That is Half 2 of an exploration into the surprising 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 depend, instruction slots, the conduct of pull noblock, and sensible 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 method that brings to thoughts a Shakespearean tragedy.
Wat 5: Inconstant constants
On this planet of PIO programming, constants must be dependable, steadfast, and, effectively, 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 should not 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 instantly utilizing set:
; In Rust, make certain '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 yApart: Don’t attempt to perceive the loopy jmp operations right here. We’ll focus on 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 your complete PIO instruction. This produces a nonsense outcome.
Workarounds for inconstant constants
To deal with this limitation, think about the next approaches:
- Learn Values and Retailer Them in a Register: We noticed this method 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 continuing of any measurement. This, nonetheless, consumes time and operations out of your 32-operation finances (see Half 1, Wat 2).
; In Rust, make certain '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, decreasing the state machine pace from 125 MHz to 343 kHz reduces the timeout fixed
182,216to500. - Use Further 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 strategy 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 should not all the time as steadfast as they appear. But, simply as their story takes surprising turns, so too does ours, transferring 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 means of the looking-glass
In most programming environments, logical conditionals really feel balanced: you possibly can check if a pin is excessive or low, or test registers for equality or inequality. In PIO, this symmetry breaks down. You possibly can soar on pin excessive, however not pin low, and on x!=y, however not x==y. The foundations are whimsical — like Humpty Dumpty in By the Trying-Glass: “After I outline a conditional, it means simply what I select it to imply — neither extra nor much less.”
These quirks power us to rewrite our code to suit the lopsided logic, making a gulf between how we want the code may very well be written and the way we should write it.
The issue: Lopsided conditionals in motion
Think about a easy state of affairs: utilizing a spread 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 until timeoutAnd 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 XSadly, PIO doesn’t allow you to check !pin or x==y instantly. You should restructure your logic to accommodate the out there conditionals, resembling pin and x!=y.
The answer: The way in which it have to be
Given PIO’s limitations, we adapt our logic with a two-step method that ensures the specified conduct regardless of the lacking conditionals:
- Bounce on the alternative conditional to skip two directions ahead.
- Subsequent, use an unconditional soar to achieve the specified goal.
This workaround provides one further soar (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 depend down
jmp measurement_complete ; if echo voltage is low, measurement is full
echo_active:
jmp y-- measure_echo_loop ; Proceed counting down until timeoutAnd right here is the code for processing the measurement such that it’ll solely output differing values:
measurement_complete:
jmp x!=y send_result ; if measurement is completely 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 the Trying-Glass, Alice learns to navigate Humpty Dumpty’s peculiar world — simply as you’ll be taught 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 data.
In Half 1’s Wat 1 and Wat 3, we noticed how jmp x-- or jmp y-- is commonly used to loop a hard and fast variety of occasions 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_loopIn case you guessed that it does not soar to measure_echo_loop and as an alternative falls by means of to the subsequent instruction, you’re completely right. 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 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 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 every time she referred to as out their identify. In PIO programming, pin and pins can discuss with utterly completely 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, exterior of PIO. Nonetheless, particular person directions usually function on a subrange of these pin ranges. The conduct varies relying on the command: the subrange may very well 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 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 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 fast predecessor are despatched, decreasing 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 implies '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
; Fundamental 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 ; Look ahead to 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 until timeout
; Y tells the place the echo countdown stopped. It
; will probably be u32::MAX if the echo timed out.
measurement_complete:
jmp x!=y send_result ; if measurement is completely 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
; Calm down interval earlier than subsequent measurement
cooldown:
wait 0 pin 0 ; Look ahead to 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_activealso 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(Course::Out, &[&trigger_pio]);
distance_state_machine.set_pin_dirs(Course::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 set_set_pins, set_in_pins, and set_jmp_pin strategies on the Config struct.
set_in_pins: Specifies the pins for enter operations, resembling 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 only pin utilized in conditional jumps, resemblingjmp(pin, ...).
As described within the desk, different non-compulsory inputs embrace:
set_out_pins: Units the consecutive pins for output operations, resembling 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 lowThis method helps you to effectively apply bit patterns to a number of pins concurrently, streamlining management for functions 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 — resembling a pin’s state. It does not imply a group of issues, because it usually does in different programming contexts. When PIO refers to a group, it often 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 in case you assume 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 out there. Let’s see simply how far we will push them.
I wish to debug with interactive breakpoints in VS Code. I additionally do print debugging, the place you insert non permanent data 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 utilize data! 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 continuing worth, resembling 7, which have to be between 0 and 31 inclusive.
.program distance
; X is the final worth despatched. Initialize it to
; u32::MAX which implies '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 taking place within the PIO code (full code and mission):
// ...
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;
data!("end_loops: {}", end_loops);
}
// ...
Outputs:
INFO Howdy, 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 in a position to 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 circulation of your PIO program, even with no conventional debugger.
Apart: In C/C++ (and probably Rust), you may get nearer to a full debugging expertise for PIO, for instance, through the use of the piodebug mission.
That concludes the 9 Wats, however let’s convey all the pieces collectively in a bonus Wat.
Now that each one the parts 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 method is a operate that maps distances (from 0 to 50 cm) to tones (roughly B2 to F5). This operate is easy to write down in Rust, leveraging Rust’s floating-point math and exponential operations. Implementing this in PIO could be just about unattainable resulting from its restricted instruction set and lack of floating-point help.
Right here’s the core monitor program to run the theremin (full file and mission):
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 => {
data!("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;
data!("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 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 instantly on a single major processor. You code in Rust slightly than a combination of Rust and PIO. Though Embassy duties don’t actually run in parallel, they swap shortly sufficient to deal with functions like a theremin. Right here’s a snippet from theremin_no_pio.rs exhibiting an analogous core loop:
loop {
match distance.measure().await {
None => {
data!("Distance: out of vary");
sound.relaxation().await;
}
Some(distance_cm) => {
let tone_frequency = distance_to_tone_frequency(distance_cm);
data!("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 parts, let’s watch the video once more of me “taking part in” the musical instrument. On the monitor display screen, 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 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 far more you possibly can be taught 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 helpful Assets
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 additionally open doorways to new potentialities in embedded techniques 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 write down about one article per 30 days.
