Abstract:
An integrated circuit and method are provided. The integrated circuit comprises: a digital core configured to output a first voltage signal: and a first input/output cell: wherein the first input/output cell is configured to convert the first voltage signal to a first current signal and provide the first current signal to circuitry external to the integrated circuit.

Description:
FIELD 
       [0001]    This invention relates to the provision of output signals of a digital integrated circuit and in particular but not exclusively to the provision of those signals under low voltage conditions. 
       BACKGROUND 
       [0002]    Input and output signals from a digital core of an integrated circuit (IC) may be provided externally to the integrated circuit through input/output cells, for example complementary metal oxide semiconductor (CMOS) IO Cells. In some systems, the power supply to the integrated circuit may differ to that of the external circuitry and the CMOS IO cell operates to convert a voltage from the digital core to a voltage in line with the power supply of the external circuitry. 
         [0003]    ICs are often tested to check their functionality. As digital cores may be expected to operate under low supply voltage conditions, these conditions may be used to test the operation of the IC. In order for the output of the digital core under these test conditions to be analysed, the CMOS IO cells should also be able to handle the low voltage supply conditions. 
         [0004]    Embodiments of the present application aim to address this. 
       SUMMARY 
       [0005]    According to a first aspect, there is provided an integrated circuit comprising: a digital core configured to output a first voltage signal; and a first input/output cell; wherein the first input/output cell is configured to convert the first voltage signal to a first current signal and provide the first current signal to circuitry external to the integrated circuit. 
         [0006]    The integrated circuit may further comprise; a second input/output cell configured to convert the first voltage signal to a second voltage signal and provide the second voltage signal to circuitry external to the integrated circuit. The first voltage may correspond to a supply voltage of the integrated circuit and the second voltage corresponds to the supply voltage of the external circuitry. The first current signal may be provided to a first circuit external to the integrated circuit, the first circuit being configured to convert the first current into a third voltage. 
         [0007]    The integrated circuit may be in a first voltage domain and the circuitry external to the integrated circuit is in a second voltage domain. The first input/output cell may be configured to operate in low voltage conditions. The first circuit may comprise a transimpedance amplifier. 
         [0008]    According to a second aspect, there is provided a method comprising: providing a first output voltage signal from a digital core to a first input/output cell; converting the first voltage signal to a first current signal by the first input/output cell; and providing the first current signal to circuitry external to the integrated circuit. 
         [0009]    The method may further comprise: further providing the first voltage output signal to is a second input/output cell; converting the first voltage signal to a second voltage signal by the second input/output cell; and providing the second voltage signal to circuitry external to the integrated circuit. The first voltage may correspond to a supply voltage of the integrated circuit and the second voltage corresponds to the supply voltage of the external circuitry. The method may further comprise: providing the first current signal to a first circuit external to the integrated circuit, the first circuit being configured to convert the first current into a third voltage. 
         [0010]    The integrated circuit may be in a first voltage domain and the circuitry external to the integrated circuit is in a second voltage domain. The first input/output cell may be configured to operate in low voltage conditions. The first circuit may comprise a transimpedance amplifier. 
         [0011]    According to a third aspect, there is provided a system comprising: the integrated circuit of the first aspect; and a first circuit external to the integrated circuit configured to convert the first current into a voltage; wherein the integrated circuit is in a first voltage domain and the circuitry external to the integrated circuit is in a second voltage domain. 
         [0012]    According to a fourth aspect, there is provided an integrated circuit comprising; a digital core; a first input/output cell configured to receive a first current from circuitry external to the integrated circuit, convert the first current to a first voltage to be provided to the digital core. 
         [0013]    The integrated circuit may further comprise: a second input/output cell configured to convert receive a second voltage signal from circuitry external to the integrated circuit and convert the second voltage signal to a third voltage signal to be provided to the digital core. 
         [0014]    According to a fifth aspect, there is provided a method comprising: receiving by a first input/output cell, a first current signal from circuitry external to an integrated circuit; converting the first current signal to a first voltage signal by the first input/output cell; and providing the first voltage signal to a digital core. 
         [0015]    The method may comprise: receiving a second voltage signal at second input/output cell from circuitry external to an integrated circuit; converting the second voltage signal to a third voltage signal by the second input/output cell; and providing the third voltage signal to the digital core. 
     
    
     
       FIGURES 
         [0016]    Embodiments will be described, by way of example only, with reference to the drawings, in which: 
           [0017]      FIG. 1  is an example of a system including an integrated circuit with a digital core; and 
           [0018]      FIG. 2  is an example of a system comprising an integrated circuit according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  shows an example of a system comprising an integrated circuit having a digital core. The system  100  comprises an integrated circuit  110  having a digital core  111  and a CMOS IO cell  112 . The integrated circuit is provided with an IC supply voltage  113  which is coupled to both the digital core  111  and the IO cell  112 . It will be appreciated that in some embodiments, the IC supply voltage  113  may be converted to another voltage before being provided to the IO cell  112 , however it will be appreciated that the source voltage for the integrated circuit is IC supply voltage  113 . 
         [0020]    The IC  110  is coupled to further circuitry  120  via a voltage output  130  of the IO cell  112 . The further circuitry is external to the IC  110 . The external circuitry  120  may also be CMOS circuitry and may be related to a testing or other functionality of the system  100 . The external circuitry  120  may be provided with an external supply voltage  121 , where is external means external to the IC. 
         [0021]    In operation, the digital core may produce an output voltage  131  and provide this output voltage  131  to the IO cell  112 . In one example the output voltage  131  is a rail-to-rail voltage signal with a maximum amplitude of the IC supply voltage  113 . The IO cell  112  receives the voltage signal  131  from the digital core  111  and converts it to be compatible with the external voltage supply  121 . The IC  113  and the external circuitry  120  may be in two voltage domains corresponding respectively to the IC supply voltage  113  and the external supply voltage  121 . The IO cell  112  may convert a first voltage signal  131  in a first voltage domain into a second voltage signal  130  compatible with the second voltage domain. The second voltage signal  130  is then provided to the external circuitry. 
         [0022]    In some systems, the IO cell  112  may convert the first voltage  131  to the second voltage  130  by level shifting the voltage to be compatible with the external supply voltage  121 . 
         [0023]    In some applications, it may be a requirement that the digital core operates at low voltage conditions. Normal operating conditions can be considered to be a typical voltage +/−10%. The typical voltage may be the ideal voltage under which the digital core was designed to operate, it will be appreciated that this may differ dependent on the application or use of the digital core. Anything less than typical voltage less 10% may be considered to be low voltage conditions. For example, for a typical supply voltage of 1.2V, normal operating conditions can be considered to be between 1.08V to 1.32V. This may be the case where the digital core is designed with a 10% tolerance for supply voltage variation. Low voltage conditions may include conditions where the IC supply voltage  113  is provided at less than 10% of the operational or typical supply voltage. In this specific above example, low voltage conditions will occur if the digital core receives a voltage less than 1.08V. For example the IC supply voltage may drop to 0.9V or even less depending on what the digital core is expected to handle. 
         [0024]    As can be seen from  FIG. 1 , the IO cell  112  is supplied from the IC supply voltage  113 . When the digital core  111  is put under low supply condition, the supply to the IO cell  112  is also lowered during the testing. 
         [0025]    Typically, suppliers of IO cells may characterize their products within 10% of the typical supply voltage and are not designed for low supply conditions. When low supply conditions are applied, voltage headroom for the IO cells may become a problem and providing the (digital) voltage signal externally to the IC may become a problem. In particular, voltage level shifter circuits in an IO cell may not be able to cope with low input voltage levels. This may introduce difficulties in the functional and POR (Power-on-Reset) level testing of a digital core under low supply voltage conditions. 
         [0026]    Embodiments of the present application may provide IO cell circuitry to convert a voltage output of a digital core to a current. This may aid the ability to provide reliable signal transfer off the IC during low voltage conditions. The IO cell may convert a signal from the digital core from a voltage domain to a current domain which may lead to robustness in terms of low voltage conditions. 
         [0027]      FIG. 2  shows an example of a system according to an embodiment. 
         [0028]      FIG. 2  depicts a system  200  comprising an IC  210  having a digital core  211  and an IO ring or block  212 . The IO ring  212  comprises a first IO cell  213  and a second IO cell  214 . The digital core  211  and the IO ring  212  are supplied by a first or IC supply voltage  215 . Again, it will be appreciated that the supply voltage  215  may be converted to another voltage before being provided to either of the IO cells  213 ,  214  or the digital core, however the IC supply voltage  215  provides the source voltage for the IC  210 . 
         [0029]    The system  200  further comprises first and second external circuitry  230  and  220 . In this example, the external circuitry may be CMOS or other circuitry used for the testing or functional operation of the system  200 . 
         [0030]    Similar to the system  100 , in operation, the digital core  211  may produce an output voltage  250  and provide this output voltage  250  to the second IO cell  214 . In one example the output voltage  250  is a rail-to-rail voltage signal with a maximum amplitude of the IC supply voltage  215 . The second IO cell  214  receives the voltage signal  250  from the digital core  211  and converts it to be compatible with the external voltage supply  240 . 
         [0031]    The IC  210  and the external circuitry  220 ,  230  may be in two voltage domains corresponding respectively to the IC supply voltage  215  and the external supply voltage  240 . The second IO cell  214  may convert a first voltage signal  250  in a first voltage domain into a second voltage signal  251  compatible with the second voltage domain. The second voltage signal  251  is then provided to the external circuitry. The second IO cell  214  may be a CMOS IO cell and may, for example, provide level shifting to convert the first voltage signal  250  to the second voltage signal  251 . 
         [0032]    The first voltage signal  250  from the digital core  211  may be provided to the first IO cell  213 . The first IO cell  213  may convert the first voltage  250  from a voltage domain to a current domain and provide a current signal  252  externally to the IC  210 . In this example the first IO cell  213  may be a virtual ground input output cell. It will be appreciated that the first IO cell  213  may operate according to any mechanism to convert the voltage signal to a current signal. For example, the circuitry may be appropriately designed and sized to fit onto the digital core  211 . 
         [0033]    The first IO cell  213  may be parallel to the second IO cell  214 . Converting the first voltage signal to a current signal may provide a more reliable transportation of the signal externally to the IC  210  as the transportation of signals in the current domain may be more robust under low supply voltage conditions. This may be due to the availability of low voltage circuit techniques, inherently used for the VI conversion. 
         [0034]    The system  200  may optionally include first external circuitry  230  for converting the current signal  252  back to a voltage signal. In this example, the current signal  252  may be provided externally to the IC  210  to the first external circuitry  230 . The first external circuitry  230  may, for example, convert the current signal  252  to a voltage signal. In one example, the first external circuitry  230  may be a transimpedance amplifier. Conversion back into the voltage domain may be useful in cases where low voltage conditions tests are developed for the voltage domain. 
         [0035]    In some examples, output from the first IO cell  213  may only be taken when the IC  210  is operating in low voltage conditions. When the IC  210  is operating under normal operating conditions the output may be taken from the second IO cell  214 . In other examples, both the first and second IO cells may provide respective outputs  251 ,  252  under all conditions and external circuitry may select which output to process. 
         [0036]    In operation the digital core  211  may provide a first voltage output  250 . The first IO cell  213  may receive the first voltage  250  and convert it to a current output  252 . The current output may optionally be received by a first external circuitry  230  and converted to a third voltage. The digital core  211  and first and second IO cells  213  and  214  may form part of an IC  210  in a first voltage domain (Vddd). The first external circuitry  230  may be part of a second voltage domain (Vddext). The first IO cell  213  and first external circuit  230  may convert the first voltage  250  from a voltage in the first digital domain to a third voltage in the second digital domain. The second IO cell  214  may receive the first voltage  250  and convert it to a second voltage  251  suitable for the second digital domain. 
         [0037]    The foregoing has exemplified providing signals from a digital core through IO of an IC to be provided off-chip (external to the IC). It will however be appreciated that embodiments may similarly work to receive signals external to the IC and provide them to the digital core. In this case the direction of the arrows in  FIG. 2  would be reversed. 
         [0038]    For example, under normal operating conditions an input voltage  251  may be provided from external circuitry  220  to the second IO cell  214 . The second IO cell  214  may convert this voltage  251  to a voltage  250  to be provided to the digital core. Under low voltage operations, the first external circuitry  230  may convert a voltage to a current  252  and provide this current to the first IO cell  213 . The first IO cell  213  may convert the current  252  to a voltage  250  to be provided to the digital core. It will be appreciated that in some embodiments, the first and second IO cells may operate in both low and normal voltage conditions. 
         [0039]    It will be appreciated that embodiments of the present application may be applied in any application where a digital core is to operate or be tested in low voltage conditions and may be provided for any digital application. In a specific example, embodiments may form part of an amplifier, for example an audio amplifier, car radio processing unit or head unit. It will be appreciated however that embodiments may be applied in further applications.