Circuit to eliminate bus contention at chip power up

A power-on control circuit for an integrated circuit of the type having plural voltage source. The circuit is powered on sequentially, thereby preventing bus contention. The power-on control circuit includes a power-on detection for generating an enabling signal and disabling signal to control output buffer. When the high voltage source is powered on and the low voltage is not, the output buffer is at a high impedance state to prevent bus contention. When the low voltage is powered on after the high voltage is powered on, the output buffer is at a normal state.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to the field of an integrated circuit technology. In particular, it relates to a power-on detection circuit at output buffer to ensure output buffer tri-state when high voltage is powered on.

2. Description of the Related Art

In order to prevent leakage and latch up in an ESD device of an IC with plural voltage supply, power-on sequence must be precise in output buffer design. The power-on sequence turns on the high voltage source first and the low voltage source second, but, bus contention results. FIG. 1A shows the function block of the I/O circuits and the bus. FIG. 1B shows a conventional output buffer 30 . The I/O voltage source VD 33 is powered on, but the core voltage source VDD is not. A PMOS 41 in an output buffer 30 of a circuit 101 is turned on such that a output terminal 40 of the circuit 101 is at high voltage level. A NMOS 40 in a output buffer 30 of a circuit 103 is turned on that an output terminal 40 of the circuit 103 is at low voltage level. Neither a PMOS 41 nor a NMOS 42 is turned on in a output buffer 30 of a circuit 105 such that a output terminal 40 of the circuit 105 is at high-impedance. A short current starts from the circuit 101 to the circuit 103 through a bus 102 . This causes a bus contention among the circuit 101 , the circuit 103 , and the circuit 105 .

The disadvantage of the conventional output buffer is that the state of the output terminal is undetermined when the I/O voltage source is powered on. There are three possible states: high voltage level, low voltage level, or high impedance, creating bus contention. It is necessary to add a power-on detect circuit in a output buffer to ensure the output buffer is at high impedance when I/O voltage source is power-on.

FIG. 2 shows a conventional power-on detect circuit. The gate of a PMOS 62 is coupled to a test signal 67 , the source is coupled to a power supply 75 , and the drain is coupled to a node B. The gate of a NMOS 63 is coupled to a power supply 75 , the source is coupled to a ground, and the drain is coupled to a node B. The input terminals of a NOR gate 65 couple to the test signal 67 and the node B respectively, the output terminal generates reset signal 66 to the reset of a standby flag 61 . The input terminals of AND gate 68 couple to a write signal 69 and node A respectively, and the output terminal of it couples to the set of the standby flag 61 . A standby flag implemented by a R/S latch flip-flop has its output coupled to a bus driver 70 . The control terminal of the bus driver is coupled to a read signal 71 , and the output terminal of it is coupled to the node A.

As shown in FIG. 3 , the power supply 75 is powered on at time t 1 , the test signal 67 is at low voltage level, such that the PMOS 62 turns on. At time t 2 , the voltage of the power supply increases gradually such that the NMOS turns on and the power-on detect signal 64 (i.e. the node B) is maintained at a low level until the power supply 75 becomes the inversion level. Therefore, the NOR gate 65 is supplied with two low level input signals, so that the NOR gate 65 supplies a high level reset signal 66 to the reset terminal of the standby flag 61 to be reset. At time t 3 , the power supply 75 increases to the inversion level, power-on detecting signal 64 reaches a high level, so that the NOR gate 65 supplies a low level reset signal 66 to the standby flag 61 . The standby flag 61 is set when the AND gate 68 supplies a high logic level on the condition that the write signal 69 is active and the bus 72 is at a high level. An output signal of the standby flag 61 is supplied to the bus 72 through the bus driver 70 when a read signal 71 supplied to the bus 72 becomes active. At time t 4 , the test signal 67 is set be high, so that the PMOS 62 is turned off and the power-on detecting signal 64 is at a low level. During the times t 3 to t 4 , a constant current I 1 flows through the PMOS 62 and the NMOS 63 .

There are two disadvantages in the conventional power-on detect circuit. First, an extra test signal is needed to control the duration of the power-on detecting signal. Second, a constant current flows through the power-on detect circuit when the power-on detecting signal is active.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention utilizes a power-on detection circuit to ensure an output buffer is at high impedance when only high voltage is powered on. The output buffer operates normally when low voltage is powered on.

FIG. 4 illustrates the operation between a power-on detection circuit and a output buffer. The architecture in FIG. 4 includes a power-on detection circuit 10 , a pull up circuit 22 , a pull down circuit 21 , an output buffer 30 . The power-on detection circuit is coupled to I/O voltage source VD 33 and core voltage source VDD, generating two control signals coupled to a pull-down circuit and a pull-up circuit through signal lines 11 a and 11 b respectively. The pull-down circuit 21 is coupled between the input terminal 27 and a low voltage level. The pull up circuit 22 is coupled between the input terminal 28 and a high voltage level. The power-on detection circuit detects the power-on sequence. When the I/O voltage source is powered on and the core voltage is not, the two control signals enable the pull down circuit 21 and the pull up circuit 22 . The input terminal 27 is coupled to the low voltage level, so that a PMOS 41 of the output buffer 30 is turned off. The input terminal 28 is coupled to the high voltage level, so that a NMOS 42 of the output buffer 30 is turned off. Therefore, the output buffer 30 is at high impedance to prevent bus contention.

The core voltage source VDD is powered on after the I/O voltage source is powered on. The power-on detection circuit 10 outputs a control signal to disable the pull down circuit 21 and pull up circuit 22 . Thus, the input terminal 27 and input terminal 28 of the output buffer 30 are enabled, that is, they can receive the internal signal.

Accordingly, a schematic of the present invention is shown in FIG. 5 . The power-on detection circuit 10 includes a PMOS 12 , a PMOS 14 , a NMOS 13 , an inverter 15 , and an inverter 16 . A source of the PMOS 12 to the I/O voltage source VD 33 . The NMOS 13 is connected at a drain to a node 17 , the drain of the PMOS 12 , at a gate to core voltage source VDD, and a source to a low voltage level VSS. The PMOS 14 , acting as a capacitance is connected at a drain and a source both to the I/O voltage source VD 33 , and at a gate to the node 17 . An inverter 15 is connected at the input terminal to the node 17 , and at an output terminal 19 b to the gate of the PMOS 12 and to the signal line 11 b . An inverter 16 is connected at an input terminal to the output terminal 19 b , and at an output 19 a to the signal line 11 a . A PMOS 24 is connected at a drain to the input terminal 28 , at a source to a high voltage level, and a gate to the signal line 11 b . A NMOS 23 is connected at a drain to the input terminal 27 , at source to a low voltage level, and a gate to the signal 11 a.

The PMOS 12 , the NMOS 14 , and the inverter 15 form a positive feedback loop. When the core voltage source VDD is not powered on, the NMOS 13 is turned off. In such a state, when the I/O voltage source VD 33 is powered on, the capacitance of the PMOS 14 and the parasitic capacitance of the NMOS 13 couple a voltage drop to the node 17 , so that the inverter 15 inverts the voltage to supply a low voltage to turn on the PMOS 12 to charge the node 17 . Therefore, the node 17 is latched at a high voltage, and the output terminal 19 b is at low voltage supplied to the inverter 16 such that the output terminal 19 a is at high voltage.

The NMOS 23 is turned on by the signal line 11 a , the input terminal 27 is pulled down to a low voltage level, and the PMOS 41 is turned off. The PMOS 24 is turned on by the signal line 11 b , the input terminal 28 is pulled up to a high voltage level, and the NMOS 42 is turned off. Thus, the output terminal 40 of the output buffer 30 is at high impedance to prevent bus contention.

When the core voltage source VDD is powered on after the I/O voltage source VD 33 is powered on, the NMOS 13 is turned on thereby. The NMOS 13 pulls down the voltage of the node 17 to a low level such that the inverter 15 supplies a high voltage to the gate of the PMOS 12 to turn off the PMOS 12 to break off the positive feedback loop. The gate of the PMOS 24 is turned off by a high voltage of the output terminal 19 b . The output terminal 19 a of the inverter 16 is inverted to a low voltage level owing to the output terminal 19 b being at a high voltage level. The gate of the NMOS 23 is turn off by a high voltage of the output terminal 19 b . Thus, the input terminal 27 and the input terminal 28 are enabled, such that they can receive the internal signal.

There are four advantages concluded from the above description of the present invention. First, it can detect the state when only the high voltage source is powered on. Second, when only high voltage source is powered on, it ensures the output buffer is at high impedance. Third, the powered on detection circuit utilizes the couple capacitance, so that it responds quickly. Fourth, the power-on detection circuit requires a small layout area.

While the invention has been described with reference to various illustrative embodiments, the description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to those persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modification or embodiments as may fall within the scope of the invention defined by the following claims and their equivalents.