Patent Publication Number: US-9892085-B2

Title: Control device for I2C slave device

Description:
RELATED APPLICATION INFORMATION 
     The present application claims priority to and the benefit of German patent application no. 10 2014 223 362.3, which was filed in Germany on Nov. 17, 2014, the disclosure of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a method for controlling at least one I 2 C slave device. The present invention also relates to a control device for at least one I 2 C slave device. 
     BACKGROUND INFORMATION 
     The I 2 C interface (inter-integrated circuit) uses two bidirectional lines in order to provide communication between one or multiple masters and one or multiple slaves. The two aforementioned lines, the serial data line (SDA) and the serial clock line (SCL), are used to send data and to display the beginning and an end of transactions on the interface or on the bus. A beginning and an end of the transaction are designated as “START” and “STOP”. Every slave device on the I 2 C bus must have a device which detects the START/STOP states and/or sends and/or receives data. 
     However, the slave device must also distinguish between correct and incorrect transactions and is only permitted to respond to correct transactions. Although the aforementioned illegal transfers do not conform to the I 2 C specification, they are often used by various microcomputer devices on the market in order to reset all devices on the I 2 C bus. 
     Circuits which are already believed to be understood, which are discussed, for example, in U.S. Pat. No. 6,530,029 or CN 202600693, have problems with the aforementioned transfers which do not conform to the standard and may result in states in which the device no longer functions correctly. 
     A watchdog timer described in EP 1607864 could solve the stated problem. It is possible, however, that some of the transactions will not be detected or will be discarded, which may result in a loss of data. 
     Another approach, which is provided in GB 2313987, functions only when there is a system clock having a substantially higher frequency in the device, which may be used to oversample the SDA and SCL signals. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is therefore to provide a method which supports reliable functioning of I 2 C slave devices on the I 2 C bus. 
     The object is achieved, according to a first aspect, by a method for controlling an I 2 C slave device, including the steps of:
         evaluating states on a data line and on a clock line of the I 2 C bus;   assigning the states on the data line and on the clock line to states of a state diagram, control signals for the I 2 C slave device being generated with the aid of the control device from the states in the state diagram.       

     Advantageously, the method according to the present invention makes it possible to “hide” prohibited or illegal actions on the I 2 C bus from bus users. If an illegal state occurs on the bus, a reset state is always active. Advantageously, this is easy to implement with the aid of a synchronous circuit. The I 2 C slave may therefore always function correctly regardless of the illegal action taking place on the I 2 C bus. In the end, the I 2 C slave is therefore protected against incorrect responses to the bus, whereby, advantageously, all I 2 C slave devices connected to the I 2 C bus profit from the present invention. 
     According to a second aspect, the object is achieved by a control device for controlling an I 2 C slave device, including:
         a feed from a data line of an I 2 C bus;   a feed from a clock line of the I 2 C bus;   the control device being configured to assign states in a state diagram to states on the data line and on the clock line;   control signals for the I 2 C slave device being generatable from the states in the state diagram.       

     Refinements of the method and the control device are the subject matter of the further descriptions herein. 
     One advantageous refinement of the method provides that a start control signal is activated on a first control line if the clock line has a high level and the data line has a falling flank, followed by a falling flank on the clock line. In this way, a control signal for the I 2 C slave device may be generated, after the transmission of which a valid transfer on the I 2 C bus is possible. 
     One advantageous refinement of the method provides that the start control signal is deactivated on the first control line if the clock line has a high level. In this way, a control signal for the I 2 C slave device may be generated, after the transmission of which a valid transfer on the I 2 C bus is possible. 
     One further advantageous refinements of the method provides that a reset control signal is activated on a second control line if a falling flank occurs on the data line and the clock line has a high level. In this way, the reset control signal may be generated, in a defined way, from signal states on lines of the I 2 C bus. 
     One further advantageous refinement of the method provides that the reset control signal is deactivated after the start control signal has been generated. 
     One advantageous refinement of the control device provides that the data line is connected to an input of a first inverter, to an inverted clock input of a second D flip-flop, and to a clock input of a fourth D flip-flop; 
     an output of the first inverter is connected to a second input of an OR gate; 
     the clock line is connected to an input of the second D flip-flop, to a reset input of the second D flip-flop, to a clock input of a first D flip-flop, to a data input of the fourth D flip-flop, and to a clock input of a third D flip-flop; 
     an output of the second D flip-flop is connected to an input of a second inverter, to an input of the third D flip-flop, and to a first input of a NOR gate; 
     an output of the second inverter is connected to a reset input of the first D flip-flop; 
     an output of the fourth D flip-flop is connected to a second input of the NOR gate; 
     an output of the third D flip-flop is connected to a first control line of the I 2 C slave device, to a first input of an OR gate, and to an input of a third inverter; 
     an output of the third inverter is connected to a reset input of the fourth D flip-flop; 
     an output of the OR gate is connected to an input of the first D flip-flop; 
     an output of the first D flip-flop is connected to an input of a fourth inverter; 
     an output of the fourth inverter is connected to a reset input of the third D flip-flop; and 
     an output of the NOR gate is connected to a second control line for the I 2 C slave device. 
     The present invention is described in detail in the following, including further features and advantages, on the basis of multiple figures. All features are the subject matter of the present invention, irrespective of their representation in the description and in the figures, and irrespective of their back reference in the patent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a functional status diagram for illustrating the functional principle of the present invention. 
         FIG. 2  shows a specific embodiment of a control device according to the present invention. 
         FIG. 3  shows a time diagram of control signals, which are implemented according to the method according to the present invention. 
         FIG. 4  shows a basic sequence diagram of a specific embodiment of the method according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a functional status diagram showing the functional principle of the present invention. 
     Depicted therein are five different states DL, DH, SP, ST, I, which are permitted to occur due to electrical states on data line SDA and on clock line SCL, the aforementioned states having the following meanings:
     DL . . . 0 or GND potential on data line SDA during an on-going data transfer   DH . . . 1 or HIGH potential on data line SDA during an on-going data transfer   SP . . . STOP state is detected on the I 2 C bus   ST . . . START state is detected on the I 2 C bus   I . . . illegal state is detected on the I 2 C bus.   

     The arrows shown between aforementioned states DL, DH, SP, ST, I depict transitions between the individual states, the first term referring to the state of data line SDA and the second term referring to the state of clock line SCL. 
     An “L” in the first term represents a falling flank on data line SDA and a “0” in the first term represents a constant low level on data line SDA. 
     An “H” in the first term represents a rising flank on data line SDA and a “1” in the first term represents a constant high level on data line SDA. 
     A “0” in the second term represents a constant low level on clock line SCL and a “1” in the second term represents a constant high level on clock line SCL. 
     An “H” in the second term represents a rising flank on clock line SCL and an “L” in the second term represents a falling flank on clock line SCL. 
     If a state change does not occur on the SDA and SCL lines, then a transition also does not occur in the status diagram in  FIG. 1 . 
     For example, L1 therefore means there is a falling flank on data line SDA and there is a constant HIGH level on clock line SCL. 
       FIG. 2  shows an exemplary embodiment of a control device  100  according to the present invention for controlling an I 2 C slave device. Control device  100  is configured as a synchronous logic having D flip-flops, each of which has an asynchronous reset input. The two lines SCL and SDA are apparent, the electrical states or levels of which may be used as stimuli for control device  100 . 
     Data line SDA is routed to an input of a first inverter  70 , to an inverted clock input of a second D flip-flop  20 , and to a clock input of a fourth D flip-flop  40 . Clock line SCL is routed to a data input of second D flip-flop  20 , to a reset input of second D flip-flop  20 , to a clock input of first D flip-flop  10 , to an inverted clock input of third D flip-flop  30  and to a data input of fourth D flip-flop  40 . 
     The output of second D flip-flop  20  is routed to an input of third D flip-flop  30 , to an input of a second inverter  71 , and to an input of a NOR gate  60 . An output of fourth D flip-flop  40  is routed to a second input of a NOR gate  60 . 
     The output of third D flip-flop  30  forms a first control line LTG 1 , on which a START signal for the I 2 C slave device (not depicted) is output. The output of third D flip-flop  30  is further routed to a first input of OR gate  50  and also to an input of a third inverter  72 . The output of third inverter  72  is routed to a reset input of fourth D flip-flop  40 . 
     The output of first inverter  70  is routed to a second input of OR gate  50 , the output of OR gate  50  being routed to a data input of first D flip-flop  10 . The output of second inverter  71  is routed to a reset input of first D flip-flop  10 . The output of first D flip-flop  10  is routed to an input of a fourth inverter  73 , the output of which is routed to a reset input of third D flip-flop  30 . The output of third inverter  72  is routed to a reset input of fourth D flip-flop  40 . 
     The output of NOR gate  60  represents a second control line LTG 2 , on which a reset control signal RESET is provided for the I 2 C slave device. If at least one of the outputs of second D flip-flop  20  (state ST is detected on the I 2 C bus) or of fourth D flip-flop  40  (state SP is detected on the I 2 C bus) is active, NOR gate  60  generates the reset control signal RESET. This means the low-active reset control signal RESET is active only when state ST is detected on the I 2 C bus or when a bus action which does not conform to the I 2 C specification occurs on the I 2 C bus. 
     In the end, start control signal START is generated with the aid of control device  100  on first control line LTG 1  after a correct ST state, i.e., when a falling flank occurs on data line SDA and clock line SCL has a high level, followed by a falling flank on clock line SCL. 
     Start control signal START is subsequently reset with a HIGH level on clock line SCL. Start control signal START is used for the I 2 C slave device which may be in order to indicate a valid transfer on the I 2 C bus. 
     Reset control signal RESET is generated on second control line LTG 2  every time states ST or SP or I have been detected on the I 2 C bus with the aid of the status diagram in  FIG. 1 . This means reset control signal RESET is generated every time a falling flank occurs on data line SDA and the clock line has a high level (state ST), or when a rising flank occurs on data line SDA and clock line SCL has a high level (state SP), or when the data line has a high level and a falling flank occurs on the clock line (transition to state I), when state SP is present. 
     Reset control signal RESET is then reset with the subsequent start control signal START. Reset control signal RESET may therefore be used as an asynchronous reset signal for the I 2 C slave device. 
     In the end, start control signal START and reset control signal RESET, which are conveyed to the I 2 C slave device (not depicted), are controlled in this way with the aid of control device  100 . 
     Second D flip-flop  20  detects the beginning of state ST according to the I 2 C specification with the aid of a falling flank on data line SDA, while clock line SCL is at a HIGH level. 
     Third D flip-flop  30  detects the end of state ST on the I 2 C bus with the aid of a falling flank on clock line SCL after state ST was detected. 
     Fourth D flip-flop  40  detects state SP according to the I 2 C specification with the aid of a rising flank on data line SDA, while clock line SCL is at a constant high level. 
     In this way, it is possible to install the entire control device  100  into a scan chain, thereby supporting cost-effective, automated, digital testing of the entire control device  100 , because there is no need to implement any additional tests with scan chain patterns. Advantageously, the implementation of the control device with D flip-flops supports a geometrically small configuration of control device  100 . 
     First D flip-flop  10  generates the reset of start control signal START, whereby the preparations for it are carried out by OR gate  50 . OR gate  50  therefore prepares that which is supposed to be sampled by first D flip-flop  10  when a rising flank is conveyed to the clock input of first D flip-flop  10  on clock line SCL. 
     Control device  100  may also be configured as part of the I 2 C slave device. In the end, behavior which conforms to the specification is made possible on the I 2 C bus with the aid of control device  100 . In particular, it is possible for the I 2 C slave device connected to the I 2 C bus to function properly even if an operation by other bus users connected to the I 2 C does not conform to the I 2 C specification. 
     A mode of operation of control device  100  is described in the following on the basis of four scenarios on the I 2 C bus with reference to  FIG. 3 .  FIG. 3  shows, as an example, a signal behavior over time of the aforementioned components  10  through  60  in response to the stimuli on the SDA and SCL lines. 
     Depicted underneath  FIG. 3  is a state diagram FSM corresponding to the status diagram in  FIG. 1 , which represents the states on the I 2 C bus, whereby start control signal START and reset control signal RESET are generated in conformance with the I 2 C specification. 
     In the end, the behavior of the START and RESET control signals is configured in such a way that the I 2 C slave device always responds in a way conforming to the I 2 C specification, irrespectively of a behavior of signals on the SDA, SCL lines, which possibly does not conform to the specification. As an example of a permitted behavior, it is shown in section A of state diagram FSM that start control signal START is correctly generated, whereby reset control signal RESET is reset. 
     Reset control signal RESET therefore supports an inactivity of the I 2 C slave device in the event that a non-specification-compliant action was detected on the I 2 C bus. 
     In a section B of state diagram FSM in  FIG. 3 , it is apparent that the I 2 C bus switches from state SP to state I, which is an illegal action on the I 2 C bus. As a result, reset control signal RESET becomes active, i.e., the low-active reset control signal RESET assumes the low level. 
     Subsequently, a transition from the SP state into the ST state takes place on the I 2 C bus in a section C, which also does not conform to the I 2 C specification. In this case, a master or slave connected to the I 2 C bus, for example, specifies that a state ST be generated which, however, does not conform to the I 2 C specification. Therefore, control device  100  also does not generate a start control signal START on control line LTG 1  in accordance with the I 2 C specification, and the states of control lines LTG 1 , LTG 2  remain unchanged. 
     In section D, a permitted transition from state ST into state DL takes place on the I 2 C bus. Therefore, a start control signal START is also generated on first control line LTG 1  and second control line LTG 2  is reset, i.e., control line LTG 2  assumes a high level. 
       FIG. 4  shows a basic sequence of a specific embodiment of the method according to the present invention. 
     In a step  200 , an evaluation of states on a data line SDA and on a clock line SCL of the I 2 C bus is carried out. 
     In a step  210 , an assignment of the states on data line SDA and on clock line SCL to states DL, DH, I, SP, ST in a state diagram FSM is carried out, whereby control signals START, RESET for the I 2 C slave device are generated with the aid of control device  100  from states DL, DH, I, SP, ST in state diagram FSM. 
     After step  210 , the process returns to step  200 , whereby steps  200  and  210  may be executed cyclically. 
     In summary, the present invention provides a method and a device, which make it possible for a behavior which does not conform to the specification to remain on the I 2 C bus without adversely affecting the slave devices connected to the I 2 C bus. Due to the fact that the I 2 C specification is very stringent and states that some combinations are prohibited on the bus, it is possible, in this way, for a behavior of bus users on the I 2 C bus to take place free from interference by any type of erratic behavior on the I 2 C bus. 
     Advantageously, the switching behavior according to the present invention may be achieved by a large number of alternative circuit variations, which are neither depicted nor described. The variation of the control device in  FIG. 2  is therefore understood to be merely an example. 
     The present invention has a number of advantages: 
     For example, the control device according to the present invention is robust with respect to interferences or improper uses of the I 2 C protocol on the I 2 C bus. Furthermore, it is advantageously not necessary to provide additional external clocks for oversampling the I 2 C bus for the purpose of detecting states. In addition, an additional RESET control signal is not necessary, except for the PoR (power-on reset). Furthermore, it is possible to use standardized digital elements, such as, e.g., flip flops and logic gates, to implement the control device, which makes it possible to implement the circuit in RTL (Register Transfer Language) such as, e.g., VHDL or Verilog®, and to implement the control device with the use of standard tools. The control device may be tested using standardized scan and IDDQ methods. 
     Those skilled in the art will also implement non-depicted specific embodiments of the present invention without departing from the core of the present invention.