US Patent 10991444 Tiered read reference calibration

Calibrating read reference voltages is disclosed. In an aspect, a control die calibrates read reference voltages for reading the non-volatile memory cells. The control die is bonded to a memory die that contains memory cells. In one aspect, a tiered approach to calibrating read reference voltages is taken. For example, first the control die may attempt to determine new values for read reference voltages. If the new read reference voltages are satisfactory, then the control die may use the new read reference voltages. The control die could use one or more different techniques to determine new read reference voltages. If the new read reference voltages determined by the control die are unsatisfactory, then a memory controller in communication with the control die may calibrate the read reference voltages. By the control die determining the new read reference voltages, the memory controller is substantially less burdened with such tasks.