Digilent 410-183P-KIT - Manual (Page 8)

Basys3™ FPGA Board Reference Manual
Artix-7
C17
PIC24FJ128
PS2_CLK
B17
USB HOST (J2)
2
PS2_DAT
FPGA
Config
7
FPGA
Config
Figure 7. Basys3 PIC24 connections.
6.1 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 Basys3, 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 Fig. 8.
T
CK
T
SU
Clock time
Data-to-clock setup time 30us
5us
50us
25us
Symbol Parameter Min Max
T
HLD
Clock-to-data hold time 5us 25us
Edge 0
‘0’ start bit ‘1’ stop bit
Edge 10
Tsu
T hld
Tck Tck
CLOCK
DATA
Figure 8. PS/2 device-to host timing diagram.
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 Basys3, 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.
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