R
XC5200 Series Field Programmable Gate Arrays
7-110 November 5, 1998 (Version 5.2)
Configuration
The l ength cou nter b egins c ounting immediat ely upo n entr y
into the configuration state. In slave-mode operation it is
important to wait at least two cycles of the internal 1-MHz
clock oscillator after INIT is recognized before toggling
CCLK and feeding the serial bitstream. Configuration will
not begin until the internal configuration logic reset is
released, which happens two cycles after INIT goes High.
A master device’s configuration is delayed from 32 to 256
µs to ens ure proper o peration with an y slave de vices dri ven
by the master device.
The 0010 preamble code, included for all modes except
Express mode, indicates that the following 24 bits repre-
sent t he len gt h c ount . Th e le ngt h c ou nt i s the t ot al numbe r
of co nf i gu ra tio n c l ock s ne ed ed to l o ad t he compl e te c on fig -
uration data. (Four additional configuration clocks are
required to complete the configuration process, as dis-
cussed below.) After the preamble and the length count
have be en pas sed t hrou gh to all de vices in t he dais y ch ain,
DOUT is held High to prevent frame start bits from reaching
any daisy-chained devices. In Express mode, the length
count bits are ignored, and DOUT is held Low, to disable
the ne xt device in the pseudo dai sy chain.
A spe cifi c conf igu ratio n bi t, ea rly i n th e firs t fr ame of a mas-
ter device, controls the configuration-clock rate and can
increase it by a factor of eight. Therefore, if a fast configu-
ration clock is selected by the bitstream, the slower clock
rate is used until this configuration bit is detect ed.
Each frame has a start field followed by the frame-configu-
rati on da ta b its a nd a frame err or f ield. If a f rame data erro r
is detected, the FPGA halts loading, and signals the error
by pullin g the open -drain INIT pin Low. After all configura-
tion frames have been loaded into an FPGA, DOUT again
follows the input data so that the remaining data is passed
on to the next device. In Express mode, when the first
device is fully programmed, DOUT goes High to enable the
next device in the chai n.
Delaying Configuration After Power-Up
To delay master mode configuration after power-up, pull
the bidirectional INIT pin Low, using an open-collector
(open-drain) driv er. (See Figure 12.)
Using an open-collector or open-drain driver to hold INIT
Low before the beginning of master mode configuration
causes the FPGA to wait after completing the configuration
memory clear operation. When INIT is no longer he ld L ow
externally, the device determine s its configuration mode by
captu ri n g its mod e pins , and is rea dy to st ar t t he conf i g ura -
tion proces s. A maste r device waits up to an addi tional 250
µs to make sure that any slaves in the optional daisy chain
have seen that INIT is High.
Start-Up
Start-up is the transition from the configuration process to
the intended user operation. This transition involves a
change from one clock source to another, and a change
from interfacing parallel or serial configuration data where
most ou tputs are 3-sta ted, t o normal operati on with I/O pi ns
active in the user-system. Start-up must make sure that
the user-logic ‘wakes up’ gracefully, that the outputs
become ac t ive w i tho ut c au sin g c onten t io n wi th t he c o nf igu-
ration signals, and that the internal flip-flops are released
from the global Reset at the right time.
Figure 25 describes start-up timing for the three Xilinx fam-
ilies in detail. Express mode configuration always uses
either CCLK_S YNC or UCLK_SYNC timing, the other con-
figur ati on mod es can use a ny of t he f our t iming sequ ences.
To acce ss the i nterna l start- up signal s, place the STARTUP
library symbol.
Start-up Timing
Different F PGA fa milie s ha ve different star t-u p seque nc es .
The XC2 000 fami ly g oes t hrou gh a fi xed seque nce. D ONE
goes H igh and the i ntern al g lobal Reset is de -act ivat ed on e
CCLK period after the I/O become active.
The XC3 000A family offers som e flexibility. DONE can be
programmed to go High one CCLK period before or after
the I/O beco me active. Independent of DONE, the internal
global Reset is de-activated one CCLK period before or
after the I/O become active.
The XC4000/XC5200 Series offers additional flexibility.
The three ev ents — DO NE going High, the internal Re set
being de-activa ted, and the user I/O going active — can all
occur in any arbitrary sequence. Each of them can occur
one CCLK pe ri o d befo re or af t er, or simultan eou s w it h, any
of the others. This relative timing is selected by means of
software options in the bi tstream generati on software.
The default option, and the most practical one, is for DONE
to go Hig h first, di sconnecti ng the conf iguratio n data s ource
and avoiding any contention when the I/Os become active
one cl ock l ater. Re set i s then rel eased an othe r cloc k pe riod
later to make sure that user-operation starts from stable
internal conditions. This is the most common sequence,
shown with heavy lines in Figure 25, but the designer can
modify it to meet particular requirements.
Normal ly, the start -up se quence is cont rolled by the intern al
device oscillator output (CCLK), which is as ynchronous to
the system clock.
XC4000/XC5200 Series offers another start-up clocking
option, UCLK_NOSYNC. The three events described
above ne ed not be trigger ed by CCLK. They can , as a con-
figuration opti on, be triggered by a user clock. This means
that the device can wake up in synchronism with the user
system .