Hid Controller - Digilent Genesys 2 Reference Manual

Page 19 of 35
The Auxiliary Function microcontroller (Microchip PIC24FJ128) provides the Genesys 2 with USB HID host capability. After
power-up, the microcontroller is in configuration mode, either downloading a bitstream to the FPGA, or waiting on it to be
programmed from other sources. Once the FPGA is programmed, the microcontroller switches to application mode, which is
USB HID Host in this case. Firmware in the microcontroller can drive a mouse or a keyboard attached to the type A USB
connector at J7 labeled "USB HID". J7 is a dual-row USB A receptacle, with the top row connected to the Auxiliary Function
microcontroller. Hub support is not currently available, so only a single mouse or a single keyboard can be used. The PIC24
drives several signals into the FPGA – two are used to implement a standard PS/2 interface for communication with a mouse
or keyboard, and the others are connected to the FPGA's two-wire serial programming port, so the FPGA can be programmed
from a file stored on a USB pen drive or microSD card.

11.1. HID Controller

The Auxiliary Function microcontroller hides the USB HID protocol from the FPGA and emulates an old-style PS/2 bus. The
microcontroller behaves just like a PS/2 keyboard or mouse would. This means new designs can re-use existing PS/2 IP cores.
Mice and keyboards that use the PS/2 protocol use a two-wire serial bus (clock and data) to communicate with a host. On the
Genesys 2, the microcontroller emulates a PS/2 device, while the FPGA plays the role of the host. Both the mouse and the
keyboard use 11-bit words that include a start bit, data byte (LSB first), odd parity, and stop bit, but the data packets are
organized differently, and the keyboard interface allows bi-directional data transfers (so the host device can illuminate state
LEDs on the keyboard). Bus timings are shown in Figure 11.
The clock and data signals are only driven when data transfers occur; otherwise they are held in the idle state at logic '1.' This
requires that when the PS/2 signals are used in a design, internal pull-ups must be enabled in the FPGA on the data and clock
pins. The clock signal is normally driven by the device, but may be held low by the host in special cases. The timings define
signal requirements for mouse-to-host communications and bi-directional keyboard communications. A PS/2 interface circuit
can be implemented in the FPGA to create a keyboard or mouse interface.
When a keyboard or mouse is connected to the Genesys 2, a "self-test passed" command (0xAA) is sent to the host. After this,
commands may be issued to the device. Since both the keyboard and the mouse use the same PS/2 port, one can tell the type
of device connected using the device ID. This ID can be read by issuing a Read ID command (0xF2). Also, a mouse sends its
ID (0x00) right after the "self-test passed" command, which distinguishes it from a keyboard.
11.2. Keyboard
PS/2 uses open-collector drivers so the keyboard or an attached host can drive the two-wire bus (if the host will not send data
to the keyboard, then the host can use input-only ports).
PS/2-style keyboards use scan codes to communicate key press data. Each key is assigned a code that is sent whenever the key
is pressed. If the key is held down, the scan code will be sent repeatedly about once every 100ms. When a key is released, an F0
key-up code is sent, followed by the scan code of the released key. If a key can be shifted to produce a new character (like a