies, and debugging — there’s nonetheless way 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 a part of, and extra.
Beneficial 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 challenge’s calls for but additionally open doorways to new potentialities in embedded methods programming.
Please observe Carl on In the direction of Knowledge Science and on @carlkadie.bsky.social. I write on scientific programming in Rust and Python, machine studying, and statistics. I have a tendency to put in writing about one article per 30 days.
