AR# 15054 Virtex-II Pro RocketIO - Behavior of AVCCAUX (RX/TX) supply currents at device power up before VCCINT and VCCAUX supplies are powered up
Description
At power up, AVCCAUX (RX/TX) supplies can sink current when VCCINT and VCCAUX are not powered up.
Solution
At power up, the RocketIO are placed in power down mode by default. However, the default POWERDOWN mode control signal originates from the digital circuitry of the RocketIO in the VCCINT supply domain. In addition, the default power down control signal passes through a translation stage in the VCCAUX supply domain. Consequently, in order for the default power down mode to take effect at power up, both the VCCINT and VCCAUX supplies must be present.
RocketIO POWERDOWN Control
For this reason, you can see current in the AVCCAUX (RX/TX) supplies because until both VCCINT and VCCAUX supplies are present, the RocketIO might be fully alive.
If the RocketIO is not used in your design and the potential current is not needed, you can limit the current with a current-limiting resistor.
To minimize the current draw, a range of 8-12 mA is recommended.
For example:
Current Limit (I) = 10 mA
Worst Case Voltage Drop across Resistor (v)= 2.5V
Resistor Value = 2.5/10 mA = 250 ohms
NOTE: Once VCCINT and VCCAUX power up, the default POWERDOWN mode signal is active and AVCCAUX (RX/TX)s will not draw current.