This invention relates to the detection of the connection of a display device. It more particularly concerns the detection of the connection of a television to a mobile device which has a composite video output.

Today's mobile devices such as mobile telephones offer numerous multimedia capabilities to users, including access to audiovisual programs.

For easier viewing, it is possible to connect an external display device such as a screen or television set equipped with a composite video input. Such equipment usually has a much larger display area than what is generally available on the screens of mobile devices.

Mobile devices generally provide a composite video output for this purpose.

When a cable is connected between an external display device and a mobile device, the mobile device must be configured to direct the video stream, initially intended for display on its own screen, to its composite video output.

A first known approach to configuring the mobile device consists in activating or deactivating the redirection to the composite video output on user request. For example, the user navigates through the menus of a graphical interface of the mobile device until he or she accesses a redirection activation/deactivation command. This first approach therefore requires user intervention.

Another known approach is based on detecting a change in the resistance value and possibly the capacitance value at the composite video output. When a standard cable is connected to the input of a display device, the standards currently in effect specify the resistance value and possibly the corresponding capacitance value. One can therefore measure these values periodically and when they correspond to the standardized values, the video signals can be redirected to the composite video output. These periodic measurements, generally controlled by a central processor or a video coprocessor of the mobile device, result in significant power consumption which ultimately impacts the autonomy of the mobile equipment.

However, it is highly advantageous to be able to detect the connection of an audiovisual device automatically, including when the mobile device is in an inactive or "idle" state. There is therefore a need to improve the situation.

A first aspect of the invention proposes a method for detecting the coupling of a display device to a video output of a first device. The first device comprises a processor adapted to manage video streams, and a detection unit separate from the processor. The method comprises the following steps, implemented in the detection unit:

IM detecting a modification of at least one measurable electrical quantity at the video output after the display device is coupled to the video output;

121 notifying the processor of the coupling of the display device to the video output;

during the step IM, the coupling of the display device to the video output resulting in a modification of the measurable capacitance value at the video output.

The process according to the first aspect enables the detection of the coupling of an external device, even if the processor of the first device is in an idle or standby state. This event is detected by the detection unit, which implements the necessary steps and thus relieves the processor of this responsibility. The processor can reduce its power consumption considerably because it is no longer required to perform this periodic detection.

In addition, the use of a detection unit designed to accomplish this task allows optimizing the power consumption of this feature. The process is then estimated to require less than 2 μΑ to be implemented.

To meet other requirements, the detection unit can, during the step 121, notify of the coupling of the display device to the video output by sending an interrupt on an interrupt line connecting the detection unit to the processor. This notification method is useful in particular when the processor is in an idle or standby state. During the step IM, the coupling of the display device to the video output can result in a modification of the measurable resistance value at the video output.

The modification of the measurable resistance value at the video output is detected for example by:

• periodically generating a first electrical pulse on the video output;

• comparing, on the rising edge of the first electrical pulse, the value of the voltage at the video output to a first threshold, with the first threshold being chosen such that the value of the voltage is:

· greater than the first threshold if the display device is not coupled to the video output;

• less than the first threshold if the display device is coupled to the video output.

The modification of the measurable capacitance value at the video output is detected for example by:

• generating a second electrical pulse on the video output;

• comparing, on the falling edge of the second electrical pulse, the value of the voltage at the video output to a second threshold, the second threshold being chosen such that it is non-zero and is less than the voltage produced by a capacitance element being the cause of the modification in the measurable capacitance value at the video output.

A second aspect of the invention proposes a device comprising a video output for coupling a display device, a processor adapted to manage video streams, and a unit for detecting the coupling of a display device to the video output and comprising means for implementing a detection method according to the first aspect of the invention, the coupling of the display device to the video output resulting in a modification of the measurable capacitance value at the video output. The video output can be a composite video output.

A third aspect of the invention proposes a system comprising a device according to the second aspect, a display device able to be coupled to the video output of the device, and possibly a cable connecting the display device to the video output of the device.

Other features and advantages of the invention will be apparent from the following detailed description. This description is purely illustrative and is to be taken in connection with the accompanying drawings, in which:

figure 1 is a schematic diagram of a mobile device comprising a unit for detecting the connection of an external display device, according to one embodiment;

figures 2A and 2B are timing charts representing the main signals of the detection unit during operation, according to one embodiment, particularly as a function of the connection state of the external display device and the type of cable used;

figure 3 is general block diagram of the process for detecting the connection of an external display device, according to one embodiment.

In the following sections, we will consider an example, for illustrative purposes only, of a mobile device 10 comprising a composite video output 12 able to be coupled to the composite video input 14 of an external display device 16 via a cable 18, as represented in figure 1 . The term composite video is commonly designated by the acronym CVBS, which stands for "Chroma, Video, Blanking, and Sync". As a non-limiting example, the mobile device 10 can be a mobile telephone with a composite video output 12 which is a female connector known as an RCA ("Radio Corporation of America") connector. The display device 16 can be a video screen or a television having a female RCA connector as the composite video input. The cable 1 8 is, for example, a cable having an RCA male connector at each end. Depending on the display device 16 and the audiovisual standards in effect, the cable 18 may comprise a capacitance element 18a, for which the capacitance value C is typically between 47 and 2200 μΡ. At the composite video output 12, when the cable 18 connects the composite video output 12 to the composite video input 14:

• the resistance value R is substantially equal to 75 ohms;

• the capacitance value is substantially equal to the capacitance value C of the capacitance element 18a, if said capacitance element is present. The mobile device 10 comprises a video processor 20, adapted to process video streams and in particular to direct to the composite video output 12 a video signal Sv when the cable 18 is coupled to the composite video output 12.

The mobile device 10 comprises a detection unit 22 for detecting the connection of the display device 10 via the cable 18 to the composite video output 12. The detection unit 22 has an input 24 coupled to the composite video output 12, and an interrupt line 26 to the video processor 20. "Interrupt line" is understood to mean an active electrical line in which a change in level indicates a change in the state of the system. The detection unit 22 can generate an interrupt IT on the interrupt line 26 in the form of an electrical signal intended for the video processor 20. In particular, the video processor can be in an inactive or idle state, or even in a power-saving standby state, while remaining able to receive the interrupt IT for processing. A resistor 25, whose resistance value RE is for example chosen to be equal to 75 ohms, is coupled to the input 24.

In the present description, provided for illustrative purposes only, the architecture of the mobile device 10 is based on two chips, one for the video processor 20 and one for the detection unit 22, connected to each other via the interrupt line 26. The present embodiment easily applies to any other architecture of the mobile device 10, however. No limitation is placed on this aspect. For example, the present invention can be applied to a single-chip architecture in which the video processor 20 and the detection unit 22 are directly connected to each other.

The detection unit 22 comprises a logic circuit 27, a pulse generator 28, and a comparator 30. The output from the pulse generator 28 is coupled to the input 24 and therefore to the composite video output 12. The inputs of the comparator 30 are coupled to the input 24 while the outputs of the comparator 30 are coupled to the inputs of the logic circuit 27. The logic circuit 27 is adapted to send an interrupt IT on the interrupt line 26 when the display device 16 is coupled to the composite video output 12. The logic circuit 27 is designed to control the pulse generator 28, in particular by controlling the activation of said pulse generator. The logic circuit 27 can comprise a register TVD for storing the occurrence of the event indicating the detection of the presence of the cable 18, and possibly a register TVDRC for storing the occurrence of the event indicating the detection of the presence of the cable 18 if said cable additionally comprises the capacitance element 18a.

The pulse generator 28 is able to generate as output, at the command of the logic circuit 27, a first square pulse PUL1 of a given intensity Ip1 and of a given duration TDP. Typically, the intensity Ip1 can be chosen as equal to 8 mA with a duration TDP of 125 ms.

The pulse generator 28 is able to generate as output, at the command of the logic circuit 27, a second square pulse PUL2 of a given intensity Ip2 and of a given duration TDRC. Typically, the intensity Ip2 can be chosen to be equal to 8 mA with a duration TDRC of 256 ms. The duration TDRC is chosen as a function of the value C of the capacitance element 18a of the cable 18, in particular such that the duration TDRC is long enough to allow sufficient load on the capacitance element 18a for the effects, on the value VE, of the discharging of said capacitance element 18a to be identifiable.

The comparator 30 is adapted to compare the value VE of the voltage at the input 24 to a first threshold S1 and to a second threshold S2. The first threshold S1 is chosen such that when one of the first pulses PUL1 is produced:

• the value VE is greater than the first threshold S1 when the display device 16 is not coupled to the input 24;

• the value VE is less than the first threshold S1 when the display device

16 is coupled to the input 24.

Thus, if the following hypotheses are applied:

• the resistance value RE of the resistor 25 is equal to 75 ohms; and

• the resistance value R on the composite video output 12, resulting from connecting the cable 18 between said output and the composite video input 14 is equal to 75 ohms; and

· the intensity Ip1 of the pulses PUL1 is equal to 8 mA;

it is found that the value of VE is substantially equal to 600 mV when the cable 18 is absent, while the value of VE is substantially equal to 300 mV when the cable is present. The first threshold S1 is therefore chosen to be greater than 300 mV and less than 600 mV, typically 500mV. The second threshold S2 is chosen such that, after the end of one of the second pulses PUL2:

• the value VE is greater than the second threshold S2 when the display device 16 is coupled to the input 24 via the cable 18 comprising the capacitance element 18a;

• the value VE is less than the second threshold S2 when the display device 16 is coupled to the input 24 via the cable 18 not comprising the capacitance element 18a.

In particular, the second threshold S2 is chosen such that it is strictly greater than 0 and less than the voltage produced by the capacitance element 18a of the cable 18 after the discharge of said element following the end of one of the second pulses PUL2.

Using the hypotheses defined in the above example to calculate the first threshold S1 , with a capacitance element 18a whose value C is between 47 μΡ and 2200 μΡ, the second threshold S2 can be chosen to be equal to 100 mV.

Figures 2a and 2b show timing charts for the main signals of the detection unit 22 during operation, according to one embodiment, as a function of the connection state of the display device 16 to the mobile device 10 and as a function of the type of cable 18 connected. The x axis in the timing charts corresponds to the time since to. In figures 2a and 2b, A charts the voltage value VE of the input 24, B represents the binary value of the register TVD in the logic circuit 27, C shows the level of the interrupt line 26, and D represents the binary value of the register TVDRC in the logic circuit 27.

When the detection unit 22 is in operation, the logic circuit 27 controls the pulse generator 28 such that said generator sends the first pulses PUL1 spaced apart by a period PTT. The period PTT is for example chosen to be equal to a value of between 2 and 8 seconds.

At time to, corresponding to the leading edge of one of the first pulses

PUL1 , the value VE is greater than the first threshold S1 and than the second threshold S2. The level of the interrupt line 26 is zero, and the binary value of the register TVD and the register TVDRC is zero.

At time t1 the event Ev occurs corresponding to the connection of the cable 18 between the mobile device 10 and the display device 16. The time t1 occurs between two first pulses PUL1 , meaning during one of the periods in between.

At time t1 when the pulse generator 28 sends another first pulse PUL1 , the comparator 30 detects on the rising edge of said pulse PUL1 that the value VE is less than the threshold S1 and so informs the logic circuit 27. Said logic circuit then commands the pulse generator 28 to send the second pulse PUL2 immediately. The second pulse PUL2 ends at time t2.

Then at time t2', on the trailing edge of said second pulse PUL2 or at a short delay after time t2, the comparator 30 compares the value VE to the second threshold S2.

Figure 2a illustrates an example corresponding to the use of the cable 18 not comprising the capacitance element 18a. At time t2', the value VE is less than the second threshold S2, as no capacitance element is producing discharge current at that time. The level of the interrupt line 26 is then not zero so the interrupt IT is sent to the video processor 20. The binary value of the register TVD is set to 1 and the binary value of the register TVDRC remains 0.

Figure 2b illustrates an example corresponding to the use of the cable 18 comprising the capacitance element 18a. At time t2', the value VE is greater than the second threshold S2, as the capacitance element 18a is producing a discharge current at that time. The level of the interrupt line 26 is then not zero so the interrupt IT is sent to the video processor 20. The binary value of the register TVD and the value of the register TVDRC are set to 1 .

Figure 3 illustrates the main steps of a process for detecting the coupling of a display device to a video output of a first device, implemented according to one embodiment. The first device comprises a processor. The process according to one embodiment comprises, in a detection unit comprised in the first device:

• a first step 100 for detecting a modification of at least one measurable electrical quantity at the video output following the coupling of the display device to the video output;

• a second step 200 for notifying the processor of the coupling of the display device to the video output. During the second step, the detection unit can notify of the coupling of the display device to the video output, by sending an interrupt on an interrupt line connecting the detection unit to the processor.

When the coupling of the display device to the video output causes a modification of the measurable resistance value at the video output, this modification can be detected by:

• periodically generating a first electrical pulse PUL1 on the video output;

• comparing, on the rising edge of the first electrical pulse PUL1 , the value of a voltage VE at the video output to a first threshold S1 , the first threshold S1 being chosen such that the value of the voltage VE is:

• greater than the first threshold S1 if the display device is not coupled to the video output;

· less than the first threshold S1 if the display device is coupled to the video output.

When the coupling of the display device to the video output causes a modification of the measurable capacitance value at the video output, this modification can be detected by:

· generating a second electrical pulse PUL2 on the video output;

• comparing, on the falling edge of the second electrical pulse PUL2, the value of a voltage VE at the video output to a second threshold S2, the second threshold S2 being chosen such that it is non-zero and is less than the voltage produced by a capacitance element from which the modification in the measurable capacitance value at the video output originated.