A simple design with Wishbone bus

This article is intended to explain how to design Wishbone compatible components with simple example. The VHDL source code can be found in sourceforge tree.

Description of the Wishbone structure for ARMadeus can be found here (in french).

General structure

The main functionality of this component is to do the same things that benoît's project : switch on a LED when a button is pressed.

But to learn more about Wishbone component design and Linux driver devt, we have make the thing a little more complicated (!).

When button is pressed, the component button send interrupt signal to irq_mngr. irq_mngr will toggle a flag and send interruption to i.MX processor. A Linux driver on i.MX will read irq_mngr and acknowledge irq by writing '1' on a register. And finally, Linux driver will toggle LED value by writing on led register.

figure 1 - Schematics of wishbone example

imx_wrapper, syscon and irq_mngr are standards ARMadeus-Wishbone IPs that just been instantiated in our design.

button and led are simple slave component we want to integrate in the FPGA.

All these components are connected together with the 'glue logic' component intercon.

Wrapper

The wrapper is used to convert i.MX interface signals into Wishbone signals. Table above show signals from i.MX and signals to wishbone conversion :

i.MX signals	function	Wishbone signals
imx_address(12)	Address vector	wbm_address(13)
imx_data(16)	Data vector	wbm_writedata(16) and wbm_readdata(16)
imx_cs_n	Chip select	wbm_strobe and wbm_cycle
imx_oe_n	Read signal	/wbm_write
imx_eb3_n	Write signal	wbm_write
-	Acknowledge	wbm_ack

Intercon

The intercon is a component used to manage signals between Wishbone master and slave components. This component decode Wishbone-master addresses and dispatch them to Wishbone-slave components.

figure 2 - Intercon internal structure

Wishbone slave application components

In this example there are 3 Wishbone-slave components :

irq manager

Some components (here, only button) generate interrupts, irq manager is used to mux these interrupts for i.MX. The irq_mngr can manage up to 16 internal interrupts.

IRQ manager component has tree registers, one to enable interrupts, one for flags/acknowledge interrupts and one identification register :

register	function
mask	write '1' to allow irq
ack/pend	read for pending irq, write to acknowledge irq
id	identification register

wb_led

Note: On apf9328_devfull and on apf27_dev a LED is already soldered on fpga. For apf27_dev the FPGA bank1 must be powered with 3.3V (connect pin 39 and pin 1 on J20 header (near Ethernet one))

This component is a simple 16-bit Wishbone slave output port, from wishbone specification example (p110).

figure 3 - LED internal structure

It is a simple register, that can be read and write. The LED is controlled with register pin 0.

The two registers are :

register	function
LED	Write '1' in LSB to shutdown LED
id	read identification number

wb_button

Wb_button component, is like led but in read only and with an edge detector to rise irq.

Note: On apf27_dev a button is already soldered on fpga. For apf9328_dev a button must be plugged on IO_L01N_0 fpga pin (connector X7/FPGA2, pin 1)

figure 4 - Button internal structure

The two registers are:

register	function
Button	read LSB to know button state
id	read identification number

Components drivers

Each component is driven by a Linux driver described above. All driver code is in armadeus directory in target/linux/module/fpga/wishbone_example/.

Each component has an identification register with unique number. This number is used by driver when modprobed to unsure that device is present.

irq manager

The description of IRQ management module is available here. The module's source code can be found here.

LED

LED driver is seen in Linux like a character driver. Writing in a /dev file will enable or disable the LED.

The driver is composed of two modules :

• g_led : this module implement generic LED driver mechanisms.
• board_led : This module describe specific datas for each LED-component available in design. These datas are described in structure plat_led_port and plat_led_device. The module will register each LED with platform_device_register() function. When a plat_led_port device is registered, g_led driver will detect it and will probe it with led_probe function.

button

Button driver is seen in Linux like a character driver. When a process want to read value in button register, the driver will block reading until an interrupt occur.

Structure of button driver is similar to LED driver (two modules) :

• gbutton : This module implements generic button driver mechanisms.
• board_button : Like LED module, this module describes specific data for each button-component available in design.

Using the design

All code for this design is available in ARMadeus tree, firmware (VHDL) and software (Linux drivers).

Make the FPGA bitstream

ISE Webpack is mandatory to generate the bitstream, so its installation is required. Once bitstream has been generated, it can be downloaded in FPGA with U-Boot or Linux.

Compile Linux drivers

To compile the drivers for this design select them in Linux menuconfig:

 $ make linux26-menuconfig

    Device Drivers --->
        Armadeus specific drivers --->
            FPGA Drivers --->
                [*] Drivers for wishbone example 
                <M>     Wishbone Button driver
                <M>     Wishbone LED driver
                <M>     IRQ manager with wishbone interface

and compile them:

$ make linux26 && make

Then reflash Linux and rootfs images.

Play with button and LED

Load modules

Modules must be loaded in right order with modprobe command :

• IRQ manager :

# modprobe irq_ocore

• Button generic module must be loaded before board module:

# modprobe gbutton
# modprobe board_buttons
BUTTON0: MAJOR: 251 MINOR: 0
button: irq registered : 193
BUTTON0 insered

• LED generic module must be loaded before board module:

# modprobe g_led
# modprobe board_leds
LED0: MAJOR: 252 MINOR: 0
LED module LED0 insered

Device access

Devices access are done with special caracters file. These file must be created with major and minor number given when modules are loaded (see Load modules above) :

• Make LED access :

# mknod /dev/led0 c 252 0

• Make button access :

# mknod /dev/button0 c 251 0

Test LED

A test program is available in module directory, simply compile it with arm-linux-gcc compiler :

$ arm-linux-gcc testled.c -o testled

Download it on your APF, then test it :

# ./testled /dev/led0 
Testing led driver
Read 1
Write 0
Read 0
Write 1
Read 1
Write 0
Read 0

LED is blinking slowly.

Test Button

A test program is available in target/linux/modules/fpga/wishbone_example/wb_button/ directory, simply compile it with:

$ make -C target/linux/modules/fpga/wishbone_example/wb_button/ test

copy it to your board and then:

# ./testbutton /dev/button0
Testing button driver
Read 1
Read 0
Read 1
Read 0
Read 1
Read 0
Read 0
Read 1
Read 0
Read 1

Each time button is pushed or released, button state is printed out.

Switching LED with button

A simple program is available in wishbone example directory to switch on the LED when button is pressed.

To compile :

arm-linux-gcc push-led.c -o push-led

To use it, simply type :

# ./push-led /dev/button0 /dev/led0 
Blink a led pushing button

Push the button to switch on/off the LED.