R
Functional Description
If not using the SPI Flash PROM after configuration, drive
CSO_B High to disable the PROM. The MOSI, DIN, and
CCLK pins are then available to the FPGA application.
Similarly, the SPI bus can be expanded to additional SPI
peripherals. Because SPI is a common industry-standard
interface, various SPI-based peripherals are available, such
as analog-to-digital (A/D) converters, digital-to-analog (D/A)
converters, CAN controllers, and temperature sensors.
However, if sufficient I/O pins are available in the applica-
tion, Xilinx recommends creating a separate SPI bus to con-
trol peripherals. Creating a second port reduces the loading
on the CCLK and DIN pins, which are crucial for configura-
tion.
Because all the interface pins are user I/O after configura-
tion, the FPGA application can continue to use the SPI
Flash interface pins to communicate with the SPI Flash
PROM, as shown in Figure 56. SPI Flash PROMs offer ran-
dom-accessible, byte-addressable, read/write, non-volatile
storage to the FPGA application.
SPI Flash PROMs are available in densities ranging from
1 Mbit up to 128 Mbits. However, a single Spartan-3E FPGA
requires less than 6 Mbits. If desired, use a larger SPI Flash
PROM to contain additional non-volatile application data,
such as MicroBlaze processor code, or other user data such
as serial numbers and Ethernet MAC IDs. In the example
shown in Figure 56, the FPGA configures from SPI Flash
PROM. Then using FPGA logic after configuration, the
FPGA copies MicroBlaze code from SPI Flash into external
DDR SDRAM for code execution. Similarly, the FPGA appli-
cation can store non-volatile application data within the SPI
Flash PROM.
The MOSI, DIN, and CCLK pins are common to all SPI
peripherals. Connect the select input on each additional SPI
peripheral to one of the FPGA user I/O pins. If HSWAP = 0
during configuration, the FPGA holds the select line High. If
HSWAP = 1, connect the select line to +3.3V via an external
4.7 kΩ pull-up resistor to avoid spurious read or write oper-
ations. After configuration, drive the select line Low to select
the desired SPI peripheral.
During the configuration process, CCLK is controlled by the
FPGA and limited to the frequencies generated by the
FPGA. After configuration, the FPGA application can use
other clock signals to drive the CCLK pin and can further
optimize SPI-based communication.
The FPGA configuration data is stored starting at location 0.
Store any additional data beginning in the next available SPI
Flash PROM sector or page. Do not mix configuration data
and user data in the same sector or page.
Refer to the individual SPI peripheral data sheet for specific
interface and communication protocol requirements.
Spartan-3E FPGA
SPI Serial Flash PROM
FFFFF
User Data
MOSI
DATA_IN
FPGA-based
SPI Master
MicroBlaze
DIN
DATA_OUT
Code
CCLK
CLOCK
FPGA
Configuration
CSO_B
SELECT
+3.3V
0
User I/O
SPI Peripherals
- A/D Converter
DATA_IN
DATA_OUT
CLOCK
- D/A Converter
- CAN Controller
- Displays
- Temperature Sensor
- ASSP
SELECT
DS312-2_47_082009
To other SPI slave peripherals
Figure 56: Using the SPI Flash Interface After Configuration
DS312-2 (v3.8) August 26, 2009
www.xilinx.com
83
Product Specification
XILINX [ XILINX, INC ]
