docs: Add util_var_fifo IP core

Signed-off-by: Jorge Marques <jorge.marques@analog.com>

docs/library/util_var_fifo/index.rst

@@ -0,0 +1,86 @@
+.. _util_var_fifo:
+
+Util VAR FIFO
+===============================================================================
+
+.. hdl-component-diagram::
+
+The :git-hdl:`Util VAR FIFO <library/util_var_fifo>` core
+controls an external BRAM memory through which it allows
+the storage of a variable number of samples before triggering.
+
+Features
+--------------------------------------------------------------------------------
+
+* Variable depth
+
+Files
+--------------------------------------------------------------------------------
+
+.. list-table::
+   :header-rows: 1
+
+   * - Name
+     - Description
+   * - :git-hdl:`library/util_var_fifo/util_var_fifo.v`
+     - Verilog source for the peripheral.
+
+Configuration Parameters
+--------------------------------------------------------------------------------
+
+.. hdl-parameters::
+
+   * - DATA_WIDTH
+     - Data width of the FIFO. The BRAM generator parameters should match this.
+   * - ADDRESS_WIDTH
+     - The BRAM generator parameters should match this. Gives the maximum depth
+       of the FIFO
+
+Interface
+--------------------------------------------------------------------------------
+
+.. hdl-interfaces::
+
+   * - clk
+     - Clock input. Should be synchronous to the input and the output data.
+   * - rst
+     - Reset input. Should be synchronous clk clock
+   * - depth
+     - Controls the depth of the FIFO. Should be less than the maximum depth.
+       Controlled by an outside IP.
+   * - data_in
+     - Data to be stored.
+   * - data_in_valid
+     - Valid for the input data.
+   * - data_out
+     - Data forwarded to the DMA.
+   * - data_out_valid
+     - Valid for the output data.
+   * - wea_w
+     - Write signal.
+   * - en_w
+     - Write enable signal.
+   * - addr_w
+     - Address for the write pointer.
+   * - din_w
+     - Data to be written to the BRAM.
+   * - en_r
+     - Read enable signal.
+   * - addr_r
+     - Address for the read pointer.
+   * - dout_r
+     - Data read from the BRAM.
+
+Detailed Description
+--------------------------------------------------------------------------------
+
+This IP controls an external BRAM. It has a two clock cycle latency even if
+bypassed.
+If valid is not always asserted, the latency is only one word instead of two.
+
+Design Guidelines
+--------------------------------------------------------------------------------
+
+The IP should be used with an external BRAM, which can be optimized for power
+or for speed, depending on the design requirements. It uses only one clock
+domain, so everything should be synchronous to that clock domain.