The signal drive strength from the DRAM can be controlled by setting mode register MR1. Every PCB layout is different so this tuning capability is required to improve signal integrity, maximize the signal's eye-size and allow the DRAM to operate at high-speeds. Having a bank of parallel 240Ω resistors allows you to tune the drive strength (for READs) and termination resistance (for WRITEs). Next, you may wonder why the DQ pins even have this parallel network of 240Ω resistors in the first place! At this point the calibration has been complete and the VOH values are transferred all the DQ pins.įigure 7: DQ calibration block (Source: Micron datasheet) When a ZQCL command is issued during initialization, this DQ calibration control block is enabled and an internal comparator within the DQ calibration control block tunes the p-channel devices using VOH until the voltage is exactly VDDq/2 (A classic resistor divider). Now, the circuit connected to the DQ calibration control block is essentially a resistor divider circuit with one of the resistors being the poly and the other is the precision 240Ω. These little transistors are set based on input VOH.įigure 6: DQ driver/receiver circuit (Source: Micron datasheet) The figure below zooms into one 240Ω leg of the DQ circuit and shows 5 p-channel devices connected to the poly-resistor. There are number of p-channel devices that are connected in parallel to this poly-resistor so that it can be tuned exactly to 240Ω. The 240Ω resistor leg within a DQ circuit is a type of resistor called "Poly Silicon Resistor" and is, typically, slightly larger than 240Ω (Poly silicon resistor is a type of resistor that is compatible with CMOS technology). If you're itching for more details, read on. If you're satisfied, proceed to the next section. The above explanation is a quick overview of ZQ calibration. This value is then copied over to each DQ's internal circuitry. ![]() When a ZQCL command is issued during initialization, this DQ calibration control block gets enabled and it produces a tuning value. This external precision resistor is the "reference" and it remains at 240Ω at all temperatures.
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