DIGITALLY CONTROLLED DELAY CIRCUIT

Abstract: A delay circuit (10) includes a first inverter circuit (20) coupled between a first voltage source (VCC) and a reference potential by a plurality of parallel-connected transistors (Q5-Q19). Each of the transistors (Q5-Q19) has a control gate (DY1-DY15) which may be selected to bring the resistance of the transistor into circuit with the inverter circuit (20) to control the charging rate of a distributed capacitance. An output circuit (40) coupled to the first inverter circuit (20) provides the distributed capacitance and an inverted buffered output (DBUF). The output circuit also includes an output (DOUT) which may be connected to another circuit identical to the delay circuit (10) to form a cascaded delay circuit which can receive a reset signal for resetting the cascaded delay circuit.

Description: DIGITALLY CONTROLLED DELAY CIRCUIT Technical Field This invention relates to delay circuits. Background Art In a known delay circuit, an input trigger signal is delayed by a prescribed interval. Disclosure of the Invention It is an object of the present invention to provide a delay circuit wherein the amount of delay can be readily and precisely controlled. Therefore, according to the invention, there is provided a delay circuit, characterized by: a first inverter circuit coupled between voltage supply means and a reference potential by a first resistance means and having input means adapted to receive an input signal to be delayed; a plurality of different value second resistance means adapted to be selectively coupled in parallel with said first resistance means; and output means including a second inverter circuit including a distributed capacitance coupled to an output of said first inverter circuit and to said voltage supply means and adapted to invert and delay the signal on said output of said first inverter circuit as a function of the current-flow through said first and second resistance means. It will be appreciated that a delay circuit according to the invention has the advantage that -the precise increments of delay can be selected in response to a digital control signal. Another advantage is that the circuit can be easily cascaded to provide longer delays. Brief Description of the Drawings One embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figs. 1A and IB are a schematic diagram of the preferred embodiment of the invention. Best Mode for Carrying Out the Invention Referring to Figs. 1A and IB, the delay circuit 10 is comprised of a first inverter circuit 20 having an input labeled IN and an output labeled DLY, a source of power VQ * a reference potential (ground), a cascade 30 of parallel connected transistors and an output circuit 40. The output circuit 40 is connected to the output DLY of the first inverter 20 and provides two outputs, one a buffered delay, DBUF, and the other a delay out, DOUT * One or more of the delay circuits 10 may be cascaded by connecting the DOUT terminal of the first delay circuit 10 to the IN labeled input of the first inverter 20 of a second delay circuit 10. The first inverter circuit 20 is a CMOS device having a P transistor, 03, and an N transistor, Q4. Next to transistor Q3 is the labeling P30/2.5 which denotes that transistor Q3 is a P-type device having a gate width, , equal to 30 microns and a gate length, L, equal to 2.5 microns. As is well known in the art of MOS design, the magnitude of the drain- source current is a function of the ratio /L. The type of transistor, N or P, and the associated gate ratio for each of the remaining transistors is shown in the drawings adjacent the transistor. The control gates of transistors Q3 and Q4 are connected to the input terminal IN for receiving the signal that is to be delayed. The drain of transistor Q3 is connected to the voltage source VQQ and the source of transistor Q4 is placed in circuit to ground through the conductive resistance of transistor Q20 when the gate of transistor Q20 is connected to the voltage source Vcc With the gate ratios shown, and only Q20 active, the delay for the input signal is 4.07 nano seconds. Each of the transistors Q5 through Q19 have gate electrodes labeled DY1 through DY15, respectively. Each input is switchable between a ground potential or the potential Vcc b Y a control signal. The longest delay is provided when all DY inputs are connected to ground. The shortest intended delay is when DY1 is at the potential Vcc and DY2 through DY15 are grounded. The output circui 40 comprises a second inverter circuit having transistors Q21 and Q22' which inverts the signal on line DLY and buffers the inverted signal to its output DBUF. The signal on line DLY is also directed to transistors Q23/ Q24, Q25 and Q26, which transistors Q23' Q24 Q.25 and Q26 perform an inverting function along with a reset function that is utilized when two or more delay lines are cascaded for increased delay. In operation, the conducting resistance of Q20 draws a certain amount of current from the line DLY when transistor Q4 is conducting so as to limit the current available to charge/discharge the combined gate capacitance of transistors Q21, Q22* Q24 and Q25. When another transistor gate DY is activated, the conducting resistance of that transistor is placed in parallel with the conducting resistance of Q20 thereby lowering the resistance between the drain of Q4 and ground. The lower resistance increases the current flow through Q4 thereby increasing the amount of current available for charging/discharging the combined gate capacitance of Q2ir Q22* Q24, and Q25. As is well known, the shorter the time it takes to charge/discharge the gate capacitance of a MOS transistor (such as transistors Q21, 0.22* Q24' and Q25) the shorter the delay in the transistors turning "on" and "off* 1 . Conversely, the longer it takes to charge the gate capacitors the longer it will take a properly biased transistor to turn "on" or "off". The rising edge of an input signal is delayed based on which DY input is activated. The falling edge has a minimal delay. The DBUF output, taken from the source of Q21 and the drain of Q22' provides the delayed timing output of the delay circuit 10. As previously stated, transistors Q23, 0.24' Q25 and Q26 form an inverter circuit and a means for coupling the output of a first delay circuit 10 to the input of a second delay circuit 10 in order to increase the amount of delay beyond that available with one delay circuit. The function of transistors Q23 and Q26 is to provide a clear signal for clearing the cascaded delay circuit upon receipt of a RESET signal on their gate electrodes. In the absence of the RESET signal transistors Q24 and Q25 operate as an inverter and a buffer to invert the signal on the line DLY and to buffer the signal to the next cascaded delay circuit.

Claims: Claims : 1. A delay circuit, characterized by: a first inverter circuit (20) coupled between voltage supply means and a reference potential by a first resistance means (Q20) and having input means adapted to receive an input signal to be delayed; a plurality of different value second resistance means (Q5-Q19) adapted to be selectively coupled in parallel with said first resistance means (Q20); and output means including a second inverter circuit (Q21, Q22) including a distributed capacitance coupled to an output of said first inverter circuit (20) and to said voltage supply means and adapted to invert and delay the signal on said output of said first inverter circuit (20) as a function of the current flow through said first and second resistance means (Q20; Q5-Q19). 2. A delay circuit according to claim 1, characterized in that said first inverter circuit (20) is a two transistor CMOS inverter. 3. A delay circuit according to claim 2, characterized in that said first resistance means includes an MOS transistor (Q20) having a gate electrode connected to said voltage supply means. 4. A delay circuit according to claim 3, characterized in that each of said plurality of different value second resistance means includes an MOS transistor (Q5-Q19) having a gate electrode adapted to receive a selection signal. 5. A delay circuit according to claim 1, characterized in that said output means includes a third inverter circuit (Q23-Q26) including a further distributed capacitance and reset means adapted to reset a further cascade-connected delay circuit.

Inventors: STEWART JOHN WAYNE, STEWART, JOHN, WAYNE
Applicants: NCR CO, NCR CORPORATION
Application year: 1988
Main classification: H03K5
IPC codes: H03K5