Enhanced EPP module to correctly forward read addresses

Implemented proper address forwarding for read requests to the FIFO interface, ensuring correct functionality during data read operations. Documented the usage procedures for data reads and writes and refined the history and timing diagrams in the associated documentation for clarity. This update includes reorganizing comments for better readability and updating signal controls to synchronize address handling with data output.

code/DEPP.vhd

@@ -1,6 +1,6 @@
 ----------------------------------------------------------------------------------
 -- @name        Digilent EPP Interface
--- @version     0.3.1
+-- @version     0.3.2
 -- @author      Maximilian Passarello (mpassarello.de)
 --@             An EPP interface for Digilent FPGA boards
 --@             This interface is designed to be used with the Digilent EPP interface
@@ -8,14 +8,28 @@
 --@             
 --@             **Measured data rate ≈ 4.68 kByte/s**
 --@             
---@             Below are diagrams of the EPP bus:
+--@             ## Usage
--- @history
+--@             The module is designed to be used with a FIFO interface.
--- - 0.2.0 (2010.05.30) Initial version
+--@             Either the data & address (**write**) are transferred via the FIFO interface 
--- - 0.3.0 (2024.03.06) Refactored and commented
+--@             **or** the requested address is transferred first and then the corresponding data is expected.
--- - 0.3.1 (2024.03.09) Complet overhaul of the module
+--@             ### Data Write:
+--@             With a data write request, the module transfers the **data** and the **address** 
+--@             to the two corresponding FIFO interfaces.
+--@             ### Data Read:
+--@             With a data read request, the module transfers the **requested address** 
+--@             to the FIFO interface. It then expects the corresponding data 
+--@             via the data input FIFO.
+--@
+--@             
+--@ ## History:
+--@ - 0.2.0 (2010.05.30) Initial version
+--@ - 0.3.0 (2024.03.06) Refactored and commented
+--@ - 0.3.1 (2024.03.09) Complet overhaul of the module
+--@ - 0.3.2 (2024.03.13) The forwarding of the address to be read to the FIFO is now implemented correctly. 
+--@                      A usage description has been added. Documentation improved.
 ----------------------------------------------------------------------------------
--- Timing Diagram's
+--@ ## Timing diagrams of the EPP bus
--- EPP Address Write
+--@ ### EPP Address Write
 --@ {
 --@   "signal": [
 --@     { "name": "DEPP_Bus", "wave": "xx3....xxx", "data": ["Adress"] },
@@ -34,7 +48,7 @@
 --@   },
 --@   "edge": ["A+B min. 80 ns", "C+D min. 40ns", "E+F 0 to 10ms", "H+I 0 to 10ms"]
 --@ }
--- EPP Data Write
+--@ ### EPP Data Write
 --@ {
 --@   "signal": [
 --@     { "name": "DEPP_Bus", "wave": "xx3....xxx", "data": ["Data"] },
@@ -53,7 +67,7 @@
 --@   },
 --@   "edge": ["A+B min. 80 ns", "C+D min. 40ns", "E+F 0 to 10ms", "H+I 0 to 10ms"]
 --@ }
--- EPP Data Read
+--@ ### EPP Data Read
 --@ {
 --@   "signal": [
 --@     { "name": "DEPP_Bus", "wave": "zz...3...x", "data": ["Data"] },
@@ -157,7 +171,7 @@ architecture Behavioral of DEPP is
     --@ The current state of the main state machine
     signal Mode : ModeType := Idle;
 
-    --@ The output signals for the output data fifo
+    --@ The output signals for the output data fifo; also controls the address fifo.
     signal InterWriteEnableOut : std_logic := '0';
     --@ The output signals for the output address fifo
     signal InterRequestEnable : std_logic := '0';
@@ -169,7 +183,7 @@ begin
 
     DataInFifo_DataAviable     <= not DataInFifo_EmptyFlag;
     DataOutFifo_WriteEnable    <= InterWriteEnableOut;
-    AddressOutFifo_WriteEnable <= InterRequestEnable;
+    AddressOutFifo_WriteEnable <= InterRequestEnable or InterWriteEnableOut;
 
     --@ Shifts the value from the `DEPP_AddressEnable` signal into the `EPP_AddressEnableShiftRegister`
     --@ for the rising/falling edge detection.

docs/DEPP/DEPP.md

@@ -4,7 +4,7 @@
 
 ## Diagram
 
-![Module Diagram](DEPP.svg "DEPP Module")
+![Diagram](DEPP.svg "Diagram")
 
 ## Description
 
@@ -14,13 +14,44 @@ and the Digilent Adept software.
 
 **Measured data rate ≈ 4.68 kByte/s**
 
-Below are diagrams of the EPP bus:
+## Usage
 
-![Waveform of the EPP Address Write Cycle](wavedrom_HWwR0.svg "EPP Address Write")
+The module is designed to be used with a FIFO interface.
+Either the data & address (**write**) are transferred via the FIFO interface
+**or** the requested address is transferred first and then the corresponding data is expected.
 
-![Waveform of the EPP Data Write Cycle](wavedrom_ypdi1.svg "EPP Data Write")
+### Data Write:
 
-![Waveform of the EPP Data Read Cycle](wavedrom_91sO2.svg "EPP Data Read")
+With a data write request, the module transfers the **data** and the **address**
+to the two corresponding FIFO interfaces.
+
+### Data Read:
+
+With a data read request, the module transfers the **requested address**
+to the FIFO interface. It then expects the corresponding data
+via the data input FIFO.
+
+## History:
+
+- 0.2.0 (2010.05.30) Initial version
+- 0.3.0 (2024.03.06) Refactored and commented
+- 0.3.1 (2024.03.09) Complet overhaul of the module
+- 0.3.2 (2024.03.13) The forwarding of the address to be read to the FIFO is now implemented correctly.
+  A usage description has been added. Documentation improved.
+
+## Timing diagrams of the EPP bus
+
+### EPP Address Write
+
+![alt text](wavedrom_fLwf0.svg "title")
+
+### EPP Data Write
+
+![alt text](wavedrom_lvks1.svg "title")
+
+### EPP Data Read
+
+![alt text](wavedrom_J3G12.svg "title")
 
 ## Ports
 
@@ -79,7 +110,7 @@ Below are diagrams of the EPP bus:
 | EPP_AddressEnableShiftRegister | std_logic_vector(1 downto 0) | Shift register for the rising/falling edge detection of the `DEPP_AddressEnable` signal |
 | EPP_DataEnableShiftRegister    | std_logic_vector(1 downto 0) | Shift register for the rising/falling edge detection of the `DEPP_DataEnable` signal    |
 | Mode                           | ModeType                     | The current state of the main state machine                                             |
-| InterWriteEnableOut            | std_logic                    | The output signals for the output data fifo                                             |
+| InterWriteEnableOut            | std_logic                    | The output signals for the output data fifo; also controls the address fifo.            |
 | InterRequestEnable             | std_logic                    | The output signals for the output address fifo                                          |
 | InterAddressEnable             | std_logic                    | Intermediary signal to start the address write cycle                                    |
 | DataInFifo_DataAviable         | std_logic                    | Negated `DataInFifo_EmptyFlag` signal                                                   |