How do I get the ISR bit counts value for the micropython app

[sloshyou]

Hello,
I have a project composed of two state machines rp2.asm_pio().

The state machine n1 reads a data frame by executing the in_(pins, 1) instruction in which the beginning and end of reading (using the jmp(pin) instruction) depends on a GPIO input from the state machine n2.

The n1 state machine configurations are:
autopush = True
push_thres = 32
in_shiftdir=rp2.PIO.SHIFT_LEFT

State machine n2 has the function of detecting the beginning and end of the data frame and sends a corresponding GPIO output to state machine n2.

My issue is that when the state machine n2 detects the end of the data frame, sometimes it doesn't completely fill the RX_FIFO and I need to know how many bits it actually shifted. In the RP2040 datasheet they refer to the ISR register, but I don't understand how I can extract the value from the bit counts in RX_FIFO. Is there another register? Anybody can help me. Thanks.