7I92 14
OPERATION
LEDS
The 7I92 has 4 FPGA driven user LEDs (User 0 through User 3 = Green), and 2
FPGA driven status LEDs (red) and a power LED. The user LEDs can be used for any
purpose, and can be helpful as a simple debugging feature. A low output signal from the
FPGA lights the LED. See the 7I92IO.PIN file for FPGA pin locations of the LED signals.
The status LEDs reflect the state of the FPGA’s DONE, and /INIT pins. The /DONE LED
lights until the FPGA is configured at power-up. The /INIT LED lights when the power on
reset is asserted, when there has been a CRC error during configuration. When using
Mesas configurations, the /INIT LED blinks when the fallback configuration has been
loaded.
PULLUP RESISTORS
All I/O pins are provided with pull-up resistors to allow connection to open drain,
open collector, or OPTO devices. These resistors have a value of 4.7K so have a
maximum pull-up current of ~1.07 mA (5V pull-up) or ~.7 mA (3.3V pull-up).
IO LEVELS
The Xilinx FPGAs used on the 7I92 have programmable I/O levels for interfacing
with different logic families. The 7I92 does not support use of the I/O standards that require
input reference voltages. All standard Mesa configurations use LVTTL levels.
Note that even though the 7I92 can tolerate 5V signal inputs, its outputs will not
swing to 5V. The outputs are push pull CMOS that will drive to the output supply rail of
3.3V. This is sufficient for TTL compatibility but may cause problems with some types of
loads. For example when driving an LED that has its anode connected to 5V, in such
devices as OPTO isolators and I/O module rack SSRs, the 3.3V high level may not
completely turn the LED off. To avoid this problem, either drive loads that are ground
referred, Use 3.3V as the VCC for VCC referred loads, or use open drain mode.
STARTUP I/O VOLTAGE
After power-up or system reset and before the the FPGA is configured, the pull-up
resistors will pull all I/O signals to a high level. If the FPGA is used for motion control or
controlling devices that could present a hazard when enabled, external circuitry should be
designed so that this initial state (high) results in a safe condition.