As each character of the PIN number is entered, a correct character exhibits one current pulse shape while an incorrect character exhibits a different current pulse shape.
PIN numbers used to unlock a cell phone should not exhibit any characteristics about themselves. This creates a side channel. An attacker could monitor the pulses using an oscilloscope or other method. Once the first character is correctly guessed (based on the oscilloscope readings), they can then move to the next character, which is much more efficient than the brute force method of guessing every possible sequence of characters.
(good code)
Rather than comparing each character to the correct PIN value as it is entered, the device could accumulate the PIN in a register, and do the comparison all at once at the end. Alternatively, the components for the comparison could be modified so that the current pulse shape is the same regardless of the correctness of the entered character.