CRUM unit mountable in consumable unit of image forming apparatus and image forming apparatus using the same

A customer replacement unit monitor (CRUM) unit that can be mounted in an image forming apparatus includes a decoder which receives first clock signals from the image forming apparatus and converts the first clock signals into second clock signals, a memory which stores data related with a consumable unit, and a controller which manages the memory based on data signals transmitted from the image forming apparatus and the second clock signals. The first clock signals are divided into a data section in which the data signals are transmitted and received and an idle section in which the data signals are not transmitted and received. The first clock signals have a first frequency on the data section while having a second frequency on the idle section, and the second clock signals are clock signals maintaining a high value or a low value on the idle section.