Nexys 3™ FPGA Board Reference Manual
Copyright Digilent, Inc. All rights reserved.
Other product and company names mentioned may be trademarks of their respective owners.
Page
12
of
22
Spartan 6
L12
PIC24FJ192
K_CLK
J13
“HOST”J4
2
L13
K14
K_DAT
M_CLK
M_DAT
R13
R15
DIN
CLK
FPGA Serial
programming
PS/2 Keyboard
PS/2 Mouse
L16
H17
GPIO0
GPIO1
Additional I/O
(for future use)
6
USB-UART Bridge (Serial Port)
The Nexys 3 includes an FTDI FT232 USB-UART bridge to allow PC applications to communicate with the board
using standard Windows COM port commands. Free USB-COM port drivers, available from www.ftdichip.com
under the "Virtual Com Port" or VCP heading, convert USB packets to UART/serial port data. Serial port data is
exchanged with the FPGA using a two-wire serial port
(TXD/RXD) and software flow control (XON/XOFF). After the
drivers are installed, I/O commands from the PC directed to the
COM port will produce serial data traffic on the N17 and N18
FPGA pins.
7
USB HID Host
A Microchip PIC24FJ192 microcontroller
provides the Nexys 3 with USB H ID host
capability. Firmware in the microcontroller
can drive a mouse or a keyboard attached to
the type A USB connector at J4 labeled "Host".
Hub support is not currently available, so only
a single mouse or a single keyboard can be
used. The PIC24 drives four signals into the
FPGA – two are used as a keyboard port
following the keyboard PS/2 protocol, and
two are used as a mouse port following the
mouse PS/2 protocol.
Two PIC24 I/O pins are also connected to the
FPGA's two-wire serial programming port, so the FPGA can be programmed from a file stored on a USB memory
stick. To program the FPGA, attach a memory stick containing a single .bit programming file in the root directory,
load both M0 and M1 on J8 jumper, and cycle board power. This will cause the PIC processor to program the FPGA,
and any incorrect bit files will automatically be rejected. Note the PIC24 reads the FPGA's mode, init, and done
pins, and can drive the PROG pin as a part of the programming sequence.
7.1 HID Controller
To access the USB host controller, EDK designs can use the
standard PS/2 core (non-EDK designs can use a simple state
machine).
Mice and keyboards that use the PS/2 protocol
1
use a two-
wire serial bus (clock and data) to communicate with a host
device. Both use 11-bit words that include a start, stop, and
1
Not all keyboard manufacturers strictly adhere to the PS/2 specifications; some keyboards may not produce the
proper signaling voltages or use the standard communication protocols. Compatibility with the USB host may vary
between different keyboards.
N17
TXD
N18
Micro-USB
J13
“UART”
2
RXD
Spartan 6
FT232
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
