Yes, some combinations of values for Vod, Pre-Tap, 1st Post-Tap, and 2nd Post-Tap in the Stratix® V transceiver PMA transmitter are illegal. These illegal combinations can result in an overall voltage in the transmitter that is either above Vmax or below Vmin. This voltage outside recommended operating conditions can result in compromised transceiver operation including a collapsed eye.
PMA transmitter settings are checked by Quartus® II during design compilation to ensure only legal combinations are used. This check has been implemented starting with Quartus II v11.1sp2. Additionally, a spreadsheet tool has been created and is available on the Altera Wiki to manually inspect which combinations of values result in legal / illegal values: Stratix_V_Legal_PMA_Setting_Check
Outside of Quartus II, there are two possible ways to change PMA transmitter analog values which are NOT checked for legal / illegal combinations:
- The first is via the Reconfiguration Controller IP. User logic can modify various transceiver operating characteristics via this controller, including the transmitter PMA. Users should be aware of using possible illegal values and the affect on transceiver operation.
- The second is the Transceiver Toolkit. Users can manually modify transmitter PMA analog values while tuning their link. The ability to select legal / illegal combinations of values within the Toolkit has been allowed so users can make short-duration changes while debugging links if necessary. Users should be aware of possible illegal values and ensure they do not persistently apply illegal values via the Toolkit.