DIGITAL INTERFACE WITH LOW POWER CONSUMPTION

Abstract: The invention proposes an interface circuit having a very low power consumption and generating very low interference noise in the sensitive band of radio chips. It is advantageously used to interface a microprocessor with a baseband radio processor in a telecommunication device, for example in DECT or GSM phones. The interface of the invention is current-driven. It comprises a current driver for transmitting a current in a transmission line depending on the data to be transferred. It also comprises a current receiver. The current receiver has an input node and an output node interconnected via a current mirror circuit, so that the voltage on said input node is near to the ground voltage and the voltage on said output node is changing depending on the transferred data.

Description: Digital interface with low power consumption The invention relates to a digital input / output interface, and a telecommunication device using such an interface. The invention is advantageously used for digital data transfers. For example, it may be used in digital mobile communication devices to transfer digital data from a baseband processor to an analogue radio transceiver. Such a digital input / output interface is known from US patent 5,811,984. The digital input / output interface described in this document comprises a current driver and a current receiver connected by a transmission line. The current driver is configured to pull a current from the current receiver when a digital signal is applied to the current driver. The current receiver includes a current conversion element connected to the transmission line at an input node through an input impedance Z, n , and an active termination element configured to adjust this input impedance Z m - to match the characteristic impedance of the transmission line. The drawback of this digital interface is that it is power consuming. Therefore, it would generate high interference noise in the sensitive band of radio frequency chips. This would make it impossible to transfer digital data to such chips while they are operating. It is an object of the present invention to propose a digital input / output interface having a very low power consumption and hence generating low interfering noise in the sensitive band of radio frequency chips. Another object of the invention is to propose an efficient input / output interface for use in digital mobile communication devices. This is achieved with the input / output interface as claimed in claim 1. According to the invention, the voltage at the input node of the current receiver is small and remains approximately constant. As a consequence, if the interface is operated with low currents, the power consumption is very low. In the interface of the invention, the impedance of the line is not compensated anymore. But this is not critical at the data rates used in the contemplated applications under consideration (a few Mega-samples per second). In an advantageous embodiment the proposed interface has a differential structure. In such a differential structure the same current is generated in one of the two branches depending on the data to be transferred. This leads to lower interference noise. In another advantageous embodiment, the current receiver comprises a current source and a current adding element arranged to bias said current mirror circuit. This allows to increase the speed of the interface. This is especially useful when the interface has a differential structure, because in that case the branch of the current receiver that is switched off loses its bias. The invention is explained in more detail below with the aid of illustrative embodiments, with reference to the drawings where: Fig. 1 is a diagram showing the principle of operation of the interface of the invention when the interface has a single structure, Fig. 2 is a diagram showing the principle of operation of the interface of the invention, when the interface has a symmetrical structure, Fig. 3 is a diagram showing a preferred embodiment of the invention, when the interface has a differential structure. Fig. 1 gives a schematic representation of the principle of the interface of the invention. This interface comprises a current driver 1 and a current receiver 2 connected by a transmission line 3. The current driver 1 comprises a data input D-IN, a current source SI for delivering a constant current II, a data modulator MOD for modulating the input data D-IN on constant current II and for generating a modulated current IM in the transmission line 3. The current receiver 2 comprises a current to voltage comparator COMP-S for comparing the incoming current IM with a reference current II ' generated by a constant current source S 1 ' and which is approximately equal to current II . Fig. 2 gives another schematic representation of the principle of the interface of the invention, when this interface has a symmetrical structure. In this case the current driver 1 has two branches Bl and B2. Branch Bl is directly connected to the data input D-IN and comprises a data modulator MODI . Branch B2 is connected to the data input via an inverter INV. It comprises a second data modulator MOD2. The data Dl present at the input of the data modulator MODI are inverted compared to the data D2 present at the input of the current modulator MOD2. Data Dl and D2 are modulated on a constant current 12, which is supplied by current sources S2 and S3. The transmission line 3 comprises two wires 3-1 and 3-2. The output of data modulator MODI is connected to wire 3-2. And the output of data modulator MOD2 is connected to wire 3-1. On the receiver side, both incoming currents are transmitted to a current to voltage comparator COMP-D. The result of the comparison gives the received data. Fig. 3 gives an example of a preferred embodiment of the invention, when the interface has a differential structure. In this embodiment the current source S2 comprises one current source IREFT and a current mirror Ml. The current mirror Ml comprises two P- MOS transistors MP1 and MP2. The data modulators MODI and MOD2 comprise each one P-MOS transistor MP3 and MP4, respectively. The drains of these transistors are connected to the source of transistor MP2. The source of transistor MP3 is connected to wire 3-1 via a resistor R3, and the source of transistor MP4 is connected to wire 3-2 via a resistor R4. The data input D-IN is connected to the gate of transistor MP3 via 3 data inverters INN1, IΝN2 and IΝN3, and a resistor R5. It is also connected to the gate of transistor MP4 via two inverters IΝN1 and IΝN4, and a resistor R6. Inverters IΝN1, IΝN2, IΝN3 and IΝN4 also have an amplification function. The gates of transistors MP3 and MP4 are also interconnected with one another via a capacitor CI . Resistors R5 and R6 and capacitor CI have a filtering function. Resistors R3 and R4 together with the parasitic capacitance of the wires have the function of a low-pass filter. The cut-off frequency of this low-pass filter may be reduced by having an additional capacitor across resistors R3 and R4 external to the integrated circuit. The current receiver has two input nodes IΝ1 and IΝ2, and one output node OUT. Input node INI is connected to wire 3-1. Input node IN2 is connected to wire 3-2. Input nodes INI and IN2 are connected to output node OUT via a current mirror circuit MIR. Current mirror circuit MIR comprises 3 current mirrors M2, M3 and M4. Current mirror M2 comprises two N-MOS transistors MN1 and MN2. Current mirror M3 comprises two N-MOS transistors MN3 and MN4. And current mirror M3 comprises two P- MOS transistors MP5 and MP6. The gates of transistors MN1 and MN2 are connected to input node INI. The gates of transistors MN3 and MN4 are connected to input node IN2. The drains of transistors MN4 and MP6 are connected to output node OUT. The drain of transistor MN3 is connected MISSING AT THE TIME OF PUBLICATION In the above description, metal-oxide semiconductors are used. This is not restrictive. For example, it is also possible to use bipolar semiconductors.

Claims: CLAIMS: 1. A digital input / output interface including a current driver and a current receiver connected by a transmission line, the current driver being configured to receive a digital signal and to generate a current in the transmission line depending on said digital signal, and the current receiver comprising an input node and an output node, said input node being connected to said output node through a current mirror circuit so that the voltage at said input node is small and remains approximately constant, and the voltage at said output node depends on the current generated in said transmission line. 2. A digital input / output interface as claimed in claim 1, wherein the voltage at said input node is close to a threshold voltage of a transistor. 3. A digital input / output interface as claimed in claim 1, wherein said current mirror circuit comprises at least a current mirror for copying the current generated in said transmission line in one transistor for converting said current into a voltage indicative of the value of said digital signal. 4. A digital input / output interface as claimed in claim 1, wherein said current driver and said current receiver are differential circuits and said transmission line comprises two wires, said current driver being configured to generate a current in only one of said wires at a time. 5. A digital input / output interface as claimed in claim 1 , wherein the current receiver comprises a current source and a current adding element arranged to bias said current mirror circuit. 6. A telecommunication device comprising a digital input / output interface as claimed in one of claims 1 to 5.

Inventors: BECKER ROLF F P, WOLFF THOMAS, BECKER, ROLF, F., P., WOLFF, THOMAS
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2001
Main classification: H04L27
IPC codes: H03K17, H03K19, H04L25