Latching input buffer circuit with variable hysteresis

• US 7,420,394 B2
• Filed: 11/17/2006
• Issued: 09/02/2008
• Est. Priority Date: 11/17/2006
• Status: Active Grant

• Current Assignee: NXP USA, Inc. (NXP Semiconductors NV)
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: Kase, Kiyoshi, Tran, Dzung T.
• Primary Examiner(s): Barnie; Rexford
• Assistant Examiner(s): Tran; Thienvu V
• Application Number:
  US11/561,209
• Publication Number:
  US 20080116952A1
• Time in Patent Office: 655 Days
• Field of Search: 326/82, 326/83, 326/86, 326/87, 327/108, 327/109
• US Class Current: 326/83
• CPC Class Codes: H03K 3/3565 Bistables with hysteresis, e.g. Schmitt trigger

Abstract

An input buffer circuit with hysteresis includes a first stage and a second stage. The first stage includes a resistive device to provide a resistance between two nodes of the first stage. The two nodes are responsive to a signal input. The second stage includes four series-coupled transistors. A first node is coupled to the control electrodes of two of the four transistors and the second node is coupled to the control electrodes of the other two transistors. The second stage includes a signal output. In some examples, a resistance provided by the resistive device is variable and provides the buffer circuit with hysteresis.

Claims

1. A buffer circuit, comprising:

a first buffer stage, the first buffer stage comprising;

a signal input;

a first node responsive to the signal input;

a second node responsive to the signal input;

a variable resistive device having a first terminal coupled to the first node and a second terminal coupled to the second node, the variable resistive device for providing a variable resistance between the first and second nodes;

a second buffer stage, the second buffer stage comprising;

a signal output;

a first transistor including a control electrode coupled to the first node;

a second transistor including a control electrode coupled to the second node;

a third transistor including a control electrode coupled to the first node;

a fourth transistor including a control electrode coupled to the second node;

wherein the first transistor, the second transistor, the third transistor, and the fourth transistor are series coupled transistors coupled between a first voltage terminal and a second voltage terminal.

2. The buffer circuit of claim 1 wherein:

the variable resistive device includes a fifth transistor including a first current electrode connected to the first node and a second current electrode connected to the second node;

the fifth transistor includes a control electrode coupled to a mode logic output for selectively controlling the resistance between the first current electrode and the second current electrode of the fifth transistor.

3. The buffer circuit of claim 1 wherein the resistive variable device includes a sixth transistor including a first current electrode connected to the first node and a second current electrode connected to the second node, the sixth transistor being of an opposite conductivity than the fifth transistor, the sixth transistor including a control electrode coupled to a second mode logic output for selectively controlling the resistance between the first current terminal and the second current terminal of the sixth transistor, wherein the second mode logic output is a complementary signal to the mode logic output.

4. The buffer circuit of claim 1 wherein the variable resistive device includes a first operating mode where the resistance between the first node and the second node is at a first resistive value and a second operating mode where the resistance between the first node and the second node is at a second resistance value, the first resistance value being different than the second resistance value.

5. The buffer circuit of claim 4 wherein when the variable resistive device is in the first operating mode, the signal output changes state from a low voltage state to a high voltage states in response to a change in state of the signal input in a first amount of time, when the variable resistive device is in the second operating mode, the signal output changes state from the low voltage state to the high voltage state in response to a change in state of the signal input in a second amount of time, the second amount of time being different than the first amount of time.

6. The buffer circuit of claim 1 wherein the variable resistive device includes:

a fifth transistor including a first current electrode connected to the first node and a second current electrode connected to the second node;

a sixth transistor including a first current electrode connected to the first node and a second current electrode connected to the second node;

wherein the fifth transistor is conductive and the sixth transistor is non conductive during a first operating mode of the variable resistive device, wherein when in the first operating mode, the variable resistive device provides a first resistive value between the first node and the second node;

wherein the sixth transistor is conductive during a second operating mode of the variable resistive device, wherein when in the second operating mode, the variable resistive device provides a second resistive value between the first node and the second node, the second resistive value being different than the first resistive value.

7. The buffer circuit of claim 1 wherein the first and second transistors are of a first conductivity type and the third and fourth transistors are of a second conductivity type, wherein the first conductivity type is of an opposite conductivity type from the second conductivity type.

8. The buffer circuit of claim 1 further comprising:

a latch circuit coupled to the signal output to latch a state of the signal output.

9. The buffer circuit of claim 1 wherein the first stage buffer further includes:

a fifth transistor including a current terminal connected to a first terminal of the variable resistive device and the first node;

a sixth transistor including a current terminal connected to a second terminal of the resistive device and the second node;

the fifth transistor and the sixth transistor each including a control terminal connected to the signal input.

10. The buffer circuit of claim 9 wherein:

the fifth transistor includes a second current terminal coupled to the first voltage terminal;

the sixth transistor includes a second current terminal coupled to the second voltage terminal.

11. The buffer circuit of claim 1 wherein the first transistor, the second transistor, the third transistor, and the fourth transistor are series coupled transistors coupled between the first voltage terminal and the second voltage terminal such that:

the first transistor includes a first current terminal coupled to the first voltage terminal and a second current terminal connected to a first current terminal of the second transistor;

the second transistor includes a second current terminal connected to a first current terminal of the third current terminal and the signal output;

the third transistor includes a second current terminal connected to a first current electrode of the fourth transistor;

the fourth transistor including a second current electrode coupled the second voltage terminal.

12. The buffer circuit of claim 1 further comprising:

a tri-state circuit configured that when enabled, pulls a voltage level of the first node to a voltage level of the first voltage terminal and pulls a voltage level of the second node to a voltage level of the second voltage terminal.

13. The buffer circuit of claim 1 wherein:

the first node is configured to transition from a high voltage state to a low voltage state in response to the signal input transitioning from a low voltage state to a high voltage state;

the second node is configured to transition from a high voltage state to a low voltage state in response to the signal input transitioning from a low voltage state to a high voltage state;

wherein the first node transitions to the low voltage state from the high voltage state in response to the signal input changing state to the high voltage state faster than the second node transitions to the low voltage state from the high voltage state in response to the signal input changing state to the high voltage state.

14. The buffer circuit of claim 1 wherein the first and second transistors are characterized as P-Channel type FETS and the third and fourth transistors are characterized as N-Channel type FETS.

15. The buffer circuit of claim 1 wherein the buffer circuit is configured to provide a hysteresis in the signal output transitioning between voltage states in response to the signal input transitioning between voltage states.

16. The buffer circuit of claim 15, wherein an amount of hysteresis provided by the buffer circuit is dependent upon a resistance level of the variable resistive device.

17. A buffer circuit comprising:

a signal input;

a first transistor including a control electrode connected to the signal input;

a second transistor including a control electrode connected to the signal input;

a first node connected to a first current terminal of the first transistor;

a second node connected to a first current terminal of the second transistor;

a resistive device coupled to provide a resistance between the first node and the second node;

a third transistor including a control electrode connected to the first node;

a fourth transistor including a control electrode connected to the second node;

a fifth transistor including a control electrode connected to the first node;

a sixth transistor including a control electrode connected to the second node;

a signal output connected to a current electrode of the fourth transistor and a current electrode of the fifth transistor;

wherein the third, fourth, fifth, and sixth transistors are coupled in series;

wherein the third transistor and the fourth transistor are of a first conductivity type and the fifth transistor and sixth transistor of a second conductivity type opposite the first conductivity type.

18. The buffer circuit of claim 17 wherein the resistive device is characterized as a variable resistive device.

19. A buffer circuit, comprising:

a first buffer stage, the first buffer stage comprising;

a signal input;

a first node responsive to the signal input;

a second node responsive to the signal input;

a resistive device coupled between the first node and the second node;

a second buffer stage, the second buffer stage comprising;

a signal output;

a first transistor including a control electrode coupled to the first node;

a second transistor including a control electrode coupled to the second node;

a third transistor including a control electrode coupled to the first node;

a fourth transistor including a control electrode coupled to the second node;

wherein the first transistor, the second transistor, the third transistor, and the fourth transistor are series coupled transistors coupled between a first voltage terminal and a second voltage terminal;

a latch circuit coupled to the signal output to latch a state of the signal output;

a tri-state circuit configured that when enabled, pulls a voltage level of the first node to a voltage level of the first voltage terminal and pulls a voltage level of the second node to a voltage level of the second voltage terminal;

wherein the latch circuit is configured to retain the previous state of the signal output prior to the tri-state circuit being enabled.