EP2C8T144C8N Datasheet, PDF (100/470 Pages) Altera Corporation – Cyclone II Device Handbook, Volume 1

Hot-Socketing Feature Implementation in Cyclone II Devices
Figure 4–1. Hot-Socketing Circuit Block Diagram for Cyclone II Devices
Output
Power-On
Reset
Monitor
Weak
R
Pull-Up
Resistor
PAD
Output Enable
Voltage
Tolerance
Control
Hot Socket
Output
Pre-Driver
Input Buffer
to Logic Array
f
The POR circuit monitors V_CCINT voltage level and keeps I/O pins
tri-stated until the device is in user mode. The weak pull-up resistor (R)
from the I/O pin to V_CCIO keeps the I/O pins from floating. The voltage
tolerance control circuit permits the I/O pins to be driven by 3.3 V before
V_CCIO and/or V_CCINT are powered, and it prevents the I/O pins from
driving out when the device is not in user mode.
For more information, see the DC Characteristics & Timing Specifications
chapter in Volume 1 of the Cyclone II Device Handbook for the value of the
internal weak pull-up resistors.
Figure 4–2 shows a transistor level cross section of the Cyclone II device
I/O buffers. This design ensures that the output buffers do not drive
when V_CCIO is powered before V_CCINT or if the I/O pad voltage is higher
than V_CCIO. This also applies for sudden voltage spikes during hot
socketing. The V_PAD leakage current charges the voltage tolerance control
circuit capacitance.
4–4
Cyclone II Device Handbook, Volume 1
Altera Corporation
February 2007