JP3217079B2 - Semiconductor integrated circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal transmission technique and a technique particularly effective when applied to a differential transmission by a current driving method, for example, a CMOS-LS on a printed circuit board.
The present invention relates to an effective technique used for signal transmission between I.

[0002]

2. Description of the Related Art Conventionally, as a method of transmitting a signal between LSIs, a voltage driving method in which the potential of a signal line is shifted between a high level and a low level, and a signal level is detected by comparing the potential with a threshold voltage on a receiving side. Was common. On the other hand, in recent years, in a high-speed system such as an ATM switch composed of a CMOS-LSI, a method has been proposed in which a signal is transmitted as a differential signal by a current driving method using a circuit as shown in FIG. 1989 IEICE Autumn National Convention C-133, "A Study of CMOS High-Speed I / O Circuits", p. 5-119).

That is, the circuit shown in FIG. 6 comprises a pair of differential transistors MP2 and MP3 connected to a common source and a constant current transistor MP1 connected to the common source terminal.
And the active load transistors MN1 and MN2 connected to the drain terminal, amplify the signals D and D 'to be output, and output mirror MN3 which is current mirror-connected to the load transistors MN1 and MN2. , MN4, and the differential current signal Iou
t and Iout 'are output.

[0004]

In the transmission circuit of the conventional current drive system, the output transistors MN3, MN
Since the conductance of N4 depends on the variation of the manufacturing process, the output current signals Iout and Iout 'fluctuate greatly when the manufacturing process varies, the power consumption of the transmitting LSI varies, and the amplitude of the transmitted signal varies. It has been clarified by the present inventors that there is a problem that a signal is not transmitted to the LSI on the receiving side and a noise margin is reduced.

An object of the present invention is to reduce the amount of current fluctuation of a differential current signal to be output in a system in which signal transmission between a CMOS and an LSI is performed by a current driving method using a differential signal. An object of the present invention is to provide a differential transmission circuit capable of stabilizing the amplitude. Regarding the above and other objects and novel features of the present invention,
It will be apparent from the description of this specification and the accompanying drawings.

[0006]

The outline of a typical invention among the inventions disclosed in the present application is as follows. That is, the signal transmission circuit is constituted by a differential circuit including a pair of differential transistors connected to a common source and a constant current source connected to a common source terminal of the differential transistors, and the constant current sources are connected in parallel. , And the output voltage of the circuit is fed back to the gate terminals of these constant current transistors directly or via an amplifier circuit.

[0007]

In the signal transmission of the differential current drive system, a terminating resistor matched to the characteristic impedance of the transmission line is connected to the receiving end of the transmission line. This terminating resistor is usually composed of a highly accurate external resistor of ± 1% or less. Therefore, when the drive current fluctuates due to the variation in the conductance of the differential output transistor of the signal transmission circuit, the amplitude of the transmission signal changes. However, according to the above-described means, since the output voltage of the signal transmission circuit is fed back to the gate terminal of the constant current transistor, the current of the constant current source decreases when the drive current increases due to variation in the conductance of the differential output transistor. As described above, when the driving current decreases, the negative feedback is applied so that the current of the constant current source increases. Therefore, the above-described object of stabilizing the amplitude of the transmission signal can be achieved. Further, even when the fluctuation amount of the drive current is small, the feedback can be performed via the amplifier circuit, whereby higher stability can be achieved.

[0008]

FIG. 1 shows an embodiment of a transmission circuit according to the present invention. The transmission circuit of this embodiment is shown as a configuration in which two CMOS-LSIs 1 and 2 mounted on a printed circuit board are connected by transmission lines la and lb formed of printed wiring. A transmission circuit provided in the transmission-side LSI 1 includes a pair of NMs connected to a common source.
The differential transistors Q1 and Q2 are composed of OSFETs, and the differential amplifier circuit is composed of a constant current source CC connected to a common source terminal of the differential transistors Q1 and Q2. Terminals are connected to signal output terminals Ta and Tb of the LSI. In this embodiment, the constant current source CC includes a pair of N-MOSFETs Q3, Q4 connected in parallel with each other.
And these constant current transistors Q
The output signals a and b of the circuit are directly fed back to the gate terminals of Q3 and Q4. A switching MOSFET Q5 is connected between the source terminals of the constant current transistors Q3 and Q4 and the ground point, and a selection signal φs for activating a transmission circuit is applied to the gate terminal of the MOSFET Q5. Have been.

The output terminals TA and Tb are connected to a transmission line l.
Input terminals Tc and Td of the receiving side LSI are connected via a and lb. The receiving ends of the transmission lines la and lb are connected to the power supply voltage terminal Vcc via the terminating resistors R1, R2 and R3.
It is connected to the. The values of the terminating resistors R1, R2 and R3 are determined so as to match the characteristic impedance of the transmission line. Note that AMP1 is a differential sense amplifier as a receiving circuit provided in the receiving LSI. The levels of the transmission signals a and b are determined by the resistance values of the transmission lines la and lb, the values of the terminating resistors R1, R2, and R3 on the receiving end and the driving current of the transmission circuit. The resistance values of the transmission lines la and lb are considerably low at about 100Ω in the case of wiring on a printed circuit board. In this embodiment, the drive current is determined by using a high-sensitivity sense amplifier for the LSI on the receiving side so that the amplitude of the transmission signal may be about 300 mV.
The constant of each element of the transmission circuit is set so that the current value can be obtained.

Next, the operation of the signal transmission circuit will be described with reference to the timing chart of FIG. In this example,
When transmission data in is generated in the LSI 1 on the transmission side, complementary signals D 'and D are formed by the inverter INV and the non-inverter NIV and supplied to the gate terminals of the differential transistors Q1 and Q2 of the output circuit. At the same time, when the selection signal φs supplied from the internal control circuit is changed to a high level, the transmission circuit enters an operating state, and
Signals a and b having waveforms as shown in (C) and (D) are output from output terminals Ta and Tb onto transmission lines la and lb.

In this embodiment, since the transmission signals a and b are fed back to the gate terminals of the constant current transistors Q3 and Q4 in the transmission circuit, the level of the transmission signal becomes higher than a predetermined level. The current flowing through the constant current transistors Q3 and Q4 increases, the drain voltages of the differential transistors Q1 and Q2 decrease, and the level of the transmission signal decreases. On the other hand, when the levels of the transmission signals a and b become lower than expected levels, the currents flowing through the constant current transistors Q3 and Q4 decrease, and the drain voltages of the differential transistors Q1 and Q2 rise, and the transmission signal a ,
The level of b rises. By applying negative feedback to the input side of the transmission circuit in this manner, the drive current of the transmission line is made constant, and the amplitudes of the transmission signals a and b become stable. On the other hand, the driving transistors Q1 and Q2 in the transmitting circuit have higher resistance values than the transmission lines la and lb and the resistance values of the terminating resistors R1, R2 and R3. Therefore, the received signals c and d
Is swung around a level close to the power supply voltage Vcc as the center of the amplitude. When the reception signals c and d are input to the reception-side sense amplifier AMP1 activated by the selection signal φs', they are amplified and a signal OUT as shown in FIG. 5F is output.

FIG. 2 shows another embodiment of the present invention. This embodiment is different from the above-described embodiment in that
1 from the output terminals Ta and Tb to the constant current transistor Q
3, in which an amplifier AMP2 is inserted in the middle of the feedback path to the gate terminal of Q4. With this configuration, even when the amount of change in the drive current is small, the amount of change can be amplified and fed back, so that the level of the transmission signal can be kept constant.

FIG. 3 shows a receiving circuit A in the above embodiment.
One configuration example of MP1 is shown. The receiving circuit of this embodiment includes two stages of a differential amplifier stage SA1 composed of MOSFETs Q6-Q12 and a differential amplifier stage SA2 composed of MOSFETs Q13-Q18. That is, the output signal of the preceding differential amplifier stage SA1 is applied to the gate terminals of the differential transistors Q16 and Q17 constituting the subsequent differential amplifier stage SA2, and the preceding differential amplifier stage SA1 performs a level shift to perform the subsequent stage. Of the differential amplifier stage SA2. Thus, the input signals c and d at the bias level close to the power supply voltage Vcc can be efficiently amplified. In addition, since the level shift is performed in the preceding differential amplifier stage SA1, the same power supply voltage as the power supply voltage of the receiving-side LSI can be used as the pull-up voltage Vcc at the end of the transmission line. Although not particularly limited, it is preferable to set the constant of each element in the preceding differential amplifier stage SA1 so as to perform amplification along with level shift.

In the receiving circuit shown in FIG. 3, the constant current source of the preceding differential amplifier stage SA1 is connected to a parallel type MOSFET.
Q10 and Q11, and a differential M
By applying the drain voltages of the OSFETs Q8 and Q9, variations in output voltage are prevented. The differential amplifier stages SA1 and SA2 are provided with switch MOSFETs Q12 and Q18, respectively, and are activated by a selection signal φs'. Further, the load MOS of the subsequent differential amplifier stage SA2
Switch MOSFET Q15 in parallel with FET Q14
And the output voltage OUT can be fixed at a high level by turning on when not selected by the selection signal φs ′.

FIG. 4 shows a circuit example of the amplifier AMP2 on the feedback path in the embodiment shown in FIG. The configuration of the amplifier AMP2 is the same as that of the preceding-stage differential amplifier SA in the receiving circuit AMP1 shown in FIG.
1, which is configured to amplify and output a signal while performing a level shift.

As described above, in the above-described embodiment, the signal transmission circuit includes a pair of differential transistors connected to a common source and a constant current source connected to a common source terminal of the differential transistors. And with
The constant current source is constituted by a pair of transistors connected in parallel, and the output voltage of the circuit is fed back to the gate terminals of these constant current transistors directly or through an amplifier circuit, so that signal transmission is performed. As the drive current increases due to the variation in the conductance of the differential output transistor of the circuit, the current of the constant current source decreases, and when the drive current decreases due to the variation in the conductance of the differential output transistor, the current of the constant current source increases. Since the negative feedback is applied as described above, the amplitude of the transmission signal is made constant. Further, since the amplitude of the transmission signal is small, the power consumption can be kept low. Therefore, it is possible to use the present invention even when it is desired to perform high-speed coupling between LSIs having a large number of signal terminals.

Further, in the above embodiment, the switch MOSFETs are provided in the transmission circuit and the reception circuit, respectively, and activated by the selection signals φs and φs ′. To transmit signals in a bus system by connecting to a transmission line.

Although the invention made by the present inventor has been specifically described based on the embodiments, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the invention. Needless to say. For example, in the above embodiment, the case where the transmitting circuit and the receiving circuit are configured by CMOS circuits has been described, but they may be configured by NMOS circuits.

In the above description, the case where the invention made by the inventor is mainly applied to signal transmission between LSIs on a printed circuit board, which is the background of the application, has been described, but the invention is not limited to this. Instead, it can be used for signal transmission between LSIs using twisted pair lines.

[0020]

The effects obtained by typical ones of the inventions disclosed in the present application will be briefly described as follows. That is, in a system in which signal transmission between a CMOS and an LSI is performed by a current driving method using a differential signal, the amount of current fluctuation of the differential current signal to be output is reduced, and thereby the amplitude of the transmitted signal is made constant. Can be.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of a signal transmission circuit using a semiconductor integrated circuit according to the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the signal transmission circuit.

FIG. 3 is a circuit diagram illustrating a specific configuration example of a receiving circuit.

FIG. 4 is a circuit diagram showing a specific configuration example of an amplifier circuit on a feedback path.

FIG. 5 is a time chart showing operation timing of the signal transmission circuit.

FIG. 6 is a circuit diagram showing an example of a conventional current drive type signal transmission circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sending side LSI 2 Receiving side LSI la, lb Transmission line R1, R2, R3 Termination resistance AMP1 Receiving circuit Q1, Q2 Differential transistor Q3, Q4 Constant current transistor

──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Yoichi Sato 5-20-1, Josuihoncho, Kodaira-shi, Tokyo Inside Hitachi Ultra-SII Engineering Co., Ltd. (56) References JP 62 -47897 (JP, A) JP-A-2-177724 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04B 3/00-3/44 H04B 3/50-3/60 H03K 19/00

Claims (3)

(57) [Claims] 1. A differential current is transmitted to the outside from a pair of output terminals.
A semiconductor integrated circuit having an output circuit for outputting a signal.
In the output circuit, each of the drains is connected to the pair of output terminals.
Pair of differential transistors with their sources connected together
And the source and drain of each other are connected in common.
A common drain is connected to the common source of the differential transistor.
And a pair of parallel type constant current transistors.
Ri, the drain voltage of the pair of differential transistors, respectively
The feedback terminals are connected to the control terminals of the pair of constant current transistors.
A semiconductor integrated circuit characterized by being locked . 2. The semiconductor integrated circuit according to claim 1, wherein the feedback of the drain voltage to the control terminals of the pair of constant current transistors is performed via an amplifier circuit. 3. The semiconductor integrated circuit according to claim 1, wherein switch means is connected in series with said pair of constant current transistors.