EP2C20F256C8N Datasheet, PDF (368/470 Pages) Altera Corporation – Cyclone II Device Handbook, Volume 1

Active Serial Configuration (Serial Configuration Devices)
During initialization, the initialization clock source is either the
Cyclone II 10 MHz (typical) internal oscillator (separate from the AS
internal oscillator) or the optional CLKUSR pin. By default, the internal
oscillator is the clock source for initialization. If the internal oscillator is
used, the Cyclone II device provides itself with enough clock cycles for
proper initialization. The advantage of using the internal oscillator is you
do not need to send additional clock cycles from an external source to the
CLKUSR pin during the initialization stage. You can also make use of the
CLKUSR pin as a user I/O pin, which means you have an additional user
I/O pin.
If you want to delay the initialization of the devices in the chain, you can
use the CLKUSR pin option. The CLKUSR pin allows you to control when
your device enters user mode. This feature also allows you to control the
order of when each device enters user mode by feeding a separate clock
to each device’s CLKUSR pin. By using the CLKUSR pins, you can choose
any device in the multiple device chain to enter user mode first and have
the other devices enter user mode at a later time.
Different device families may require a different number of initialization
clock cycles. Therefore, if your multiple device chain consists of devices
from different families, the devices may enter user mode at a slightly
different time due to the different number of initialization clock cycles
required. However, if the number of initialization clock cycles is similar
across different device families or if the devices are from the same family,
then the devices enter user mode at the same time. See the respective
device family handbook for more information about the number of
initialization clock cycles required.
If an error occurs at any point during configuration, the FPGA with the
error drives the nSTATUS signal low. If you enable the Auto-restart
configuration after error option, the entire chain begins reconfiguration
after a reset time-out period (a maximum of 40 µs). If the Auto-restart
configuration after error option is turned off, a microprocessor or
controller must monitor nSTATUS for errors and then pulse nCONFIG low
to restart configuration. The microprocessor or controller can pulse
nCONFIG if it is under system control rather than tied to V_CC.
1 While you can cascade Cyclone II devices, serial configuration
devices cannot be cascaded or chained together.
1 If you use the optional CLKUSR pin and the nCONFIG is pulled
low to restart configuration during device initialization, make
sure the CLKUSR pin continues to toggle while nSTATUS is low
(a maximum of 40 µs).
13–14
Cyclone II Device Handbook, Volume 1
Altera Corporation
February 2007