Abstract:
In a microcomputer including a CPU, at least one peripheral unit and a bus control unit connected therebetween, the bus control unit is constructed by a bus control circuit for controlling transfer of data, a strobe signal generating circuit for generating a strobe signal and transmitting the strobe signal to the peripheral unit, a flip-flop for sampling a retry requesting signal from the peripheral unit in synchronization with the strobe signal to generate a strobe requesting signal, and a strobe requesting signal detecting circuit for detecting the strobe requesting signal to reset the flip-flop. The bus control circuit receives the strobe requesting signal to transfer data from the CPU to the peripheral unit. The strobe signal generating circuit receives the strobe requesting signal to generate another strobe signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a microcomputer, and more particularly, to the improvement of a sampling function of a retry requesting signal from peripheral units. 
     2. Description of the Related Art 
     In a prior art microcomputer including a central processing unit (CPU), at least one peripheral unit and a bus control unit connected therebetween, the bus control unit is constructed by a bus control circuit for controlling transfer of data, a strobe signal generating circuit for generating a strobe signal and transmitting the strobe signal to the peripheral unit, and a flip-flop for sampling a retry requesting signal from the peripheral unit in synchronization with a clock signal to generate a strobe requesting signal. The bus control circuit receives the strobe requesting signal to transfer data from the CPU to the peripheral unit. The strobe signal generating circuit receives the strobe requesting signal to generate another strobe signal. This will be explained later in datail. 
     In the above-described prior art microcomputer, however, if the peripheral unit is not synchronized with the clock signal while the CPU and the bus control unit are synchronized with the clock signal, unnecessary retry operations may be carried out and required retry operations may not be carried out. That is, the retry requesting signal may be sampled by the flip-flop to carry out a retry operation. On the other hand, in an abnormal state where the write operation fails, the sampling of the retry requesting signal by the flip-flop may be delayed which would delay the generation of an waiting signal for the CPU, so that a retry operation is not carried out. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to be able to sample only required retry requesting signals in a microcomputer. 
     According to the present invention, in a microcomputer including a CPU, at least one peripheral unit and a bus control unit connected therebetween, the bus control unit is constructed by a bus control circuit for controlling transfer of data, a strobe signal generating circuit for generating a strobe signal and transmitting the strobe signal to the peripheral unit, a flip-flop for sampling a retry requesting signal from the peripheral unit in synchronization with the strobe signal to generate a strobe requesting signal, and a strobe requesting signal detecting circuit for detecting the strobe requesting signal to reset the flip-flop. The bus control circuit receives the strobe requesting signal to transfer data from the CPU to the peripheral unit. The strobe signal generating circuit receives the strobe requesting signal to generate another strobe signal. 
     If the active time period of the strobe signal is suitably adjusted, a retry requesting signal having a short period time in a normal state can not be sampled, while a retry requesting signal having a long period time in an abnormal state can be sampled. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the description set forth below, as compared with the prior art, with reference to the accompanying drawings, wherein: 
     FIG. 1 is a block circuit diagram illustrating a prior art microcomputer; 
     FIG. 2 is a block circuit diagram illustrating a first embodiment of the microcomputer according to the present invention; 
     FIGS. 3 and 4 are timing diagrams for explaining the operation of the microcomputer of FIG. 2; 
     FIG. 5 is a block circuit diagram illustrating a second embodiment of the microcomputer according to the present invention; and 
     FIG. 6 is a block circuit diagram illustrating a third embodiment of the microcomputer according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before the description of the preferred embodiments, a prior art microcomputer will be explained with reference to FIG.  1 . 
     In FIG. 1, reference numeral  10  designates a CPU,  20  designates a bus control unit,  30  designates a peripheral unit, and  40  designates an oscillator for generating a clock signal CLK. The clock signal CLK is supplied to the CPU  10  and the bus control circuit  20 , but is not supplied to the peripheral unit  30 . Therefore, the CPU  10  and the bus control unit  20  are synchronized with the clock signal CLK, while the peripheral unit  30  is not synchronized with the clock signal CLK. 
     The bus control unit  20  is constructed by a bus control circuit  21 , a strobe signal generating circuit  22  for generating a strobe signal ST 1  in response to a bus cycle start signal STA from the CPU  10 , a strobe signal generating circuit  23  for generating a strobe signal ST 2  in response to a strobe requesting signal SREQ, an OR circuit  24  for receiving the strobe signals ST 1  and ST 2  to generate a strobe signal ST, and a T-type flip-flop  25  for generating the strobe requesting signal SREQ. 
     The peripheral unit  30  is constructed by an interface circuit  31  and a register  32 . The interface circuit  31  performs a read/write operation of data upon the register  32 . 
     The bus control circuit  21  receives the strobe requesting signal SREQ to control a bus DB connected between the CPU  10  and the interface circuit  31 . 
     The strobe signal generating circuit  22  is constructed by a delay circuit  221  and an RS-type flip-flop  222 . In this case, the delay circuit  221  delays the bus cycle start signal STA from the CPU  90  by a predetermined number of pulses of the clock signal CLK which corresponds to a time τ. Therefore, the RS-type flip-flop  222  is set by a falling edge of the bus cycle start signal STA and is reset by its delayed falling edge, to thereby generate a strobe signal ST 1  having a time duration of τ. 
     Similarly, the strobe signal generating circuit  23  is constructed by a delay circuit  231  and an RS-type flip-flop  232 . In this case, the delay circuit  231  delays the strobe requesting signal SREQ by a predetermined number of pulses of the clock signal CLK which also corresponds to a time τ. Therefore, the RS-type flip-flop  232  is set by a falling edge of the strobe requesting signal SREQ and is reset by its delayed falling edge, to thereby generate a strobe signal ST 2  having a time duration of τ. 
     Note that each of the delay circuits  221  and  231  can be constructed by a counter for counting the pulses of the clock signal CLK and a comparator for comparing the content of the counter with a predetermined value. Therefore, if this predetermined value is changed, the time duration τ can be changed. 
     The T-type flip-flop  25  samples a retry requesting signal RREQ from the interface circuit  31  of the peripheral unit  30  in synchronization with the clock signal CLK to generate the strobe requesting signal SREQ. 
     The retrying operation of the micro computer of FIG. 1 is explained below. 
     Upon receipt of a strobe signal ST, the interface circuit  31  fetches data from the bus DB and writes the data into the register  32 , and simultaneously, the interface circuit  31  activates a retry requesting signal RREQ. In this case, if such a write operation succeeds normally, the interface circuit  31  immediately deactivates the retry requesting signal RREQ. On the other-hand, if the write operation fails, the interface circuit  31  continues to activate the retry requesting signal RREQ. 
     In the microcomputer of FIG. 1, the T-type flip-flop  25  is synchronized with the clock signal CLK and the retry requesting signal RREQ is not synchronized with the clock signal CLK. Therefore, even in a normal state, the retry requesting signal RREQ may be sampled by the T-type flip-flop  25  to carry out a retry operation. On the other hand, in an abnormal state where the write operation fails, the sampling of the retry requesting signal RREQ by the T-type flip-flop  25  may he delayed which would delay the generation of a waiting signal WT, so that a retry operation is not carried out. 
     In FIG. 2, which illustrates a first embodiment of the present invention, a D-type flip-flop  26  and a strobe requesting signal detecting circuit  27  are provided instead of the T-type flip-flop  25  of FIG.  1 . 
     The D-type flip-flop  26  has a data input terminal D for receiving a retry requesting signal RREQ, a clock input terminal C for receiving a strobe signal ST, an output terminal Q for generating a strobe requesting signal SREQ and a reset terminal R for receiving a reset signal RST from the strobe signal requesting signal detecting circuit  27 . Note that the D-type flip-flop  26  samples the retry requesting signal RREQ in synchronization with a falling edge of the strobe signal ST. 
     The operation of the microcomputer of FIG. 2 where no retry operation is carried out will be explained next with reference to FIG.  3 . 
     First, at time t 1 , the CPU  10  generates a bus cycle start signal STA, and also generates data on the bus DB. 
     Next, at time t 2 , upon receipt of the bus cycle start signal STA, the strobe signal generating circuit  22  generates a strobe signal ST 1  having an active duration τ. As a result, the strobe signal ST 1  is transmitted as a strobe signal ST via the OR circuit  24  to the interface circuit  31 . Therefore, the interface circuit  31  initiates a data write operation upon the register  32 , and later at time t 3 , the interface circuit  31  generates a retry requesting signal RREQ and transmits it to the D-type flip-flop  26 . In a normal state, since the data is quickly written into the register  32 , at time tit, the interface circuit  31  stops the generation of the retry requesting signal RREQ. 
     Next, at time t 5 , the strobe signal ST 1 (ST) is deactivated. In this case, however, since the retry requesting signal RREQ is deactivated, the D-type flip-flop  26  does not sample the retry requesting signal RREQ. Therefore, no retry operation is carried out. 
     The operation of the microcomputer of FIG. 2 where a retry operation is carried out will be explained next with reference to FIG.  4 . 
     First, at time t 1 , the CPU  10  generates a bus cycle start signal STA, and also generates data on the bus DB. 
     Next, at time t 2 , upon receipt of the bus cycle start signal STA, the strobe signal generating circuit  22  generates a strobe signal ST 1  having an active duration τ. As a result, the strobe signal ST 1  is transmitted as a strobe signal ST via the OR circuit  24  to the interface circuit  31 . Therefore, the interface circuit  31  initiates a data write operation upon the register  32 , and later at time t 3 , the interface circuit  31  generates a retry requesting signal RREQ and transmits it to the D-type flip-flop  26 . In an abnormal state, since the data is not written into the register  32 , the interface circuit  31  never stops the generation of the retry requesting signal RREQ. 
     Next, at time t 4 , the strobe signal ST 1 (ST) is deactivated. In this case, since the retry requesting signal RREQ is still activated, the D-type flip-flop  26  samples the retry requesting signal RREQ. Therefore, a strobe requesting signal SREQ is generated from the D-type flip-flop  26 . 
     Next, at time t 5 , the strobe requesting signal detecting circuit  27  detects the strobe requesting signal SREQ to generate a reset signal RST. As a result, the D-type flip-flop  26  is reset, so as to reset the strobe requesting signal SREQ. 
     Next, at time t 6 , upon receipt of the strobe request signal SREQ, the strobe signal generating circuit  23  generates a strobe signal ST 2  having an active duration τ. As a result, the strobe signal ST 2  is transmitted as a strobe signal ST via the OR circuit  24  to the interface circuit  31 . Simultaneously, the strobe signal ST 2  is transmitted as a waiting signal WT to the CPU  10 , so that the CPU  10  continues to generate the data. Also, the bus control circuit  21  receives the strobe requesting signal SREQ. Therefore, the interface circuit  31  retries a data write operation upon the register  32 . 
     Next, at time t 7 , the strobe signal ST 2 (ST) is deactivated. In this case, since the retry requesting signal RREQ is still activated, the D-type flip-flop  26  samples the retry requesting signal RREQ. Therefore, a strobe requesting signal SREQ is generated from the D-type flip-flop  26 . 
     Also, assume that the date is written into the register  32 , and at time t 8 , the interface circuit  31  stops the generation of the retry requesting signal RREQ. 
     Next, at time t 9 , the strobe requesting signal detecting circuit  27  detects the strobe requesting signal SREQ to generate a reset signal RST. As a result, the D-type flip-flop  26  is reset, so as to reset the strobe requesting signal SREQ. 
     Next, at time t 10 , upon receipt of the strobe requesting signal SREQ, the strobe signal generating circuit  23  generates a strobe signal ST 2  having an active duration τagain. As a result, the strobe signal ST 2  is transmitted as a strobe signal ST via the OR circuit  24  to the interface circuit  31 . Simultaneously, the strobe signal ST 2  is transmitted as a waiting signal WT to the CPU  10 , so that the CPU  10  continues generating the data. Also, the bus control circuit  21  receives the strobe requesting signal SREQ. Therefore, the interface circuit  31  retries a data write operation upon the register  32 . 
     Next, at time t 11 , the strobe signal ST 2 (ST) is deactivated. In this case, however, since the retry requesting signal RREQ is already activated, the D-type flip-flop  26  does not sample the retry requesting signal RREQ. Therefore, no strobe requesting signal SREQ is generated from the D-type flip-flop  26 . 
     Thus, the retry operation is completed. 
     In the microcomputer of FIG. 2, since a retry requesting signal RREQ generated from the peripheral unit  30  is sampled by the strobe signal ST, only required retry operations can be surely carried out. 
     In FIG. 5, which illustrates a second embodiment of the present invention, a plurality of peripheral units such as  30 - 1  and  30 - 2  which have the same configuration as the peripheral unit  30  of FIG. 2 are provided, and an OR circuit  28  for receiving retry requesting signals RREQ 1  and RREQ 2  from the peripheral units  30 - 1  and  30 - 2  is provided in the bus control unit  20 . That is, if one of the peripheral units  30 - 1  and  30 - 2  generates a retry requesting signal RREQ 1  or RREQ 2 , the OR circuit  28  generates a retry requesting signal RREQ. Therefore, the bus control unit  20  of FIG. 5 can operate in the same way as in FIG.  2 . 
     In FIG. 6, which illustrates a third embodiment of the present invention, a pull-down resistor  29  is provided instead of the OR circuit  28  of FIG.  5 . Even in FIG. 6, if one of the peripheral units  30 - 1  and  30 - 2  generates a retry requesting signal RREQ 1  or RREQ 2 , the output of the pull-down resistor  29  becomes a retry requesting signal RREQ. Therefore, the bus control unit  20  of FIG. 6 can operate in the same way as in FIG.  2 . Note that the value of the pull-down resistor  29  depends upon the number of peripheral units connected to the bus control unit  20 . 
     As explained herein above, according to the present invention, since a retry requesting signal is sampled by a strobe signal, only required retry operations can be surely carried out.