docs: Document the cross-port Timer hard= option.

Update the main machine.Timer specification, and any references to
hard/soft interrupts in port-specific documentation. There is a separate
copy of the machine.Timer documentation for the pyboard, so update that
too to keep everything consistent.

Signed-off-by: Chris Webb <chris@arachsys.com>

docs/esp32/quickref.rst

@@ -287,6 +287,10 @@ with a timer ID of 0, 0 and 1, or from 0 to 3 (inclusive)::
 The period is in milliseconds. When using UART.IRQ_RXIDLE, timer 0 is needed for
 the IRQ_RXIDLE mechanism and must not be used otherwise.
 
+Timer callbacks are scheduled as soft interrupts on this port; hard
+callbacks are not implemented. Specifying ``hard=True`` will raise
+a ValueError.
+
 Virtual timers are not currently supported on this port.
 
 .. _Pins_and_GPIO:

docs/esp8266/quickref.rst

@@ -108,6 +108,9 @@ with timer ID of -1::
 
 The period is in milliseconds.
 
+By default, timer callbacks are scheduled as soft interrupts on this port.
+Specify ``hard=True`` to run them in hard interrupt context instead.
+
 Pins and GPIO
 -------------
 

docs/library/machine.Timer.rst

@@ -18,8 +18,9 @@ Timer callbacks.
 
     Memory can't be allocated inside irq handlers (an interrupt) and so
     exceptions raised within a handler don't give much information.  See
-    :func:`micropython.alloc_emergency_exception_buf` for how to get around this
-    limitation.
+    :func:`micropython.alloc_emergency_exception_buf` for how to get around
+    this limitation, which applies to all callbacks of Timers created with
+    ``hard=True``.
 
 If you are using a WiPy board please refer to :ref:`machine.TimerWiPy <machine.TimerWiPy>`
 instead of this class.
@@ -38,7 +39,7 @@ Constructors
 Methods
 -------
 
-.. method:: Timer.init(*, mode=Timer.PERIODIC, freq=-1, period=-1, callback=None)
+.. method:: Timer.init(*, mode=Timer.PERIODIC, freq=-1, period=-1, callback=None, hard=True)
 
    Initialise the timer. Example::
 
@@ -76,6 +77,19 @@ Methods
        will occur upon timer expiration:
        ``TypeError: 'NoneType' object isn't callable``
 
+     - ``hard`` can be one of:
+
+       - ``True`` - The callback will be executed in hard interrupt
+         context, which minimises delay and jitter but is subject to the
+         limitations described in :ref:`isr_rules` including being unable
+         to allocate on the heap.
+       - ``False`` - The callback will be scheduled as a soft interrupt,
+         allowing it to allocate but possibly also introducing
+         garbage-collection delays and jitter.
+
+       The default value of this option is port-specific for historical
+       reasons.
+
 .. method:: Timer.deinit()
 
    Deinitialises the timer. Stops the timer, and disables the timer peripheral.

docs/library/pyb.Timer.rst

@@ -62,7 +62,7 @@ Constructors
 Methods
 -------
 
-.. method:: Timer.init(*, freq, prescaler, period, mode=Timer.UP, div=1, callback=None, deadtime=0, brk=Timer.BRK_OFF)
+.. method:: Timer.init(*, freq, prescaler, period, mode=Timer.UP, div=1, callback=None, deadtime=0, brk=Timer.BRK_OFF, hard=True)
 
    Initialise the timer.  Initialisation must be either by frequency (in Hz)
    or by prescaler and period::
@@ -115,6 +115,18 @@ Methods
        ``mode=Pin.ALT, alt=Pin.AFn_TIMx``. The pin's GPIO input features are
        available in alt mode - ``pull=`` , ``value()`` and ``irq()``.
 
+     - ``hard`` can be one of:
+
+       - ``True`` - The callback will be executed in hard interrupt
+         context, which minimises delay and jitter but is subject to the
+         limitations described in :ref:`isr_rules` including being unable
+         to allocate on the heap.
+       - ``False`` - The callback will be scheduled as a soft interrupt,
+         allowing it to allocate but possibly also introducing
+         garbage-collection delays and jitter.
+
+       The default value of this option is True.
+
     You must either specify freq or both of period and prescaler.
 
 .. method:: Timer.deinit()