coredevice.spi, doc/manual: add spi

artiq/coredevice/spi.py

@@ -21,9 +21,25 @@ SPI_RT2WB_READ = 1 << 2
 
 class SPIMaster:
     """Core device Serial Peripheral Interface (SPI) bus master.
+    Owns one SPI bus.
+
+    **Transfer Sequence**:
+
+    * If desired, write the ``config`` register (:meth:`set_config`)
+      to configure and activate the core.
+    * If desired, write the ``xfer`` register (:meth:`set_xfer`)
+      to set ``cs_n``, ``write_length``, and ``read_length``.
+    * :meth:`write` to the ``data`` register (also for transfers with
+      zero bits to be written). Writing starts the transfer.
+    * If desired, :meth:`read_sync` (or :meth:`read_async` followed by a
+      :meth:`input_async` later) the ``data`` register corresponding to
+      the last completed transfer.
+    * If desired, :meth:`set_xfer` for the next transfer.
+    * If desired, :meth:`write` ``data`` queuing the next
+      (possibly chained) transfer.
 
     :param ref_period: clock period of the SPI core.
-    :param channel: channel number of the SPI bus to control.
+    :param channel: RTIO channel number of the SPI bus to control.
     """
     def __init__(self, dmgr, ref_period, channel):
         self.core = dmgr.get("core")
@@ -43,12 +59,69 @@ class SPIMaster:
 
     @kernel
     def set_config(self, flags=0, write_freq=20*MHz, read_freq=20*MHz):
+        """Set the configuration register.
+
+        * If ``config.cs_polarity`` == 0 (```cs`` active low, the default),
+          "``cs_n`` all deasserted" means "all ``cs_n`` bits high".
+        * ``cs_n`` is not mandatory in the pads supplied to the gateware core.
+          Framing and chip selection can also be handled independently
+          through other means, e.g. ``TTLOut``.
+        * If there is a ``miso`` wire in the pads supplied in the gateware,
+          input and output may be two signals ("4-wire SPI"),
+          otherwise ``mosi`` must be used for both output and input
+          ("3-wire SPI") and ``config.half_duplex`` must to be set
+          when reading data is desired or when the slave drives the
+          ``mosi`` signal at any point.
+        * The first bit output on ``mosi`` is always the MSB/LSB (depending
+          on ``config.lsb_first``) of the ``data`` register, independent of
+          ``xfer.write_length``. The last bit input from ``miso`` always ends
+          up in the LSB/MSB (respectively) of the ``data`` register,
+          independent of ``xfer.read_length``.
+        * Writes to the ``config`` register take effect immediately.
+
+        **Configuration flags**:
+
+        * :const:`SPI_OFFLINE`: all pins high-z (reset=1)
+        * :const:`SPI_ACTIVE`: transfer in progress (read-only)
+        * :const:`SPI_PENDING`: transfer pending in intermediate buffer
+          (read-only)
+        * :const:`SPI_CS_POLARITY`: active level of ``cs_n`` (reset=0)
+        * :const:`SPI_CLK_POLARITY`: idle level of ``clk`` (reset=0)
+        * :const:`SPI_CLK_PHASE`: first edge after ``cs`` assertion to sample
+          data on (reset=0). In Motorola/Freescale SPI language
+          (:const:`SPI_CLK_POLARITY`, :const:`SPI_CLK_PHASE`) == (CPOL, CPHA):
+
+          - (0, 0): idle low, output on falling, input on rising
+          - (0, 1): idle low, output on rising, input on falling
+          - (1, 0): idle high, output on rising, input on falling
+          - (1, 1): idle high, output on falling, input on rising
+        * :const:`SPI_LSB_FIRST`: LSB is the first bit on the wire (reset=0)
+        * :const:`SPI_HALF_DUPLEX`: 3-wire SPI, in/out on ``mosi`` (reset=0)
+
+        This method advances the timeline by the duration of the
+        RTIO-to-Wishbone bus transaction (three RTIO clock cycles).
+
+        :param flags: A bit map of `SPI_*` flags.
+        :param write_freq: Desired SPI clock frequency during write bits.
+        :param read_freq: Desired SPI clock frequency during read bits.
+        """
         write_div = round(1/(write_freq*self.ref_period))
         read_div = round(1/(read_freq*self.ref_period))
         self.set_config_mu(flags, write_div, read_div)
 
     @kernel
     def set_config_mu(self, flags=0, write_div=6, read_div=6):
+        """Set the ``config`` register (in SPI bus machine units).
+
+        .. seealso:: :meth:`set_config`
+
+        :param write_div: Counter load value to divide the RTIO
+          clock by to generate the SPI write clk. (minimum=2, reset=2)
+          ``f_rtio_clk/f_spi_write == write_div``. If ``write_div`` is odd,
+          the setup phase of the SPI clock is biased to longer lengths
+          by one RTIO clock cycle.
+        :param read_div: Ditto for the read clock.
+        """
         rtio_output(now_mu(), self.channel, SPI_CONFIG_ADDR, flags |
                     ((write_div - 2) << 16) | ((read_div - 2) << 24))
         self.write_period_mu = int(write_div*self.ref_period_mu)
@@ -57,6 +130,43 @@ class SPIMaster:
 
     @kernel
     def set_xfer(self, chip_select=0, write_length=0, read_length=0):
+        """Set the ``xfer`` register.
+
+        * Every transfer consists of a write of ``write_length`` bits
+          immediately followed by a read of ``read_length`` bits.
+        * ``cs_n`` is asserted at the beginning and deasserted at the end
+          of the transfer if there is no other transfer pending.
+        * ``cs_n`` handling is agnostic to whether it is one-hot or decoded
+          somewhere downstream. If it is decoded, "``cs_n`` all deasserted"
+          should be handled accordingly (no slave selected).
+          If it is one-hot, asserting multiple slaves should only be attempted
+          if ``miso`` is either not connected between slaves, or open
+          collector, or correctly multiplexed externally.
+        * For 4-wire SPI only the sum of ``read_length`` and ``write_length``
+          matters. The behavior is the same (except for clock speeds) no matter
+          how the total transfer length is divided between the two. For
+          3-wire SPI, the direction of ``mosi`` is switched from output to
+          input after ``write_length`` bits.
+        * Data output on ``mosi`` in 4-wire SPI during the read cycles is what
+          is found in the data register at the time.
+          Data in the ``data`` register outside the least/most (depending
+          on ``config.lsb_first``) significant ``read_length`` bits is what is
+          seen on ``miso`` (or ``mosi`` if ``config.half_duplex``)
+          during the write cycles.
+        * Writes to ``xfer`` are synchronized to the start of the next
+          (possibly chained) transfer.
+
+        This method advances the timeline by the duration of the
+        RTIO-to-Wishbone bus transaction (three RTIO clock cycles).
+
+        :param chip_select: Bit mask of chip selects to assert. Or number of
+            the chip select to assert if ``cs`` is decoded downstream.
+            (reset=0)
+        :param write_length: Number of bits to write during the next transfer.
+            (reset=0)
+        :param read_length: Number of bits to read during the next transfer.
+            (reset=0)
+        """
         rtio_output(now_mu(), self.channel, SPI_XFER_ADDR,
                     chip_select | (write_length << 16) | (read_length << 24))
         self.xfer_period_mu = int(write_length*self.write_period_mu +
@@ -65,24 +175,64 @@ class SPIMaster:
 
     @kernel
     def write(self, data):
+        """Write data to data register.
+
+        * The ``data`` register and the shift register are 32 bits wide.
+          If there are no writes to the register, ``miso`` data reappears on
+          ``mosi`` after 32 cycles.
+        * A wishbone data register write is acknowledged when the
+          transfer has been written to the intermediate buffer.
+          It will be started when there are no other transactions being
+          executed, either beginning a new SPI transfer of chained
+          to an in-flight transfer.
+        * Writes take three ``ref_period`` cycles unless another
+          chained transfer is pending and the transfer being
+          executed is not complete.
+        * The SPI ``data`` register is double-buffered: Once a transfer has
+          started, new write data can be written, queuing a new transfer.
+          Transfers submitted this way are chained and executed without
+          deasserting ``cs`` in between. Once a transfer completes,
+          the previous transfer's read data is available in the
+          ``data`` register.
+
+        This method advances the timeline by the duration of the
+        RTIO-to-Wishbone bus transaction (three RTIO clock cycles).
+        """
         rtio_output(now_mu(), self.channel, SPI_DATA_ADDR, data)
         delay_mu(3*self.ref_period_mu)
 
     @kernel
     def read_async(self):
-        # every read_async() must be matched by an input_async()
+        """Trigger an asynchronous read from the data register.
+
+        Reads always finish in two cycles.
+
+        Every data register read triggered by a :meth:`read_async`
+        must be matched by a :meth:`input_async` to retrieve the data.
+
+        This method advances the timeline by the duration of the
+        RTIO-to-Wishbone bus transaction (three RTIO clock cycles).
+        """
         rtio_output(now_mu(), self.channel, SPI_DATA_ADDR | SPI_RT2WB_READ, 0)
         delay_mu(3*self.ref_period_mu)
 
     @kernel
     def input_async(self):
-        # matches the preeeding read_async()
+        """Retrieves data written asynchronously.
+
+        :meth:`input_async` must match a preeeding :meth:`read_async`.
+        """
         return rtio_input_data(self.channel)
 
     @kernel
     def read_sync(self):
-        rtio_output(now_mu(), self.channel, SPI_DATA_ADDR | SPI_RT2WB_READ, 0)
+        """Read the ``data`` register synchronously.
-        return rtio_input_data(self.channel)
+
+        This is a shortcut for :meth:`read_async` followed by
+        :meth:`input_async`.
+        """
+        self.read_async()
+        return self.input_async()
 
     @kernel
     def _get_xfer_sync(self):

doc/manual/core_drivers_reference.rst

@@ -24,6 +24,13 @@ These drivers are for the core device and the peripherals closely integrated int
 .. automodule:: artiq.coredevice.dds
     :members:
 
+:mod:`artiq.coredevice.spi` module
+----------------------------------
+
+.. automodule:: artiq.coredevice.spi
+    :members:
+
+
 :mod:`artiq.coredevice.exceptions` module
 -----------------------------------------