AI in FabAcademy steptime1

The provided code, hello.steptime1.RP2040.py, is designed to measure the step-response loading on a single pin using the Programmable Input/Output (PIO) capabilities of the RP2040 microcontroller. This technique is useful for applications like capacitive touch sensing, where changes in capacitance can be detected by measuring the time it takes for a signal to rise or fall.

Here's a step-by-step explanation of the code:

Importing Modules and Setting Frequency:
```
from machine import Pin, freq
import rp2
```
These imports bring in the necessary modules to interact with the machine's hardware and the RP2040's PIO functionalities. The freq function is used later to set the CPU frequency.
Defining the PIO Assembly Program:
```
@rp2.asm_pio(set_init=rp2.PIO.OUT_HIGH)
def steptimer():
    # PIO assembly code
```
This decorator defines a PIO assembly program named steptimer. The set_init=rp2.PIO.OUT_HIGH parameter initializes the pin to a high state.
PIO Assembly Code: The PIO program performs the following steps:
Initialization:
- pull(): Pulls initialization data from the output shift register (OSR).
- mov(y, osr): Moves the pulled data to register y, setting the loop count.
- pull(): Pulls the settle time from the OSR.
- mov(x, null): Clears register x, which will be used as a counter.
- set(pins, 1): Sets the pin to a high state.
Main Loop ('main loop'):
- Charge Up:
- set(pindirs, 1): Sets the pin direction to output, initiating the charging phase.
- Settle Up:
- mov(isr, x): Moves the current count to the input shift register (ISR).
- mov(x, osr): Loads the settle time into x.
- 'up settle loop': A loop that decrements x until it reaches zero, allowing the signal to settle.
- Discharge:
- set(pindirs, 0): Sets the pin direction to input, starting the discharging phase.
- Time Down:
- 'down loop': A loop that waits for the pin to go low, decrementing x with each iteration.
- Once the pin goes low or x reaches zero, it exits the loop.
- Loop Control:
- jmp(y_dec, 'main loop'): Decrements y (loop counter) and jumps back to 'main loop' if y is not zero.
- After completing the loops, it pushes the count to the FIFO with mov(isr, x) and push().

Creating the steptime Class:

class steptime:
    def __init__(self, sm_id, pin):
        self._sm = rp2.StateMachine(sm_id, steptimer, jmp_pin=pin, set_base=pin)
        self._sm.active(True)
        self.get = self._sm.get
        self.put = self._sm.put

This class initializes a state machine to run the steptimer PIO program on a specified pin. It provides get and put methods to interact with the state machine's FIFO.

Setting the CPU Frequency:
```
freq(250000000)
```
This sets the CPU frequency to 250 MHz, ensuring precise timing for the measurements.
Initializing steptime Instances:
```
step0 = steptime(0, Pin(0))
step1 = steptime(1, Pin(1))
step2 = steptime(2, Pin(2))
step3 = steptime(3, Pin(4))
step4 = steptime(4, Pin(3))
```
Five instances of the steptime class are created, each associated with a different GPIO pin. Note that pins 3 and 4 are swapped in the initialization, as indicated by the comment.

Measurement Loop:

loop = 200
settle = 20000
min0 = min1 = min2 = min3 = min4 = 1e6
while True:
    # Measurement code

This loop continuously performs the following for each steptime instance:

Sends the loop and settle values to the state machine.
Retrieves the result from the state machine.
Calculates the actual result by subtracting from 4294967296 (2^32) to account for the counter's rollover.
Updates the minimum observed value for each pin.
Prints the difference between the current result and the minimum value for each pin, scaled by 7500.

In summary, this code utilizes the RP2040's PIO to measure the time it takes for a pin to charge and discharge, which correlates to the capacitance present on the pin. By monitoring changes in this timing, the system can detect variations in capacitance, enabling applications like touch sensing.