Auto power-on the SIM868 module via its PWRKEY GPIO line

Root-caused a "GPS not working" report to the SIM868 module chip being
fully powered off (the HAT's 5V rail can be live while the chip itself
stays off until PWRKEY is triggered) - confirmed by getting zero AT
response at any baud rate while the app's GPS thread had the serial
port open normally.

Discovered this specific HAT revision has the "P4" GPIO power-on mod
wire installed (Waveshare FAQ / GSM_PWRKEY.rar script), letting PWRKEY
be triggered via physical pin 7 (BCM GPIO4) instead of a manual button
press. Verified live: pulsing it low for 4s then high brought the
module from zero AT response to responding immediately.

modules/app_gps.py: added _module_responds()/_pulse_pwrkey()/
_ensure_module_powered(), wired into _poll_loop() before the existing
GNSS power-on check. Checks AT first and only pulses PWRKEY if the
module isn't already responding, since PWRKEY is a toggle (pulsing an
already-on module would turn it back off).

Also fixed a related logging gap found while debugging: print()
diagnostics from app_gps.py were invisible in yaris-obd.log for
months because Python fully-buffers stdout when it isn't a TTY.
deploy/yaris-obd.service now sets PYTHONUNBUFFERED=1.

requirements.txt: added RPi.GPIO.
PLAN.md: documented both findings and the Pi deploy gotchas (systemd
unit file changes need a manual copy + daemon-reload; requirements.txt
changes need a manual venv pip install - neither is picked up by a
plain git pull + restart).

Co-authored-by: Copilot <[email protected]>

PLAN.md

@@ -30,8 +30,14 @@ pick up the project without needing prior chat history.
 - **The SIM868 module chip itself needs its PWRKEY button pressed and held (~1-2s) to power on**
   — this is separate from the board's 5V/PWR LED, which can be lit while the module chip is
   fully off. AT commands get zero response if the module chip isn't powered, even with perfect
-  UART wiring/config. This is a manual step needed on every power cycle unless a P4
-  GPIO-power-on hardware mod (per Waveshare FAQ) is done — not pursued so far.
+  UART wiring/config. **Update 2026-08-09: this HAT revision DOES have the "P4" GPIO-power-on
+  mod wire installed** (per the Waveshare FAQ/`GSM_PWRKEY.rar` script) — confirmed by pulsing
+  physical header pin 7 (BCM GPIO4) LOW for 4s then HIGH while the module was unresponsive, after
+  which it immediately answered `AT`. This is now automated in `modules/app_gps.py`
+  (`_ensure_module_powered()`), which checks `AT` first and only pulses PWRKEY if the module
+  isn't already responding (PWRKEY is a toggle — pulsing an already-on module would turn it back
+  off, so this check-first behavior is required). No more manual button-pressing needed after a
+  Pi power cycle.
 - Confirmed AT command baud rate: **115200**.
 - GNSS workflow: `AT+CGNSPWR?` to check GNSS power state, `AT+CGNSPWR=1` to power on,
   `AT+CGNSINF` to poll for a fix. Response format:
@@ -577,7 +583,18 @@ optimization. Not scoped/designed yet.
   `~/app/yaris-obd/yaris-obd.log` (not journald - the systemd unit redirects both streams there),
   so use `tail`/`grep` on that file, not `journalctl`, to see application-level prints (upload
   progress, OBD/GNSS errors, etc.) - `journalctl -u yaris-obd.service` only shows service
-  start/stop/crash lifecycle events.
+  start/stop/crash lifecycle events. **Update 2026-08-09**: plain `print()` calls were silently
+  invisible in that log for months because Python fully-buffers stdout when it's not a TTY (a
+  file, in this case) — `logging`-based lines (e.g. the `obd` library's own logger, which uses
+  stderr) showed up fine, but bare `print()` diagnostics from `app_gps.py` never did. Fixed by
+  adding `Environment=PYTHONUNBUFFERED=1` to the systemd unit
+  (`deploy/yaris-obd.service`) — **remember to also copy the updated unit file to
+  `/etc/systemd/system/yaris-obd.service` and run `sudo systemctl daemon-reload`** on the Pi, a
+  plain `git pull` + restart does NOT pick up systemd unit file changes (the live
+  `/etc/systemd/system/` copy is independent of the repo's `deploy/` copy).
+- **Pi deploy loop, requirements.txt changes**: after any `requirements.txt` change (e.g. adding
+  `RPi.GPIO`), also run `~/app/yaris-obd/venv/bin/pip install -r requirements.txt` on the Pi — a
+  `git pull` alone doesn't install new dependencies into the venv.
 - **Pi memory budget**: ~900MB usable RAM, no swap. Any code path that might buffer a whole
   dashcam-sized file (hundreds of MB) in memory is unsafe on this hardware - stream, don't buffer
   (see Phase 10).

deploy/yaris-obd.service

@@ -8,6 +8,10 @@ Type=simple
 User=yaris
 Group=yaris
 WorkingDirectory=/home/yaris/app/yaris-obd
+# Unbuffered stdout so print() diagnostics (GPS/GNSS status, etc.) actually reach
+# the log file promptly instead of sitting in Python's stdout buffer indefinitely
+# when it's redirected to a file instead of a TTY.
+Environment=PYTHONUNBUFFERED=1
 ExecStart=/home/yaris/app/yaris-obd/venv/bin/python app.py
 Restart=on-failure
 RestartSec=5

modules/app_gps.py

@@ -3,12 +3,28 @@ from threading import Thread, Lock
 import serial
 import time
 
+try:
+    import RPi.GPIO as GPIO
+except (ImportError, RuntimeError):
+    # Not running on a Pi (e.g. local dev machine) - PWRKEY auto power-on is
+    # simply unavailable there; _pulse_pwrkey() below degrades gracefully.
+    GPIO = None
+
 # SIM868 GNSS/GSM/GPRS HAT is wired to the Pi's GPIO UART (jumper set to "B: Pi-SIM868"),
 # exposed as /dev/serial0 once the Pi's onboard Bluetooth has been disabled/freed.
 SERIAL_PORT = "/dev/serial0"
 BAUDRATE = 115200
 POLL_INTERVAL = 2  # seconds between AT+CGNSINF polls
 
+# This HAT revision has the "P4" GPIO power-on mod wire connected to the SIM868's
+# PWRKEY line (confirmed 2026-08-09 by pulsing it and seeing the module respond to
+# AT afterward) - physical header pin 7 (BCM GPIO4), same pin Waveshare's own
+# GSM_PWRKEY.py reference script uses. This lets us auto power-on the module chip
+# on every app start instead of requiring someone to press its PWRKEY button by
+# hand after every Pi power cycle.
+PWRKEY_PIN = 7  # BOARD numbering
+PWRKEY_HOLD_SECONDS = 4  # matches Waveshare's reference script
+
 LocationX = None
 LocationY = None
 
@@ -40,6 +56,65 @@ def _send_at(cmd, wait=0.5):
     return ser.read(1000).decode(errors="replace")
 
 
+def _module_responds():
+    """Check whether the SIM868 module chip is powered on and answering AT commands."""
+    try:
+        return "OK" in _send_at("AT", wait=0.3)
+    except Exception:
+        return False
+
+
+def _pulse_pwrkey():
+    """Pulse the PWRKEY line low then high, same sequence as Waveshare's own
+    GSM_PWRKEY.py reference script.
+
+    PWRKEY is a toggle, not a simple "power on" switch: pressing it while the
+    module is off powers it on, but pressing it again while already on powers
+    it back off. Callers must only invoke this after confirming via
+    `_module_responds()` that the module is NOT currently responding - never
+    unconditionally on every app start, or a normal restart while the module
+    is already up would accidentally shut it back down.
+    """
+    if GPIO is None:
+        print("RPi.GPIO not available, cannot auto-power-on the SIM868 module - "
+              "press its PWRKEY button by hand.")
+        return
+    GPIO.setmode(GPIO.BOARD)
+    GPIO.setup(PWRKEY_PIN, GPIO.OUT)
+    GPIO.output(PWRKEY_PIN, GPIO.LOW)
+    time.sleep(PWRKEY_HOLD_SECONDS)
+    GPIO.output(PWRKEY_PIN, GPIO.HIGH)
+    GPIO.cleanup(PWRKEY_PIN)
+
+
+def _ensure_module_powered():
+    """Power on the SIM868 module chip via its PWRKEY line if it isn't already on.
+
+    The HAT's 5V rail can be live (board powered) while the module chip itself
+    stays fully off until PWRKEY is pulsed - this is what previously required a
+    manual button press after every Pi power cycle. Returns True once the
+    module is confirmed responding to AT commands, False if it still isn't
+    after attempting the power-on pulse (e.g. GPIO unavailable, or this board
+    doesn't have the P4 mod wire).
+    """
+    if _module_responds():
+        return True
+
+    print("SIM868 module not responding to AT, pulsing PWRKEY to power it on...")
+    _pulse_pwrkey()
+    time.sleep(3)  # give the module a few seconds to boot after PWRKEY release
+
+    for _ in range(5):
+        if _module_responds():
+            print("SIM868 module powered on and responding.")
+            return True
+        time.sleep(1)
+
+    print("SIM868 module still not responding after PWRKEY pulse - check the "
+          "P4 wiring or press the physical PWRKEY button.")
+    return False
+
+
 def _ensure_gnss_powered():
     resp = _send_at("AT+CGNSPWR?")
     if "+CGNSPWR: 0" in resp:
@@ -76,6 +151,7 @@ def _poll_loop():
     global LocationX, LocationY
     try:
         with _serial_lock:
+            if _ensure_module_powered():
                 _ensure_gnss_powered()
     except Exception as e:
         print("Failed to initialize GNSS:", str(e))

requirements.txt

@@ -4,6 +4,7 @@ gevent
 pymongo
 python-dotenv
 pyserial
+RPi.GPIO
 obd
 requests
 requests-toolbelt