1. A method to operate a charge trapping memory comprising a first group of memory cells at a first state having a first threshold voltage range and a second group of memory cells at a second state having a second threshold voltage range, wherein the first threshold voltage range is lower than the second threshold voltage range, the method comprising: lowering threshold voltages of the second group of memory cells from the second threshold voltage range to the first threshold voltage range, and raising threshold voltages of the first group and the second group of memory cells from the first threshold voltage range to the second threshold voltage range.