More ADC Inputs using PIO Sigma Delta

[cebersp]

Hi, this can give you additional Adc channels.
It needs 2 Pins per channel, 2 resistors and 2 capacitors.

Pico Sigma Delta ADC.JPG (82.49 KiB) Viewed 2886 times

The charge of the two condensers is balanced by the current through the input resistor 1meg and the current through the feedback resistor 220k. PIO code tries to hold the voltage at the input pin at its low/high threshold.
It is possible to measure higher and negative voltages, if you use an adapted input resistor.

Have fun! Christof

Code: Select all

title= "pioAdcB" # by CWE
# Adc sigma delta type

from machine import Pin
from rp2 import PIO, StateMachine, asm_pio
from time import sleep

maxTime=1_000_000

@asm_pio(sideset_init=PIO.OUT_LOW)
def cap_prog():
    mov(y, osr) # OSR must be preloaded with max
    mov(x, osr)  
    
    label("test_pin") # 
    jmp(pin, "is_set") 
    jmp(y_dec, "not_set") # just count it 1
    label("not_set")
    #nop() .side(1)
    jmp(x_dec,"test_pin") .side(1) # 2
    jmp("save_result")
    
    label("is_set")
    # nop() .side(0)
    nop() # 1
    jmp(x_dec,"test_pin") .side(0) #2
    
    label("save_result")
    mov(isr, y) #.side(0) # save result; pin low
    push(noblock)

    


class PIOCAP:
    def __init__(self, sm_id, outPin, inPin, max_count, count_freq):
        self._sm = StateMachine(sm_id, cap_prog, freq=2 * count_freq, sideset_base=Pin(outPin), jmp_pin=Pin(inPin,Pin.IN))
        # Use exec() to load max count into ISR
        self._sm.put(max_count)
        self._sm.exec("pull()")
        #self._sm.exec("mov(isr, osr)")
        self._sm.active(1)
        self._max_count = max_count
        
    def getCap(self):
        #self._sm.exec("push()")
        return self._sm.get()

cap = PIOCAP(0, 22, 17, max_count=(maxTime), count_freq=50_000_000)

mittel=0
while True:
    onTime= maxTime-cap.getCap()
    mittel=(9*mittel+onTime)/10
    #print(onTime, mittel, (onTime/maxTime))
    print(onTime, 7.309-1.4369e-5*onTime)
    sleep(0.1)