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README for the SNESpad core
===========================
Version: $Id: README 41 2009-04-01 19:58:04Z arniml $


Description
-----------

The SNESpad core manages one or more gamepads of the Super Nintendo
Entertainment System in parallel. The button information is provided
statically at a simple interface with dedicated signal lines.
The core has to be configured to fit into the integrating system. Details
about this are given in the section "Integration" below.


Integration
-----------

The interface of the SNESpad core is straight forward. It requires:

  * a clock signal which is evaluated on the rising edge by the internal
    registers an asynchronous reset (active level is configurable)
  * connections to the gamepad(s)

The button outputs should be self-describing.

Configuration of the core is done via generics in the instantiation. There are
four generic parameters:

  * num_pads_g - Number of pads connected to this controller instance (1 to n)
  * reset_level_g - Active level of the asynchronous reset at port reset_i
    (0 = low active, 1 = high active)
  * button_level_g - Active level of the button outputs
    (0 = low active, 1 = high active)
  * clocks_per_6us_g - Number of clk_i cycles that elapse during 6 us (2 to x) 

Button outputs and pad data input are arrays of num_pads_g width. The
assignment is 1:1. i.e. the pad connected to pad_data_i(i) will propagate its
button status to but_a_o(i), but_b_o(i) etc. where i ranges from 0 to n-1.

The communication to the SNES gamepad relies on a timebase of approximately
6 us. It is therefore necessary to adjust the counters inside the core via the
clock_per_6us_g generic parameter. Let's assume an example where the system
clock is running at 20 MHz. There are 20 clock cycles during 1 us, so the
generic has to be set to 6 x 20 = 120.


Adapter Hardware
----------------

The required hardware setup is pretty simple if you reuse the connector of a
SNES console. It is quite robust and offers all connections centrally on the
bottom side of the PCB at the pins for the cable socket. In addition, you will
need an external 5V power source. Such a configuration is shown in the
following picture.

Pin B1 is the common Data Latch signal for Pad 1 and Pad 2. The pins B2 and T2
are the Data Clock for Pad 1 and Pad 2, respectively. They have to be
connected together to pad_clock_i as the core clocks both pads
simultaneously. Pin B3 is the Serial Data of Pad 1 and Pin B4 is the Serial
Data of Pad 2. Each data line requires a 10 kOhm pull-up resistor.

See snespad.png


Verification
------------

The SNESpad core comes with a simple testbench that simulates two SNES
gamepads. Serial information is sent to the core and the reported button
states are compared against the input.
You should normally not need to run the testbench. But in case you modified
the VHDL code the testbench gives some hints if the design has been broken.


Directory Structure
-------------------

The core's directory structure follows the proposal of OpenCores.org.

snespad
 |
 \--+-- rtl
    |    |
    |    \-- vhdl           : VHDL code containing the RTL description
    |                         of the core.
    |
    +-- bench
    |    |
    |    \-- vhdl           : VHDL testbench code.
    |
    \-- sim
         |
         \-- rtl_sim        : Directory for running simulations.


Compiling the VHDL Code
-----------------------

VHDL compilation and simulation tasks take place inside in sim/rtl_sim
directory. The project setup supports only the GHDL simulator (see
http://ghdl.free.fr).

To compile the code simply type at the shell

$ make

This should result in a file called tb_behav_c0 which can be executed as any
other executable.

The basic simple sequence list can be found in COMPILE_LIST. This can be
useful to quickly set up the analyze stage of any compiler or
synthesizer. Especially when synthesizing the code, you want to skip the VHDL
configurations in *-c.vhd and everything below the bench/ directory.


References
----------

  * Gamepads project at OpenCores.org
    http://www.opencores.org/projects.cgi/web/gamepads/overview

  * The Hardware Book
    http://www.hardwarebook.net/connector/userinput/snescontroller.html

  * Linux gamecon driver
    http://lxr.linux.no/source/drivers/char/joystick/gamecon.c?v=2.4.26