Recovered from modem failure: OSError: AT+SMSWBOOT=3,0

[derefnull]

My pyCom gPy device had an unresponsive lte modem.

During a failed firmware upgrade, it spit out this exception:
OSError: AT+SMSWBOOT=3,0 failed!

Sending basic AT commands appeared to fail as well..

> import LTE
> lte=lTE()
> lte.send_at_command('ATI')
''

I recovered the modem by running the script supplied here by @jmarcelino

#!/usr/bin/env python
import struct
import crc
import stp
import time
import os
from machine import UART
from machine import SD


FFF_FMT = "<4sIIIIIIIHHIHHIHHHH"
FFF_SLIM_FMT = "<4sIIIIIIIHHHH"
FFF_FEATURES_SLIM = 1 << 0
FFF_MAGIC = "FFF!"

SFFF_MAGIC = "SFFF"             # Firmware
SUFF_MAGIC = "SUFF"             # Updater
TEST_MAGIC = "TEST"             # Test
DIFF_MAGIC = "DIFF"             # Diff Upgrade
UPGR_MAGIC = "UPGR"             # Generic raw upgrade
RASR_MAGIC = "RASR"             # Generic raw rasterize


def read_rsp(s, size=None, timeout=-1):
    if timeout < 0:
        timeout = 20000
    elif timeout is None:
        timeout = 0
    while not s.any() and timeout > 0:
        time.sleep_ms(1)
        timeout -= 1

    if size is not None:
        rsp = s.read(size)
    else:
        rsp = s.read()
    if rsp is not None:
        return rsp
    else:
        return b''

def print_pretty_response(rsp):
    lines = rsp.decode('ascii').split('\r\n')
    for line in lines:
        if line:
            print(line)


def wait_for_modem(s, send=True, expected=b'OK'):
    while True:
        if send:
            s.write("AT\r\n")
        rsp = read_rsp(s, timeout=50)
        if expected in rsp:
            print()
            break
        else:
            print('.', end='', flush=True)
            time.sleep(0.5)

def run(file_path, baudrate):
    abort = False

    print('<<< Welcome to the SQN3330 firmware updater >>>')

    if '/sd' in file_path:
        sd = SD()
        time.sleep(0.5)
        os.mount(sd, '/sd')
        time.sleep(0.5)

    blobsize = os.stat(file_path)[6]
    blob = open(file_path, "rb")

    if 'FiPy' in os.uname().sysname:
        pins = ('P20', 'P18', 'P19', 'P17')
    else:
        pins = ('P5', 'P98', 'P7', 'P99')

    s = UART(1, baudrate=baudrate, pins=pins, timeout_chars=100)
    s.read()

    # disable echo
    s.write("ATE0\r\n")
    response = read_rsp(s)

#     print('Entering recovery mode')
#     s.write("AT+SMSWBOOT=3,0\r\n")
#     response = read_rsp(s)
#     if b'OK' in response:
#         print('Resetting.', end='', flush=True)
#         s.write('AT^RESET\r\n')
#         wait_for_modem(s, send=False, expected=b'+SHUTDOWN')
#         time.sleep(2)
#         wait_for_modem(s)
#         s.write("AT\r\n")
#         s.write("AT\r\n")
#     else:
#         raise OSError('AT+SMSWBOOT=3,0 failed!')
# 
#     time.sleep(1)
#     s.read()

    print('Starting STP (DO NOT DISCONNECT POWER!!!)')
    s.write('AT+SMSTPU=\"ON_THE_FLY\"\r\n')
    response = read_rsp(s, size=4)
    if response != b'OK\r\n' and response != b'\r\nOK' and response != b'\nOK':
        raise OSError("Invalid answer '%s' from the device" % response)
        blob.close()
    s.read()
    try:
        stp.start(blob, blobsize, s, baudrate, AT=False)
        print('Code download done, returning to user mode')
    except:
        blob.close()
        print('Code download failed, aborting!')
        abort = True

    time.sleep(1.5)
    s.read()
    s.write("AT+SMSWBOOT=1,0\r\n")
    response = read_rsp(s)

    print('Resetting (DO NOT DISCONNECT POWER!!!).', end='', flush=True)
    time.sleep(1.5)
    s.write("AT^RESET\r\n")
    wait_for_modem(s, send=False, expected=b'+SHUTDOWN')
    time.sleep(2)
    wait_for_modem(s, send=False, expected=b'+SYSSTART')

    if not abort:
        time.sleep(0.5)
        print('Deploying the upgrade (DO NOT DISCONNECT POWER!!!)...')
        s.write("AT+SMUPGRADE\r\n")
        response = read_rsp(s, timeout=120000)

        print('Resetting (DO NOT DISCONNECT POWER!!!).', end='', flush=True)
        time.sleep(1.5)
        s.write("AT^RESET\r\n")
        wait_for_modem(s, send=False, expected=b'+SHUTDOWN')
        time.sleep(2)
        wait_for_modem(s, send=False, expected=b'+SYSSTART')
        s.write("AT\r\n")
        s.write("AT\r\n")
        time.sleep(0.5)
        s.read()
        print('Upgrade completed!')
        print("Here's the current firmware version:")
        time.sleep(0.5)
        s.read()
        s.write("ATI1\r\n")
        response = read_rsp(s)
        print_pretty_response(response)

allowing it to run to COMPLETION (even though it appeared to hang for several minutes)

at the end of completion, the script was able to query the modem for firmware version (it was old)

I replaced the sqnsupgrade script with the original, and was able to run it to completion!

>>> import sqnsupgrade
>>> sqnsupgrade.run('/sd/FIPY_GPY_CATM1_33988.dup', 921600)
<<< Welcome to the SQN3330 firmware updater >>>
Entering recovery mode
Resetting..

Starting STP (DO NOT DISCONNECT POWER!!!)
STP started
Session opened: version 1, max transfer 8192 bytes
Sending 5665583 bytes: [########################################] 100%
Code download done, returning to user mode
Resetting (DO NOT DISCONNECT POWER!!!)..
........
Deploying the upgrade (DO NOT DISCONNECT POWER!!!)...
Resetting (DO NOT DISCONNECT POWER!!!)..
...
Upgrade completed!
Here's the current firmware version:
UE5.0.0.0c
LR5.1.1.0-33988
OK
>>>