Part 6 – resource sharing
So far we have the control module merging an on-screen display with the underlying host core’s video output, responding to keypresses and running a simple on-screen menu. The largest single addition now will be SD card access, which I will explore in the next part. In this part, however, I’m going to talk about resource sharing.
Some cores, like the test-pattern generator we’ve been using so far or the PC-Engine core, make no use of the keyboard or SD card, so giving sole access to the control module is no problem. If, on the other hand, the underlying core does make use of keyboard and SD card (such as the OneChipMSX core, for instance) we need to have some way of arbitrating for access to these resources. Continue reading
Always worth a plug, 880 Gamer is a magazine produced in the style of the half-a-dozen or so glossy news-stand magazines we could choose from back in the Amiga’s heyday.
Distributed as a PDF, it also has two coverdisks in .adf format, and printable labels for each, should you wish to write them to physical disks!
All five issues of 880 gamer produced to date can be found at: http://www.users.on.net/~stanners/
Part 5 – Adding a Menu
Having talked in depth about the hardware in previous parts, in this part I’m going to talk about the software side of things.
I mentioned before that the character ROM contains 128 characters which, from characters 32 upwards, are standard 7-bit ASCII. I’ve added some special characters in the lower 32 character slots, however, which have uses in drawing the OSD. The extra characters are as follows:
- 0 – 7: solid blocks 7 pixels high, 1 to 8 pixels wide, left aligned in the character cells. Useful for drawing progress bars.
- 8 – 15: As above but right-aligned within the character cell. May be useful in progress bar applications but I haven’t actually used these yet, so might eventually reassign them.
- 16 – 19: Arrow heads, pointing right, left, up and down, respectively. (The right arrow is used as a cursor in the menu.)
- 20: Checkmark
- 21: Cross
- 22: Cycle
- 23: ellipsis
- 24 – 31: Currently unused
For the OneChipMSX and FPGA PC Engine cores I put together a simple, lightweight data-driven menu system, which can be found in CtrlModule/Firmware/menu.[c|h]
Part 4 – Keyboard control
So far we have the control module printing a message to the screen and autonomously sending signals to the underlying core. Now it’s time to make it somewhat interactive!
To do this we need to add Keyboard support. Many FPGA platforms still have support for PS/2 keyboards simply because they’re electrically simple and don’t require much in the way of high speed transceivers or carefully-routed circuits to drive them. Thus the PS/2 keyboard is a de-facto standard for FPGA projects, even to the point that on the MIST platform (which has no PS/2 ports, but does have USB ports) there is a bridge component available which makes the keyboard masquerade as PS/2. This is taken care of in the MIST-specific toplevel file, which allows the main project files to be identical between platforms.
To add keyboard support we need three things: a hardware interface to the PS/2 socket (whether it’s real or emulated!), software to drive that hardware interface, and support for interrupts, so that the CPU doesn’t have to poll the hardware waiting for keystrokes.
Part 3 – Hello World!
This time round I’ve added the On-screen Display component, and the firmware verifies that it’s working correctly by way of the archetypal “Hello World!” message!
I’ve also added project files for the MIST board, and will add support for a Xilinx-based board in the near future.
The source tree to accompany this part is tagged in the git repo as Step2.
The OSD component itself provides a few hardware registers that can be accessed from software, along with a 512-byte character buffer.
The VHDL interface looks like this:
entity OnScreenDisplay is
reset_n : in std_logic;
clk : in std_logic;
hsync_n : in std_logic; -- Sync inputs from the main core, used to time the
vsync_n : in std_logic; -- window and pixel signals and position the OSD.
enabled : out std_logic;
pixel : out std_logic;
window : out std_logic;
addr : in std_logic_vector(8 downto 0);
data_in : in std_logic_vector(15 downto 0);
data_out : out std_logic_vector(15 downto 0);
reg_wr : in std_logic;
char_wr : in std_logic;
char_q : out std_logic_vector(7 downto 0)
The readable registers are implemented using simple combinational logic and will thus respond within a single clock, so we don’t bother with any kind of req / ack mechanism here, in the interests of keeping things simple.
Address and data from the CPU are placed on addr and data_in, and reg_wr is brought high to trigger a write to a register, and a char_wr is brought high to trigger a write to the character RAM.
data_out and char_q will output data from registers and character RAM, respectively, based on the addr input. This is a constant connection – no req signal is needed. If reading from the registers triggered some kind of action then we’d need a more complete req/ack mechanism here, but since reads are completely passive we don’t need to worry about it in this case.
Part 2 – a simple test core
To demonstrate how the control module is built, we need a core to which we can add the control module. In the interests of keeping the project as simple as possible and avoiding needless distractions, I’ve started a new project for this purpose, which can be found on github at https://github.com/robinsonb5/CtrlModuleTutorial
I shall tag this at key points, and at the time of writing there are two tags in place.
To play with this, check out a local copy of the core, like so:
> git clone https://github.com/robinsonb5/CtrlModuleTutorial.git
> cd CtrlModuleTutorial
> git submodule init
> git submodule update
> git checkout <tag name>
The first tag, called “StartingPoint” contains a VGA test pattern generator for the DE1 board, which has four slightly different test patterns selectable by the DE1’s switches. In the coming parts I shall show how to eliminate the switches and replace them with an On Screen Display.