Data Sheet
June 1999
ORCA Series 3C and 3T FPGAs
configuration, the high on the FPGA's DONE disables
the serial ROM.
FPGA Configuration Modes (continued)
Master Serial Mode
Serial ROMs can also be cascaded to support the con-
figuration of multiple FPGAs or to load a single FPGA
when configuration data requirements exceed the
capacity of a single serial ROM. After the last bit from
the first serial ROM is read, the serial ROM outputs
CEO low and 3-states the DATA output. The next serial
ROM recognizes the low on CE input and outputs con-
figuration data on the DATA output. After configuration
is complete, the FPGA’s DONE output into CE disables
the serial ROMs.
In the master serial mode, the FPGA loads the configu-
ration data from an external serial ROM. The configura-
tion data is either loaded automatically at start-up or on
a PRGM command to reconfigure. The ATT1700A
Series Serial PROMs can be used to configure the
FPGA in the master serial mode. This provides a sim-
ple 4-pin interface in a compact package.
Configuration in the master serial mode can be done at
powerup and/or upon a configure command. The sys-
tem or the FPGA must activate the serial ROM's
RESET/OE and CE inputs. At powerup, the FPGA and
serial ROM each contain internal power-on reset cir-
cuitry that allows the FPGA to be configured without
the system providing an external signal. The power-on
reset circuitry causes the serial ROM's internal address
pointer to be reset. After powerup, the FPGA automati-
cally enters its initialization phase.
This FPGA/serial ROM interface is not used in applica-
tions in which a serial ROM stores multiple configura-
tion programs. In these applications, the next
configuration program to be loaded is stored at the
ROM location that follows the last address for the previ-
ous configuration program. The reason the interface in
Figure 55 will not work in this application is that the low
output on the INIT signal would reset the serial ROM
address pointer, causing the first configuration to be
reloaded.
The serial ROM/FPGA interface used depends on such
factors as the availability of a system reset pulse, avail-
ability of an intelligent host to generate a configure
command, whether a single serial ROM is used or mul-
tiple serial ROMs are cascaded, whether the serial
ROM contains a single or multiple configuration pro-
grams, etc. Because of differing system requirements
and capabilities, a single FPGA/serial ROM interface is
generally not appropriate for all applications.
In some applications, there can be contention on the
FPGA's DIN pin. During configuration, DIN receives
configuration data, and after configuration, it is a user
I/O. If there is contention, an early DONE at start-up
(selected in ORCA Foundry) may correct the problem.
An alternative is to use LDC to drive the serial ROM's
CE pin. In order to reduce noise, it is generally better to
run the master serial configuration at 1.25 MHz (M3 pin
tied high), rather than 10 MHz, if possible.
Data is read in the FPGA sequentially from the serial
ROM. The DATA output from the serial ROM is con-
nected directly into the DIN input of the FPGA. The
CCLK output from the FPGA is connected to the CLK
input of the serial ROM. During the configuration pro-
cess, CCLK clocks one data bit on each rising edge.
TO DAISY-
CHAINED
DEVICES
DOUT
DATA
CLK
DIN
CCLK
ATT1700A
Since the data and clock are direct connects, the
FPGA/serial ROM design task is to use the system or
FPGA to enable the RESET/OE and CE of the serial
ROM(s). There are several methods for enabling the
serial ROM’s RESET/OE and CE inputs. The serial
ROM’s RESET/OE is programmable to function with
RESET active-high and OE active-low or RESET active-
low and OE active-high.
CE
DONE
INIT
RESET/OE
CEO
ORCA
SERIES
FPGA
DATA
CLK
PRGM
ATT1700A
CE
M2
M1
M0
In Figure 55, serial ROMs are cascaded to configure
multiple daisy-chained FPGAs. The host generates a
500 ns low pulse into the FPGA's PRGM input. The
FPGA’s INIT input is connected to the serial ROMs’
RESET/OE input, which has been programmed to
function with RESET active-low and OE active-high.
The FPGA DONE is routed to the CE pin. The low on
DONE enables the serial ROMs. At the completion of
RESET/OE
CEO
TO MORE
SERIAL ROMs
AS NEEDED
PROGRAM
5-4456.1(F)
Figure 55. Master Serial Configuration Schematic
Lucent Technologies Inc.
93
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