PATENT DOCUMENT

Publication Number: US-9947255-B2
Application Number: US-201615274942-A
Country: US
Kind Code: B2

Title: Electronic device display with monitoring circuitry

Abstract:
An electronic device may have a flexible display such as an organic light-emitting diode display. A strain sensing resistor may be formed on a bent tail portion of the flexible display to gather strain measurements. Resistance measurement circuitry in a display driver integrated circuit may make resistance measurements on the strain sensing resistor and a temperature compensation resistor to measure strain. A crack detection line may be formed from an elongated pair of traces that are coupled at their ends to form a loop. The crack detection line may run along a peripheral edge of the flexible display. Crack detection circuitry may monitor the resistance of the crack detection line to detect cracks. The crack detection circuitry may include switches that adjust the length of the crack detection line and thereby allow resistances to be measured for different segments of the line.

Claims:
What is claimed is: 
     
       1. A display system, comprising:
 a flexible display having a bent portion; 
 a strain-sensitive resistor on the bent portion that has a first resistance; 
 a temperature calibration resistor on the flexible display that has a second resistance; and 
 a resistance measurement circuit that measures the strain in the bent portion of the flexible display by subtracting the second resistance from the first resistance. 
 
     
     
       2. The display system defined in  claim 1  further comprising a display driver integrated circuit that supplies data to pixels in the flexible display, wherein the resistance measurement circuit forms part of the display driver integrated circuit. 
     
     
       3. The display system defined in  claim 2  wherein the flexible display comprises an organic light-emitting diode display having an array of pixels that receive the data from data lines in the flexible display, wherein the flexible display has an active area that includes the array of pixels and has a tail portion that includes the bent portion, wherein the display system further comprises a flexible printed circuit that is coupled to the tail portion, and wherein the display driver integrated circuit is mounted to the flexible printed circuit. 
     
     
       4. The display system defined in  claim 3  further comprising test pads on the flexible printed circuit that are configured to couple to probes from a test system. 
     
     
       5. A display system, comprising:
 a flexible display having an array of pixels and having a bent tail portion; 
 first and second strain sensing resistors on opposing edges of the bent tail portion; and 
 a resistance measurement circuit that is configured to measure strain in the bent tail portion by measuring resistances for the first and second strain sensing resistors. 
 
     
     
       6. The display system defined in  claim 5  further comprising:
 at least one temperature compensation resistor in the bent tail portion, wherein the resistance measurement circuit is configured to measure the strain in the bent tail portion based at least partly on a temperature compensation resistance measurement from the temperature compensation resistor. 
 
     
     
       7. The display system defined in  claim 6  wherein the bent tail portion bends about a bend axis and wherein the temperature compensation resistor comprises a meandering metal trace with a plurality of elongated lines that extend parallel to the bend axis. 
     
     
       8. The display system defined in  claim 7  wherein the first and second strain sensing resistors each have a respective meandering metal trace with a plurality of elongated lines that extend perpendicular to the bend axis. 
     
     
       9. The display system defined in  claim 5  further comprising a display driver integrated circuit that supplies data to the array of pixels through data lines that extend across the bent tail portion and wherein the resistance measurement circuit forms part of the display driver integrated circuit. 
     
     
       10. The display system defined in  claim 9  further comprising:
 first and second temperature compensation resistors in the bent tail portion; and 
 a flexible printed circuit that is coupled to the bent tail portion, wherein the display driver integrated circuit is mounted to the flexible printed circuit and wherein the flexible display comprises an organic light-emitting diode display. 
 
     
     
       11. The display system defined in  claim 5  wherein the resistance measurement circuit comprises:
 a first voltage sensor that measures a voltage across the first strain sensing resistor; and 
 a second voltage sensor that measures a voltage across the second strain sensing resistor. 
 
     
     
       12. The display system defined in  claim 5  wherein the resistance measurement circuit comprises:
 a current source that applies a current through the temperature compensation resistor; and 
 a voltage sensor that measures a voltage across the temperature compensation resistor. 
 
     
     
       13. The display system defined in  claim 12  wherein the flexible display comprises at least one pad and a positive power supply line, wherein the power supply line is coupled to the pad, and wherein the current source is coupled to the pad. 
     
     
       14. The display system defined in  claim 5  further comprising:
 first and second temperature compensation resistors in the bent tail portion, wherein the resistance measurement circuit is configured to measure the strain in the bent tail portion based at least partly on a resistance measurements from the first and second temperature compensation resistors. 
 
     
     
       15. A display system, comprising:
 a flexible display having an array of pixels and having a bent tail portion; 
 a strain sensing resistor on the bent tail portion; 
 a temperature compensation resistor on the flexible display; and 
 a resistance measurement circuit that is configured to measure strain in the bent tail portion by measuring resistances for the strain sensing resistor and temperature compensation resistor. 
 
     
     
       16. The display system defined in  claim 15  wherein the bent tail portion bends about a bend axis and wherein the strain sensing resistor has a meandering metal trace with a plurality of elongated lines that extend perpendicular to the bend axis. 
     
     
       17. The display system defined in  claim 16  wherein the temperature compensation resistor comprises a meandering metal trace with a plurality of elongated lines that extend parallel to the bend axis. 
     
     
       18. The display system defined in  claim 17  further comprising a display driver integrated circuit that supplies data to the array of pixels through data lines that extend across the bent tail portion and wherein the resistance measurement circuit forms part of the display driver integrated circuit. 
     
     
       19. The display system defined in  claim 18  wherein the resistance measurement circuit comprises:
 a current source that applies a current through the strain sensing resistor and the temperature compensation resistor; 
 a first voltage sensor that measures a voltage across the strain sensing resistor; and 
 a second voltage sensor that measures a voltage across the temperature compensation resistor. 
 
     
     
       20. The display system defined in  claim 19  wherein the flexible display comprises at least one pad and a positive power supply line, wherein the power supply line is coupled to the pad, and wherein the current source is coupled to the pad.

Description:
This application claims the benefit of provisional patent application No. 62/377,483, filed Aug. 19, 2016, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates to electronic devices, and more particularly, to electronic devices with displays. 
     Electronic devices are often provided with displays. For example, cellular telephones, computers, and wristwatch devices may have displays for presenting images to a user. 
     Displays such as organic light-emitting diode displays may have flexible substrates. This allows portions of the display to be bent. The tail of a display may, for example, be bent when mounting the display in a compact device housing. 
     Challenges can arise in providing electronic devices with bent flexible displays. If care is not taken, mishandling during fabrication or stress due to drop events may damage the display. 
     SUMMARY 
     An electronic device may have a display mounted in a housing. The display may be a flexible display such as an organic light-emitting diode display. The display may have an array of pixels and a bent tail portion. The bent tail portion may bend about a bend axis. A display driver integrated circuit may supply data to columns of the pixels using data lines that extend across the bent tail portion. The display driver circuit may be coupled to the bent tail portion through a flexible printed circuit. A gate driver circuit may supply control signals to rows of the pixels using gate lines. 
     A strain sensing resistor may be formed on the bent tail portion of the flexible display to gather strain measurements. A temperature compensation resistor may be located adjacent to the strain sensing resistor. The strain sensing resistor and temperature compensation resistor may be formed from meandering metal traces. The meandering traces of the strain sensor may run perpendicular to the bend axis. The meandering traces of the temperature compensation resistor may run parallel to the bend axis. Resistance measurement circuitry in the display driver circuit may be used to measure the resistance of the strain sensing and temperature compensation resistors. Strain measurements may be obtained by subtracting the temperature compensation resistance from the strain sensing resistance. 
     A crack detection line may be formed form an elongated pair of traces that are coupled to form a loop. The crack detection line may run along the peripheral edge of the flexible display. Crack detection circuitry in the display driver integrated circuit may monitor the resistance of the crack detection line to detect cracks. If no cracks are present, crack detection line resistance will be low. In the presence of a crack, the resistance of the crack detection line will become elevated. 
     A shift register in the gate driver circuit may be provided with switches. The switches may be positioned at various positions along the length of the crack detection line and may be selectively closed to shorten the length of the signal path in the crack detection line by various amounts. By closing the switches in sequence while simultaneously measuring the resulting resistances of the crack detection line, the resistance of each of a plurality of segments of the crack detection line can be determined. This allows the positions of cracks within the crack detection line to be identified. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional side view of an illustrative electronic device in accordance with an embodiment. 
         FIG. 2  is a top view of an illustrative flexible display with strain gauge monitoring resistors in accordance with an embodiment. 
         FIG. 3  is a top view of a portion of a flexible display with strain gauge resistors in accordance with an embodiment. 
         FIG. 4  is a cross-sectional side view of a portion of a flexible display with metal traces in accordance with an embodiment. 
         FIGS. 5, 6, and 7  are circuit diagrams illustrative strain gauge circuits in accordance with an embodiment. 
         FIG. 8  is a diagram of an illustrative display with crack detection monitoring circuitry in accordance with an embodiment. 
         FIG. 9  is a diagram of an illustrative resistance measurement circuit that may be used to detect cracks in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An electronic device such as electronic device  10  of  FIG. 1  may be provided with a flexible display having monitoring circuitry. The monitoring circuitry may include strain gauge monitoring circuitry for monitoring strain in the bent portion of a display and may include peripheral crack monitoring circuitry. The strain gauge monitoring circuitry may include strain gauge resistors on a bent portion of the flexible display and a strain gauge circuit that monitors for resistance changes arising when stress is applied to the bent portion of the flexible display. The peripheral crack monitoring circuitry may have a peripheral crack detection line formed from a loop-shaped signal path with two parallel metal traces that runs along the periphery of the active area of the display. A crack detection circuit may use resistance monitoring circuitry to measure resistance changes in one or more segments of the crack detection line that are indicative of cracking in the line and in structures elsewhere in the display. 
     Electronic device  10  may be a computing device such as a laptop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wristwatch device, a pendant device, a headphone or earpiece device, a device embedded in eyeglasses or other equipment worn on a user&#39;s head, or other wearable or miniature device, a television, a computer display that does not contain an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, or other electronic equipment. In the illustrative configuration of  FIG. 1 , device  10  is a portable device such as a wristwatch. Other configurations may be used for device  10  if desired. The example of  FIG. 1  is merely illustrative. 
     Device  10  may have a display such as display  14 . Display  14  may be mounted on the front face of device  10  in housing  12 . Housing  12 , which may sometimes be referred to as an enclosure or case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials. Housing  12  may be formed using a unibody configuration in which some or all of housing  12  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). Housing  12  may have metal sidewalls or sidewalls formed from other materials, 
     Display  14  may be a touch screen display that incorporates a layer of conductive capacitive touch sensor electrodes or other touch sensor components (e.g., resistive touch sensor components, acoustic touch sensor components, force-based touch sensor components, light-based touch sensor components, etc.) or may be a display that is not touch-sensitive. Capacitive touch screen electrodes may be formed from an array of indium tin oxide pads or other transparent conductive structures. 
     Display  14  may include an array of display pixels formed from liquid crystal display (LCD) components, an array of electrophoretic display pixels, an array of plasma display pixels, an array of organic light-emitting diode display pixels, an array of electrowetting display pixels, or display pixels based on other display technologies. Configurations in which display  14  includes organic-light-emitting diode structures may sometimes be described herein as an example. 
     Display  14  may have a thin flexible display layer (sometimes referred to as a pixel array, display, or flexible display) such as flexible display  22 . Flexible display  22  may be formed from thin-film circuitry (e.g., thin-film transistors, thin-film organic light-emitting diodes, etc.) on a polymer substrate such as a flexible polyimide substrate. The thin-film circuitry may be encapsulated using one or more encapsulation layers (e.g., moisture barrier layers formed from organic and/or inorganic films). A transparent protective layer such as display cover layer  20  may overlap flexible display  22 . Cover layer  20  may be formed from transparent glass, clear polymer, sapphire or other crystalline material, ceramic, or other transparent protective layer. 
     Flexible display  22  may have an array of pixels  24  (pixel array  22 A) that form an active area for displaying images. Flexible display  22  may also have an inactive tail region such as tail  22 T that is free of pixels  24 . Images may be displayed for a user in pixel array  22 A by pixels  24 . Pixels  24  may be, for example, organic light-emitting diode pixels formed on a flexible polymer substrate (e.g., a polyimide substrate) and may be formed from thin-film circuitry on the substrate. 
     Metal traces such as metal traces  30  in flexible display  22  (e.g., data lines, control lines, etc.) may couple the circuitry of pixel array  22 A with display driver circuitry such as display driver circuitry in display driver integrated circuit  42 . In the example of  FIG. 1 , circuit  42  has been mounted on flexible printed circuit  32  and flexible printed circuit  32  has been coupled to flexible display  22 . With this arrangement, display driver integrated circuit  42  may be coupled to pixel array  22 A using metal traces  36  in flexible printed circuit  32  and metal traces  30  in flexible display  22 . Metal traces  36  in flexible printed circuit  32  may be soldered to contact pads on integrated circuit  42 . Metal traces  36  and metal traces  30  may also form mating pads that are coupled together at bonds  34 . Bonds  34  may be anisotropic conductive film bonds or other conductive connections. If desired, display driver circuitry such as display driver circuitry  42  may be coupled to pixel array  22 A with other arrangements. The use of flexible printed circuit  32  to couple circuit  42  to display  22  is merely illustrative. 
     Flexible display  22  may have a bent portion such as bent portion  26  that bends about bend axis  28 . The inclusion of bent portion  26  in display  22  may help display  22  fit within housing  12 . Display driver integrated circuit  42  may be coupled to system circuitry such as components  48  on one or more additional printed circuits such as printed circuit  46 . Components  48  may include storage and processing circuitry for controlling the operation of device  10 . Components  48  may be coupled to display driver circuit  42  and display  22  using connectors  45  (e.g., board-to-board connectors). 
     The bending of display  22  may create stress for traces  30 . If mishandled during assembly or if subjected to stress from a drop event, there is a risk that traces  30  could become damaged. To help characterize the stresses to which display  22  is subjected, display  22  may be provided with strain monitoring circuitry. The strain monitoring circuitry may include, for example, strain gauge resistors on bent portion  26  of display  22 . Crack monitoring circuitry may also be included in flexible display  22  (e.g., peripheral crack detection lines may run along one or more of the edges of pixel array  22 A or other portions of display  22 ). 
     The monitoring circuitry may include resistors (strain gauge resistors, peripheral lines that have associated resistances, etc.) and circuitry for evaluating the resistances associated with the resistors. The resistors may be incorporated into sensitive portions of display  22  (e.g., bent portion  26 , the edges of pixel array  22 A, etc.). 
     The circuitry for measuring and evaluating the resistances may be formed in display driver integrated circuit  42 , in other display driver circuitry (e.g., thin-film gate driver circuitry or gate driver integrated circuits on the edges of pixel array  22 A), or may be formed using components  48 . If desired, probe pads  38  may be formed on printed circuit  32  and/or on display  22  and these probe pads may be contacted by probes associated with test equipment. The test equipment may include resistance monitoring circuitry for monitoring resistance changes in strain gauge resistors and/or crack detection line resistance changes. Test equipment may also be coupled to the circuitry of display  22  using connector  45  or other coupling techniques (e.g., to monitor strain gauge resistors and/or crack detection resistors). During testing, test equipment may use electrically controlled actuators or other equipment to automatically apply stress to display  22  (e.g., to bend display  22  in region  26 ) and/or may otherwise manipulate display  22  while gathering data from monitoring structures in display  22 . With this type of testing arrangement, the tester may, for example, direct the actuators to apply known amounts of stress to display  22  in bent portion  26  or other region of display  22  while using the strain gauge resistors or other monitoring sensors to gather corresponding measurements (e.g., strain gauge measurements). Configurations in which resistance measurement circuitry and other monitoring circuitry is incorporated into display driver integrated circuit  42  (see, e.g., resistance measurement circuitry such as circuit  44  in display driver integrated circuit  42  of  FIG. 2 ) so that strain measurements and crack detection measurements may be made during fabrication or during normal use of device  10  by a user may sometimes be described herein as an example. 
       FIG. 2  is a top view of flexible display  22  in an unbent configuration. As shown in  FIG. 2 , pixel array  22 A may include rows and columns of pixels  24 . Gate driver circuitry (e.g., thin-film gate driver circuitry running along the left and/or right edges of pixel array  22 A) may supply horizontal control signals to each row of pixels  24 . These horizontal control signals, which may sometimes be referred to as gate line signals, may be used to control switching transistor in the pixel circuits associated with pixels  24  (e.g., for data loading, threshold voltage compensation operations, etc.). During data loading operations, data signals from display driver integrated circuit  42  may be supplied to columns of pixels  24  via respective data lines D. 
     Tail portion  22 T of flexible display  22  may bend around bend axis  28 . Strain gauge monitoring structures such as strain gauge resistors R 1  and R 2  and associated strain gauge circuitry in display driver integrated circuit  42  such as resistance measurement circuit  44  may be may be used in monitoring strain in tail portion  22 T and may form a strain gauge that can gather real time strain gauge measurements. 
     The strain gauge may include one or more strain-sensing (strain-sensitive) resistors such as resistors R 1 . Resistors R 1  may contain meandering metal traces that change resistance when bent. Resistors R 1  may be placed on tail  22 T in a location that overlaps bend axis  28 , so that resistance changes in resistors R 1  due to bending of display  22  in tail region  22 T may be maximized. 
     The strain gauge may also include one or more temperature compensation strain gauge resistors such as temperature compensation resistors R 2  (sometimes referred to as reference strain gauge resistors). Resistors R 2  may have meandering metal trace that match those of resistors R 1  so that both resistors R 1  and resistors R 2  experience the same responses to changes in operating temperature. Resistors R 2  may be placed on tail  22 T at locations that do not overlap bend axis  28  and may be oriented so that the traces in resistors R 2  run perpendicular to the traces in resistors R 1 . As a result, resistors R 1  will change resistance when tail  22 T is bent about axis  28 , but resistors R 2  will not change resistance when tail  22 T is bent about axis  28 . This allows resistance measurements made with a reference resistor R 2  to be subtracted from resistance measurements made with a strain-sensing resistor R 1  to remove temperature-dependent effects from the strain gauge resistance measurements (e.g., to remove noise due to temperature fluctuations). 
     In the example of  FIG. 2 , display  22  has been provided with two sets of strain gauge resistors. A left-hand set (formed from a first strain-sensing resistor R 1  overlapping bend axis  28  and a first associated temperature compensation resistor R 2 ) may be located along the left-hand edge of tail  22 T and may measure strain along the left side of tail  22 T. A right-hand set (formed from a second strain-sensing resistors R 1  overlapping bend axis  28  and a second associated temperature compensation resistor R 2 ) may be located along the right-hand edge of tail  22 T and may measure strain along the right side of tail  22 T. By including strain measurement circuitry along both the right and left edges of tail  22 T, strain data may be gathered that is sensitive to situations in which tail  22 T is bent unevenly along the left and right of tail  22 T (e.g., situations in which tail  22 T is twisted). 
     An illustrative trace layout for resistors R 1  and R 2  is shown in  FIG. 3 . As shown in  FIG. 3 , resistors R 1  and R 2  may have meandering paths formed from metal traces or other elongated conductive lines  50 . There may be any suitable number of parallel elongated lines in each resistor (e.g., more than 5 lines, 10-100 lines, 20-50 lines, more than 20 lines, fewer than 200 lines, fewer than 150 lines, etc.). The width of the metal traces forming lines  50  may be 2-10 microns, 4-8 microns, more than 3 microns, less than 20 microns, or other suitable width. The length of the sides of each resistor may be, for example, more than 0.05 mm, more than 0.1 mm, more than 0.5 mm, less than 1 mm, or less than 2 mm, etc. Resistors R 1  and R 2  may be rectangular or may have other shapes. Lines  50  in resistor R 1  may extend perpendicular to bend axis  28  (e.g., along dimension Y which is aligned with the longitudinal axis of tail  22 T) to maximize bending of lines  50  and therefore changes in the resistance of R 1  when tail  22 T is bent. Lines  50  in temperature compensation resistor R 2  may be parallel to lines  50  in resistor R 1  or may be arranged parallel to bend axis  28  as shown in  FIG. 3  to help reduce the sensitivity of resistor R 2  to changes in the bending of tail  22 T. 
     A cross-sectional side view of a portion of tail portion  22 T of display  22  is shown in  FIG. 4 . As shown in  FIG. 4 , tail portion  22 T may have a substrate such as substrate  52 . Substrate  52  may be formed from a flexible polymer such as a layer of polyimide. Metal traces  54  may be formed on substrate  52  and may be covered with planarization layer  56 . Metal traces  58  may be formed on planarization layer  56  and may be covered with planarization layer  60 . In pixel array  22 A, metal traces  54  and  58  may be used in forming thin-film transistor structures (e.g., source-drain terminals) and signal lines. In inactive tail portion  22 T of display  22 , metal traces  54  and  58  may form control signal lines and data lines D for carrying data from display driver integrated circuit  42  to pixels  24  in pixel array  22 A. Planarization layers  56  and  60  may be formed from polymers or other suitable materials. Polymer layer  62  may serve as a neutral stress plane adjustment layer that helps move the neutral stress plane of tail  22 T into alignment with traces  54  to minimize stress-induced cracking in traces  54  when tail  22 T is bent. With this type of configuration, traces  58  may (as an example) experience more stress than traces  54  when tail  22 T is bent. Accordingly, it may be desirable to form lines  50  for resistors R 1  and R 2  from the same metal layer that is used in forming lines  58  to maximize strain gauge sensitivity. Other layers of conductive material in display  22  may be patterned to form strain gauge resistors if desired. The use of the metal layer that is used in forming traces  58  to form strain gauge resistors is merely illustrative. 
     An illustrative strain gauge circuit is shown in  FIG. 5 . Resistance measurement circuitry  44  may be formed in display driver integrated circuit  42  (as an example) and may be coupled to resistors R 1  and R 2  on tail portion  22 T using metal traces  36  in flexible printed circuit  32  and traces  30  in display  22 . Bonds  34  between the pads formed from traces  36  and  30  and the portions of traces  30  and  36  that carry signals between resistors R 1  and R 2  and circuit  44  (shown collectively as paths  70 ) may have associated resistances Rc. For accurate strain gauge measurements, resistances Rc should be subtracted out of the strain gauge resistance measurements. Resistance changes in resistor R 1  that are due to changes in temperature and not changes in strain can be measured using temperature compensation resistor R 2  and can be subtracted from the measured resistance of resistor R 1 . 
     During operation, current source  64  may apply a known current I between terminals A and B. This causes current I to flow through resistors R 1  and R 2 , which are coupled in series between terminals A and B. Voltage sensor  66  may measure the resulting voltage V 1  between terminals C and D and voltage sensor  68  may measure the resulting voltage V 2  between terminals D and E. The resistance of resistor R 1  is equal to V 1 /I and the resistance of resistor R 2  is V 2 /I. Resistances R 1  and R 2  are therefore independent of the value of resistance Rc associated with bonds  34 . The resistance values for resistors R 1  and R 2  may be determined by resistance measurement circuitry  44  (e.g., using a processor circuit in circuitry  44 ) based on the known value of I and the measured values of V 1  and V 2 . The processor circuitry may also subtract R 2  from R 1  to isolate changes in resistance R 1  that are due to changes in the strain on resistor R 1  (e.g., bending of lines  50  about axis  26 , which can narrow lines  50  and thereby increase the resistance of lines  50 ). The measured changes in resistance R 1  due to strain may be used as strain gauge measurements that reflect the amount of strain experienced by tail portion  22 T in bend region  26 . 
     The availability of contact pads on tail portion  22 T may be limited due to the limited amount of area available on tail portion  22 T. It may therefore be desirable to coupled terminals A and B to pads that are coupled to other lines in display  22  such as lines  72 . Lines  72  may be, for example, positive power supply lines (e.g., lines that carry a positive power supply voltage Vdd to pixels  24  during normal operation of display  22 ). By piggybacking the measurement signals for measuring R 1  and R 2  through these contact pads, pad count can be minimized. 
       FIG. 6  shows how lines  72  may be omitted, if desired. 
     The number of pads used to measure resistances R 1  and R 2  may, if desired, be minimized using a resistance measurement arrangement of the type shown in  FIG. 7 . With this arrangement, resistance measurement circuitry  44  may measure the resistance RM 1  between terminals P 1  and P 2  and may measure the resistance RM 2  between terminals P 2  and P 3 . Resistor R 1  or R 2  may be coupled between terminals F and G (e.g., separate circuits of the type shown in  FIG. 7  may be used for measuring R 1  and for measurement R 2 ). After measuring RM 1  and RM 2 , resistance measurement circuitry  44  can compute the value of the resistance between terminals F and G (either R 1  or R 2  depending on which strain gauge resistor is coupled between terminals F and G) by subtracting RM 2  from RM 1 . This cancels out resistance Rc so that the measured strain gauge resistance values are independent of bond resistance. 
     In addition to measuring strain in display  22 , display  22  may incorporate crack detection circuitry. With one illustrative configuration, which is shown in  FIG. 8 , a crack detection line such as crack detection line  80  may run along some or all of the peripheral edge of display  22 . Crack detection line  80  may be formed from metal (e.g., part of one of the metal layers used in forming pixels  24  such a gate metal layer, source-drain metal layer, anode metal layer, cathode metal layer, etc.). Crack detection line  80  may also be formed from semiconductor (e.g., polysilicon or semiconducting oxide) or other conductive material. Illustrative configurations in which crack detection line  80  is formed from metal traces may sometimes be described herein as an example. 
     Crack detection line  80  may have a loop shape formed from outgoing line  80 - 1 , end connection path  80 - 2 , and return line  80 - 3  (i.e., a metal trace that is parallel to the metal trace forming path  80 - 2 ). This allows line  80  to serve as a crack detection resistor. In the absence of damage to display  22 , line  80  will be free of cracks and will be characterized by a low resistance. In the event that display  22  is subjected to stress that forms cracks in pixels  24  or other display circuity, crack detection line  80 , which is subjected to the same stress, will also develop cracks. The presence of cracks in crack detection line  80  will raise the resistance of line  80 . The change in the resistance of line  80  can detected by crack detection circuitry  44  in display driver circuit  42  (or external crack detection circuitry in a tester, etc.). The crack detection circuitry can then report this result to circuit components  48  (e.g., control circuitry in device  10 ), may report this result to external equipment, or may present warnings on display  22  (as examples). 
     If desired, the crack detection circuitry for display  22  may measure the resistance of individual segments SG of line  80  such as segments SG 1 , SG 2 , . . . SGN. As shown in  FIG. 8 , the display driver circuitry of display  22  may include gate driver circuitry  90 . Gate driver circuitry  90  may receive control signals (e.g., clock signals, start and stop pulses, etc.) from display driver circuit  42  via path  92 . Gate driver circuitry  90  may contain a shift register formed from a chain of register circuits  84 . Register circuits  84  may each supply horizontal control signals (e.g., scan signals, emission enable signals, etc.) to a corresponding row of pixels  24  (e.g., signals on illustrative gate lines G). During operation, circuit  42  initiates propagation of a control pulse through the shift register. As the control pulse propagates through the shift register, each gate line G (or other set of control signals) is activated in sequence, allowing successive rows of pixels  24  to be loaded with data from data lines D. 
     Gate driver circuitry  90  (e.g., some of register circuits  84 ) may be provided with switches SW 1 , SW 2 , . . . SWN, each of which selectively creates a short between lines (parallel metal traces)  80 - 1  and  80 - 3  at a different respective location along the length of line  80 . As the control pulse propagates through the shift register, each of switches SW 1 , SW 2 , . . . SWN is activated in sequence. As each switch is closed, resistance measurement circuitry  44  may measure the resistance of line  80 . When switch SW 1  is closed, line  80  is shorted at switch SW 1  and circuit  44  measures the resistance of segment SG 1  of line  80 . When switch SW 2  is closed, line  80  is shorted at switch SW 2  and circuit  44  measures the resistance of segments SG 1  and SG 2  together. This process continues until all switches have been closed and circuit  44  measures the resistance of all segments of line  80  (i.e., the entire length of line  80  from circuit  44  to connection path  80 - 2 . Using these resistance measurements, the resistance of each individual segment can be determined by resistance measurement circuit  44 . These resistance measurements can then be processed by the resistance measurement circuitry to determine whether the resistance of any segment is sufficiently high to reveal the presence of a crack. 
     Any suitable technique may be used by measurement circuitry  44  to measure the resistance of line  80 . For example, resistance measurement circuitry  44  may measure the resistance of line  80  by applying a known voltage to a capacitor of known capacitance C and discharging that capacitor through line  80  while incrementing a counter or otherwise timing the decay time (RC time) associated with discharging the capacitor. The RC time can then be used to extract a measured resistance value R. 
     Consider, as an example, a resistance measurement circuit such as illustrative resistance measurement circuitry  44  of  FIG. 9 . As shown in  FIG. 9 , resistance measurement circuitry  44  of display driver integrated circuit  42  may be coupled to crack detection line (resistor)  80  in display panel  22  (see, e.g., line  80  of  FIG. 8 ). Resistance measurement circuitry  44  may make measurements of the resistance of line  80  while switches  84  ( FIG. 8 ) are opened and closed so that segments of line  80  can be monitored for the presence of cracks. 
     Resistance measurement circuitry  44  may have an integrator such as integrator  100 . Integrator  100  may have a capacitor such as capacitor  104  and an operational amplifier such as operational amplifier  106 . The input of integrator  100  is coupled to line  80  and can be used to receive current that passes through reference resistor Rref or line  80  (of unknown resistance R) from reference voltage source Vref. 
     Clock  116  may supply clock signals to control logic  112  and counter  114 . The clock signals may be used to increment a count value maintained by counter  114 . When it is desired to perform a resistance measurement with integrator  100 , control logic  112  may assert a control signal on line  118  that closes switch  102 . Switch  102 , which may sometimes be referred to as an integrator reset switch, is coupled across capacitor  104  and discharges capacitor  104  when closed. While discharging capacitor  104  to reset integrator  108 , control logic  112  may also clear counter  114 . 
     When making resistance measurements, control logic  112  may place resistance selection switch  108  in either a first state in which voltage Vref is coupled to integrator  100  via resistor Rref or a second state in which voltage Vref is coupled to integrator  100  via resistor (resistance) R. In the first state, a current equal to Vref/Rref flows into integrator  100 . In the second state, a current equal to Vref/R flows into integrator  100 , where R is the resistance of the currently selected segment SG of line  80  that is being measured. 
     During integration operations, switch  102  is placed in its open state and the voltage on capacitor  104  rises in proportion to the current flowing into integrator  100 . The output of amplifier  106 , which serves as the output of integrator  100 , may be supplied to a first input of comparator  110 . A second input of comparator  110  may be provided with reference voltage V 0 . Comparator  110  may compare the signals on its first and second inputs and may produce corresponding output signals at its output. 
     When the output from integrator  100  exceeds V 0 , the output of comparator  110  will change state (i.e., the output of comparator  110  will toggle). The change in state of the output of comparator  110  may be detected by control logic  112 . In response to detection of the change of state of the comparator output, control logic  112  can obtain the current count value of counter  114 . This count value is proportional to the magnitude of the current being integrated by integrator  100 . The amount of time taken to charge the integrator output to V 0  (the count value of counter  114 ) can be measured by control logic  112  in both the first state of resistor selection switch  108  (in which current Vref/Rref flows into integrator) and in the second state of resistor selection switch  108  (in which current Vref/R flows into integrator  100 ). Control logic  112  may then obtain the unknown value of resistance R from the count value obtained when switch  108  is in the first state and the count value obtained when switch  108  is in the second state. 
     Strain resistor measurements (e.g., strain data from strain sensor resistor R 1 ) and/or crack detection resistor measurements (e.g., crack detection data such as measured resistance R from line  80 ) may be gathered during testing and analyzed to determine whether design changes should be made. Strain and crack detection measurements may be gathered by a tester having test probes that are coupled to pads in display  22  or pads in flexible printed circuit  32  and/or may be gathered by a tester that obtains digital measurements from resistance measurement circuitry  44  over a digital data communications path. Strain and crack measurements may be gathered during manufacturing to detect damaged parts so that they can be repaired or replaced. If desired, strain and crack data can be gathered during normal operation of device  10 . Any suitable action may be taken in response to abnormal strain or crack data. For example, an alert may be presented on display  22  that informs a user that display  22  has been subjected to potentially damaging amounts of stress and should be serviced, historical data can be gathered (e.g., to detect whether device  10  has been dropped), and/or other actions may be taken in response to gathered strain and crack detection information. These alert techniques may also be used during testing and manufacturing. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20160923
Publication Date: 20180417
Grant Date: 20180417
Priority Date: 20160819
Inventors: ZHANG, ZHEN
AHMED, IZHAR
KIM, HOON SIK
Assignee: APPLE INC
CPC Classifications: [{"code": "G09G3/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "G11C19/28", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2300/0426", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2300/0426", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2330/12", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01M5/0033", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01M5/0033", "inventive": true, "first": false, "tree": "[]"}, {"code": "G11C19/28", "inventive": false, "first": false, "tree": "[]"}, {"code": "G11C7/04", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2330/12", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01M5/0083", "inventive": true, "first": false, "tree": "[]"}, {"code": "G11C7/04", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01M5/0083", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2380/02", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/006", "inventive": true, "first": true, "tree": "[]"}, {"code": "Y02P70/50", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01L1/2281", "inventive": true, "first": true, "tree": "[]"}, {"code": "G09G3/035", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/006", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/035", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10K77/111", "inventive": true, "first": false, "tree": "[]"}, {"code": "H10K59/131", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01L1/2281", "inventive": true, "first": true, "tree": "[]"}, {"code": "Y02E10/549", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 61192000