Add synchronization checklist to documentation

docs/source/hardware_triggering.rst

@@ -1,3 +1,5 @@
+.. _hardware_triggering:
+
 ===================
 Hardware Triggering
 ===================

docs/source/index.rst

@@ -8,6 +8,7 @@ Welcome to HackRF's documentation!
 
    getting_help
    faq
+   synchronization_checklist
    hackrf_projects_mentions
 
 .. toctree::

docs/source/synchronization_checklist.rst

@@ -0,0 +1,94 @@
+.. _synchronization_checklist:
+
+================================================
+Synchronization Checklist
+================================================
+
+There are many scenarios where you may want to use multiple HackRF One devices
+synchronized with each other. For instance, multiple devices can be used with a
+phased antenna array to implement direction finding or beamforming.
+
+If you're having trouble achieving fully synchronized operation, the following
+checklist may be useful for troubleshooting:
+
+* **Are you running the latest firmware and host software versions?**
+
+    There have been many bug fixes, so please use the latest releases.
+
+* **Are you applying settings to the right target devices?**
+
+    With more than one HackRF device connected, you need to take care to specify
+    which device should be used where. When using our command line tools, use the
+    ``-d`` option with a serial number on the command line to specify which
+    device each command should target.
+
+* **Are all HackRFs sharing a clock?**
+
+    If the HackRFs are not sharing a clock, frequencies and sample rates will
+    not match exactly. See the
+    :ref:`external clock interface <external_clock_interface>`
+    section for how to connect the clock signals.
+
+* **Is the clock input being detected?**
+
+    Use ``hackrf_clock`` with the ``-i`` option to check for clock input.
+    Use the ``-d`` option to specify the serial number.
+
+    This requires ``2022.09.1`` or later host software.
+
+    The older way of checking using ``hackrf_debug`` will not work correctly on
+    some hardware revisions.
+
+* **If the clock source is CLKOUT of another HackRF, has it been enabled?**
+
+    Use ``hackrf_clock`` with the ``-o`` option to enable the clock output.
+    Use the ``-d`` option to specify the serial number.
+
+* **Is the CLKIN waveform correct?**
+
+    * It should be a 10MHz square wave between 0 V and 3.0 to 3.3 V.
+    * A sine wave is OK, but may give greater phase noise.
+    * An unbuffered TCXO output may not have sufficient voltage.
+    * Some hardware revisions are less tolerant of out-of-spec
+      clock input than others.
+
+* **Is your hardware faulty?**
+
+    Some HackRF clones were sold with a non-functional CLKIN port.
+
+* **Are all HackRFs being started together using hardware triggering?**
+
+    If hardware triggering is not used, start times will not match exactly.
+    See the :ref:`hardware triggering <hardware_triggering>` section for how
+    to connect the necessary trigger signals.
+
+    Use ``hackrf_transfer`` with the ``-H`` option to wait for trigger input.
+
+    If launching multiple ``hackrf_transfer`` instances, make sure that those
+    running with ``-H`` have had time to reach the ``Waiting for trigger...``
+    state before the trigger signal is sent.
+
+    .. note:: There is currently no support for hardware triggering via the
+              Osmocom or Soapy blocks in GNU Radio. The Osmocom block may look
+              like it supports this but the options have no effect. 
+
+* **Are any samples being lost due to USB throughput problems?**
+
+    If RX samples are dropped or TX samples delayed, signals will become out
+    of sync.
+
+    Use ``hackrf_debug -S`` on each HackRF after running your software to check
+    if there were throughput problems. If the shortfall count reported is
+    non-zero, some samples were dropped on RX or late for TX, and signals will
+    be out of sync as a result.
+
+    This requires ``2022.09.1`` or later host software.
+
+    To force shortfalls to stop the device at runtime, use
+    ``hackrf_debug -T 0 -R 0`` to set zero tolerance for shortfalls.
+
+* **Are your HackRFs sharing a USB bus?**
+
+    A single HackRF One at 20MHz sample rate uses practically all the bandwidth
+    of a single USB 2.0 bus. Unless using very low sample rates, each HackRF
+    should be connected to its own bus.