Removal of External Pull-up Resistor® to Vt on ODT enabled pins for QDRII+/DDRII+ and QDR IV - KBA82936

Version: *A

 

Question:

Can the external pull-up resistor® to Vt on ODT-enabled pins be removed for QDRII+, DDRII+ and QDR IV ODT parts?

 

Answer:

Yes, the external pull-up resistor is not required as ODT is enabled on the ODT-enabled pins of QDRII+, DDRII+ and QDR IV. In QDRII+ and DDRII+ devices, the value of ODT resistance can be controlled by RQ resistor (Resistance connected to ZQ pin) and the ODT pin. If the ODT pin is HIGH, then the ODT resistance will be RQ/1.66, while if the ODT pin is LOW, the ODT resistance will be RQ/3.33. For QDR IV devices, ODT resistance value depends upon the value of resistor connected to ZQ/ZT pin and the bit settings of “Termination control register” and “Impedance control register”. Details about setting each bit in these registers are explained in respective QDR IV datasheets.

An external pull-up resistor to Vt = Vddq/2 is normally recommended termination scheme for HSTL inputs for better signal integrity at high switching speeds. However, in ODT-enabled devices, the termination is available internal to the die, for the input pins with ODT feature enabled (Clock, K/K#, Data input and BWS# pins for QDRII+/DDRII+, Clock, Address, Command and Data pins for QDR IV), so termination resistances are not required on these pins. For more information, refer to the Application Notes AN42468 - On-Die Termination for QDRII+/DDRII+ SRAMs and AN84060 - QDR®-IV Design Guide