A system comprising: a voltage sensing module configured to: sense a supply voltage of a circuit block; generate a first control signal when the supply voltage is less than or equal to a first voltage; and generate a second control signal when the supply voltage is within a predetermined range of a second voltage, wherein the first voltage is less than the second voltage; and a frequency adjustment module configured to: set a frequency of a clock signal supplied to the circuit block to a first frequency, wherein the first frequency is an operating frequency less than a normal operating frequency of the circuit block, when i) the supply voltage is initially supplied to the circuit block after a power on reset operation, and ii) the first control signal or the second control signal is received; switch the frequency of the clock signal from the first frequency to the normal operating frequency when the second control signal is received again and the frequency of the clock signal is set to the first frequency; and switch the frequency of the clock signal from the normal operating frequency to the first frequency when the first control signal is received again and the frequency of the clock signal is set to the normal operating frequency. a voltage sensing module configured to: sense a supply voltage of a circuit block; generate a first control signal when the supply voltage is less than or equal to a first voltage; and generate a second control signal when the supply voltage is within a predetermined range of a second voltage, wherein the first voltage is less than the second voltage; and sense a supply voltage of a circuit block; generate a first control signal when the supply voltage is less than or equal to a first voltage; and generate a second control signal when the supply voltage is within a predetermined range of a second voltage, wherein the first voltage is less than the second voltage; and a frequency adjustment module configured to: set a frequency of a clock signal supplied to the circuit block to a first frequency, wherein the first frequency is an operating frequency less than a normal operating frequency of the circuit block, when i) the supply voltage is initially supplied to the circuit block after a power on reset operation, and ii) the first control signal or the second control signal is received; switch the frequency of the clock signal from the first frequency to the normal operating frequency when the second control signal is received again and the frequency of the clock signal is set to the first frequency; and switch the frequency of the clock signal from the normal operating frequency to the first frequency when the first control signal is received again and the frequency of the clock signal is set to the normal operating frequency. set a frequency of a clock signal supplied to the circuit block to a first frequency, wherein the first frequency is an operating frequency less than a normal operating frequency of the circuit block, when i) the supply voltage is initially supplied to the circuit block after a power on reset operation, and ii) the first control signal or the second control signal is received; i) the supply voltage is initially supplied to the circuit block after a power on reset operation, and ii) the first control signal or the second control signal is received; switch the frequency of the clock signal from the first frequency to the normal operating frequency when the second control signal is received again and the frequency of the clock signal is set to the first frequency; and switch the frequency of the clock signal from the normal operating frequency to the first frequency when the first control signal is received again and the frequency of the clock signal is set to the normal operating frequency.