That is Half 2 of an exploration into the surprising quirks of programming the Raspberry Pi Pico PIO with Micropython. In case you missed Part 1, we uncovered 4 Wats that problem assumptions about register rely, 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 challenge that reveals a few of the quirks and perplexities of PIO programming. Put together to problem your understanding of constants in a approach that brings to thoughts a Shakespearean tragedy.
Wat 5: Inconstant constants
On the earth of PIO programming, constants needs to 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 similar to 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 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 would possibly naïvely attempt to load the fixed worth straight utilizing set:
; In Rust, ensure '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 kind 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 a substitute, it silently corrupts the entire PIO instruction. This produces a nonsense consequence.
Workarounds for inconstant constants
To handle 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 ; 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’ll be able to construct up a relentless of any dimension. This, nevertheless, consumes time and operations out of your 32-operation price range (see Part 1, Wat 2).
; In Rust, ensure '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 kind 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, 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-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 technique to generate
4,294,967,295(the utmost unsigned 32-bit integer) through subtraction.
Very similar to Juliet cautioning in opposition to swearing by the inconstant moon, we’ve found that PIO constants should not at all times 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 the looking-glass
In most programming environments, logical conditionals really feel balanced: you’ll be able to 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 soar on pin excessive, however not pin low, and on x!=y, however not x==y. The principles 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 might be written and the way we should write it.
The issue: Lopsided conditionals in motion
Take into account a easy state of affairs: utilizing a variety finder, you need to rely 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’d write:
measurement_complete:
jmp x==y cooldown ; If measurement is similar, 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 will need to restructure your logic to accommodate the out there conditionals, equivalent to 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:
- Leap on the other 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 rely 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 similar, 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 have 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 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 a substitute falls by 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 a substitute 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 realized how one can 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 countless confusion each time she referred to as out their title. In PIO programming, pin and pins can confer with fully 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, exterior 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 exhibits 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 potential.
- 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 instant predecessor are despatched, lowering the output price.
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
; Most 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 ; Look forward 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 rely 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
; shall 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 similar, 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 ; Look forward to echo pin to be low
.wrap ; Restart the measurement loopConfiguring Pins
To make sure the PIO program behaves as meant:
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.frequent.make_pio_pin({hardware}.set off);
let echo_pio = pio1.frequent.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.frequent.load_program(&program_with_defines.program);
config.use_program(&program, &[]); // No side-set pins
config
});The keys listed below 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, equivalent to 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, equivalent tojmp(pin, ...).
As described within the desk, different non-obligatory inputs embrace:
set_out_pins: Units the consecutive pins for output operations, equivalent to out(pins, …).use_program: Units a) the loaded program and b) consecutive pins for sideset operations. Sideset operations enable simultaneous pin toggling throughout different directions.
Configuring A number of Pins
Though not required for this program, you’ll be able to 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.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 permits 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 — equivalent to a pin’s state. It does not imply a group of issues, because it typically does in different programming contexts. When PIO refers to a group, it often makes use of the time period “vary” as a substitute. 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 the event 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 prefer to debug with interactive breakpoints in VS Code. I additionally do print debugging, the place you insert short-term 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 embrace set and out to push a relentless worth, equivalent to 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’ll be able to 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 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:57When push-to-print debugging isn’t sufficient, you’ll be able to 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’ll be able to hint the stream of your PIO program, even with out a conventional debugger.
Apart: In C/C++ (and probably Rust), you may get nearer to a full debugging expertise for PIO, for instance, by utilizing the piodebug project.
That concludes the 9 Wats, however let’s carry every thing 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 technique is a perform that maps distances (from 0 to 50 cm) to tones (roughly B2 to F5). This perform is easy to put in writing in Rust, leveraging Rust’s floating-point math and exponential operations. Implementing this in PIO could be just about unimaginable because 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 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(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 straight on a single foremost processor. You code in Rust somewhat 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 identical 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 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 “enjoying” the musical instrument. On the monitor display screen, you’ll be able to 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 charming 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 vital floor — managing state machines, pin assignments, timing intricacies, and debugging — there’s nonetheless way more you’ll be able to 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.
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 is not going to solely meet your challenge’s calls for but additionally open doorways to new potentialities in embedded techniques 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 put in writing about one article per thirty days.
