Abstract:
The present invention relates to a FIFO (First In First Out) memory control circuit for controlling FIFO memory which is used in various electronic devices. Specifically, the present invention relates to a FIFO memory control circuit capable of performing asynchronous read/write control hen a write clock and a read clock are different and it is known or determined which of these clocks has a higher clock frequency.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a FIFO (First In First Out) memory control circuit for controlling FIFO memory which is used in various electronic devices. Specifically, the present invention relates to a FIFO memory control circuit capable of performing asynchronous read/write control when a write clock and a read clock are different and it is known or determined which of these clocks has a higher clock frequency. 
     2. Description of the Related Art 
     FIG. 11 shows a structure of a conventional FIFO memory control circuit  1100 . The FIFO memory control circuit  1100  includes a memory  101 , a write control section  102 , a read control section  103 , a write address circuit  104 , a read address circuit  105 , and a Full-Empty control circuit  106 . 
     The memory  101  is a dual-port RAM (Random Access Memory) in which reading and writing of data can be performed simultaneously, and which has a memory capacity of N words. In the memory  101 , while a write permission signal (WE) is asserted, data (WDATA) is written in an address designated by a write address (WADR) on a word-by-word basis at a clock timing of a write clock signal (WCLK). On the other hand, while a read permission signal (RE) is asserted, data (RDATA) is read from an address designated by a read address (RADR) on a word-by-word basis at a clock timing of a read clock signal (RCLK). The write permission signal (WE) is output from the write control section  102  (described later), and the read permission signal (RE) is output from the read control section  103  (described later). 
     The write address circuit  104  receives the write clock signal (WCLK) and the write permission signal (WE). While the write permission signal (WE) is asserted, the write address circuit  104  increments the write address (WADR) by one at a clock timing of the write clock signal (WCLK). 
     The read address circuit  105  receives the read clock signal (RCLK) and the read permission signal (RE). While the read permission signal (RE) is asserted, the read address circuit  105  increments the read address (RADR) by one at a clock timing of the read clock signal (RCLK). 
     The Full-Empty control circuit  106  is formed by an up-down counter  107  and a signal generator  108 . The Full-Empty control circuit  106  obtains the number of effective data, which is the difference between the number of data words written in the memory  101  and the number of data words read from the memory  101 . That is, the “number of effective data words” means the number of data words in the memory  101  which have not yet read therefrom. Based on the number of effective data, the Full-Empty control circuit  106  generates control signals for writing and reading operations. 
     The up-down counter  107  receives the write permission signal (WE) as a count-up enable signal (UPEN) which permits a count-up operation and the read permission signal (RE) as a count-down enable signal (DNEN) which permits a count-down operation. While one of the count-up enable signal (UPEN) and the count-down enable signal (DNEN) is asserted, the up-down counter  107  performs a count operation at a clock timing of the write clock signal (WCLK). A count value (CNT) of the up-down counter  107  is equal to the number of effective data words, which is output to the signal generator  108 . 
     The signal generator  108  receives the count value (CNT) from the up-down counter  107 . When the received count value (CNT) is 0, the signal generator  108  outputs to the read control section  103  an empty signal (EMP) which indicates that the memory  101  has no data to be read. When the received count value (CNT) is N (the number of words storable in the memory  101 ), the signal generator  108  outputs to the write control section  102  a full signal (FLL) which indicates that the memory  101  has no more capacity to store data. 
     The write control section  102  receives the full signal (FLL). While the full signal (FLL) is asserted, the write control section  102  prohibits writing data in the memory  101 , thereby preventing the memory  101  from losing data due to overwriting. 
     The read control section  103  receives the empty signal (EMP). While the empty signal (EMP) is asserted, the read control section  103  prohibits reading data from the memory  101 , thereby preventing one data word from being read twice from the memory  101 . 
     FIG. 12 shows a structure of a conventional FIFO memory control circuit  1200  in which an up-down counter  107 ′ performs a count operation at a clock timing of a read clock signal (RCLK). In other respects, the FIFO memory control circuit  1200  has the same structure as the conventional FIFO memory control circuit  1100 , and descriptions thereof are omitted. 
     In the conventional FIFO memory control circuit  1100 , when the write clock signal (WCLK) and the read clock signal (RCLK) have the same frequency, the up-down counter  107  uses the write permission signal (WE) as a count-up enable signal (UPEN) which permits a count-up operation and the read permission signal (RE) as a count-down enable signal (DNEN) which permits a count-down operation. While one of the count-up enable signal (UPEN) and the count-down enable signal (DNEN) is asserted, the up-down counter  107  performs a count operation at a clock timing of the write clock signal (WCLK). In the conventional FIFO memory control circuit  1200 , when the write clock signal (WCLK) and the read clock signal (RCLK) have the same frequency, the up-down counter  107 ′ uses the write permission signal (WE) as a count-up enable signal (UPEN) which permits a count-up operation and the read permission signal (RE) as a count-down enable signal (DNEN) which permits a count-down operation. While one of the count-up enable signal (UPEN) and the count-down enable signal (DNEN) is asserted, the up-down counter  107 ′ performs a count operation at a clock timing of the read clock signal (RCLK). 
     In these conventional FIFO memory control circuits  1100  and  1200 , when the write clock signal (WCLK) and the read clock signal (RCLK) have different frequencies, a count operation cannot be correctly performed. 
     For example, in the conventional FIFO memory control circuit  1100  shown in FIG. 11, the up-down counter  107  performs a count operation at a clock timing of the write clock signal (WCLK). In the case where the write clock signal (WCLK) has a higher frequency than that of the read clock signal (RCLK), as shown in FIG. 13, in one read cycle, the count-down enable signal (DNEN=RE) is asserted for a period longer than one cycle of the write clock signal (WCLK). In such a case, although only one data is actually read out, the count value of the up-down counter  107  may be decremented by 2 or more. 
     On the other hand, in the conventional FIFO memory control circuit  1200  shown in FIG. 12, the up-down counter  107 ′ performs a count operation at a clock timing of the read clock signal (RCLK). In the case where the write clock signal (WCLK) has a higher frequency than that of the read clock signal (RCLK), as shown in FIG. 14, although data is actually written in, the count value of the up-down counter  107 ′ may not be incremented. 
     Alternatively, when the read clock signal (RCLK) has a higher frequency than that of the write clock signal (WCLK), in the conventional FIFO memory control circuit  1100  shown in FIG. 11, although data is actually read out, the count value of the up-down counter  107  may not be decremented; in the conventional FIFO memory control circuit  1200  shown in FIG. 12, although only one data is actually written in, the count value of the up-down counter  107 ′ may be incremented by 2 or more. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a FIFO memory control circuit includes: a write address circuit for generating a write address which is an operation address; a read address circuit for generating a read address which is another operation address; a memory which receives a write permission signal, a read permission signal, a write clock signal, and a read clock signal and which has a memory capacity of a predetermined number of words, wherein, while the write permission signal is asserted, data is written into an address in the memory designated by the write address in synchronization with the write clock signal, and while the read permission signal is asserted, data is read from an address in the memory designated by the read address in synchronization with the read clock signal; a first count control enable signal generation circuit for generating a first count control enable signal based on a first clock signal and a least significant bit of a said operation address corresponding to a second clock signal, the first clock signal being one of the write clock signal and the read clock signal which has the higher frequency, and the second clock signal being one of the write clock signal and the read clock signal which has the lower frequency; and an up-down counter which has a count value and receives a count-up enable signal, a count-down enable signal, and the first clock signal, wherein while the count-up enable signal is asserted, the count value is incremented in synchronization with the first clock signal, and while the count-down enable signal is asserted, the count value is decremented in synchronization with the first clock signal, wherein the first count control enable signal is one of the count-up enable signal and the count-down enable signal. 
     In one embodiment of the present invention, the first clock signal is the write clock signal; the first count control enable signal generation circuit is a count-down enable signal generation circuit; and the first count control enable signal is a count-down enable signal. 
     In another embodiment of the present invention, the count-down enable signal is asserted for one cycle of the write clock signal in response to one reading operation. 
     In still another embodiment of the present invention, the first clock signal is the read clock signal; the first count control enable signal generation circuit is a count-up enable signal generation circuit; and the first count control enable signal is a count-up enable signal. 
     In still another embodiment of the present invention, the count-up enable signal is asserted for one cycle of the read clock signal in response to one writing operation. 
     In still another embodiment of the present invention, the FIFO memory control circuit further includes a second count control enable signal generation circuit for generating a second count control enable signal based on the first clock signal and a least significant bit of the operation address which corresponds to the first clock signal, wherein the second count control enable signal is the other of the count-up enable signal and the count-down enable signal. 
     In still another embodiment of the present invention, each of the count-up enable signal and the count-down enable signal is asserted for one cycle of the first clock signal in response to one of a writing operation and a reading operation which corresponds to the first clock signal. 
     In still another embodiment of the present invention, the FIFO memory control circuit further includes a memory capacity monitoring section, wherein: when the count value of the up-down counter is 0, the memory capacity monitoring section generates an EMPTY signal which indicates that the memory has no data to be read, and when the count value of the up-down counter is equal to the predetermined number of words storable in the memory, the memory capacity monitoring section generates a FULL signal which indicates that the memory has no more capacity to store data. 
     In still another embodiment of the present invention, the FIFO memory control circuit includes: a write control section for controlling writing of data into the memory based on the FULL signal; and a read control section for controlling reading of data from the memory based on the EMPTY signal. 
     Hereinafter, functions of the present invention will be described. 
     According to the present invention, a first count control enable signal is generated based on a first clock signal and a least significant bit of the operation address corresponding to a second clock signal. The first clock signal is one of the write clock signal (WCLK) and the read clock signal (RCLK) having the higher frequency. The second clock signal is one of the write clock signal (WCLK) and the read clock signal (RCLK) having the lower frequency. With such a structure, the first count control enable signal can be asserted in an up-down counter in synchronization with a timing of the first clock signal in response to one reading operation or one writing operation. 
     Specifically, according to embodiment 1 of the present invention, in the case where the frequency of the write clock signal (WCLK) is higher than that of the read clock signal (RCLK), a count-down enable signal generation circuit generates a count-down enable signal (DNEN) based on a least significant bit (RADR 0 ) of the read address (RADR) and the write clock signal (WCLK). As a result, the count-down enable signal (DNEN) is asserted in the up-down counter in synchronization with a timing of the write clock signal (WCLK) in response to one reading operation. 
     While the count-down enable signal (DNEN) is asserted in the up-down counter, the up-down counter performs a count-down operation (decrementation) in synchronization with the write clock signal (WCLK). Thus, the up-down counter can decrement the count value (CNT) once in response to one reading operation. While the write permission signal (WE) is asserted in the up-down counter, the up-down counter performs a count-up operation (incrementation) using the write permission signal (WE) as a count-up enable signal (UPEN) in synchronization with the write clock signal (WCLK). Thus, the count value (CNT) can be incremented once in response to one writing operation. 
     The count-down enable signal (DNEN) only needs to be asserted for one cycle of the write clock signal (WCLK). 
     Specifically, according to embodiment 2 of the present invention, in the case where the frequency of the read clock signal (RCLK) is higher than that of the write clock signal (WCLK), a count-up enable signal generation circuit generates a count-up enable signal (UPEN) based on a least significant bit (WADR 0 ) of the write address (WADR) and the read clock signal (RCLK). As a result, the count-up enable signal (UPEN) is asserted in the up-down counter in synchronization with a timing of the read clock signal (RCLK) in response to one writing operation. 
     While the count-up enable signal (UPEN) is asserted in the up-down counter, the up-down counter performs a count-up operation (incrementation) in synchronization with the read clock signal (RCLK). Thus, the up-down counter can increment the count value (CNT) once in response to one writing operation. While the read permission signal (RE) is asserted in the up-down counter, the up-down counter performs a count-down operation (decrementation) using the read permission signal (RE) as a count-down enable signal (DNEN) in synchronization with the read clock signal (RCLK). Thus, the count value (CNT) can be decremented once in response to one reading operation. 
     The count-up enable signal (UPEN) only needs to be asserted for one cycle of the read clock signal (RCLK). 
     Specifically, according to embodiment 3 of the present invention, a count-down enable signal generation circuit generates a count-down enable signal (DNEN) based on a least significant bit (RADR 0 ) of the read address (RADR) and one of a write clock signal (WCLK) and a read clock signal (RCLK) having the higher frequency. As a result, the count-down enable signal (DNEN) is asserted in the up-down counter in response to one reading operation in synchronization with a timing of one of the write clock signal (WCLK) and the read clock signal (RCLK) having the higher frequency. On the other hand, a count-up enable signal generation circuit generates a count-up enable signal (UPEN) based on a least significant bit (WADR 0 ) of the write address (WADR) and one of the write clock signal (WCLK) and the read clock signal (RCLK) having the higher frequency. As a result, the count-up enable signal (UPEN) is asserted in the up-down counter in response to one writing operation in synchronization with a timing of one of the write clock signal (WCLK) and the read clock signal (RCLK) having the higher frequency. While the count-down enable signal (DNEN) is asserted in the up-down counter, the up-down counter performs a count-down operation (decrementation) in synchronization with one of the write clock signal (WCLK) and the read clock signal (RCLK) having the higher frequency. Thus, the up-down counter can decrement the count value (CNT) once in response to one reading operation. While the count-up enable signal (UPEN) is asserted in the up-down counter, the up-down counter performs a count-up operation (incrementation) in synchronization with one of the write clock signal (WCLK) and the read clock signal (RCLK) having the higher frequency. Thus, the up-down counter can increment the count value (CNT) once in response to one writing operation. 
     Each of the count-down enable signal (DNEN) and the count-up enable signal (UPEN) only needs to be asserted for one cycle of one of a write clock signal (WCLK) and a read clock signal (RCLK) having the higher frequency. 
     When the count value (CNT) of the up-down counter is 0, a memory capacity monitoring section generates an empty signal (EMP), and a read control section controls reading of data from the memory based on the empty signal (EMP). On the other hand, when the count value (CNT) of the up-down counter is equal to the predetermined number of words storable in the memory, the memory capacity monitoring section generates a FULL signal (FLL), and a write control section controls writing of data into the memory based on the FULL signal (FLL). 
     According to the present invention, the up-down counter can correctly count the amount of effective data in the memory. Thus, data in the memory can be prevented from being lost by being overwritten, and data in the memory can be prevented from being read out twice. 
     Thus, the invention described herein makes possible the advantage of providing a FIFO memory control circuit in which the amount of effective data in a memory can be correctly counted so that when the frequencies of a read clock and a write clock are different, data is prevented from being lost by being overwritten, and data is prevented from being read out twice. 
     This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a structure of a FIFO memory control circuit according to embodiment 1 of the present invention. 
     FIG. 2 shows an example of a count-down enable signal generation circuit in the FIFO memory control circuit according to embodiment 1. 
     FIG. 3 is a timing chart showing signals in the FIFO memory control circuit according to embodiment 1. 
     FIG. 4 is a block diagram showing a structure of a FIFO memory control circuit according to embodiment 2 of the present invention. 
     FIG. 5 shows an example of a count-up enable signal generation circuit in the FIFO memory control circuit according to embodiment 2. 
     FIG. 6 is a timing chart showing signals in the FIFO memory control circuit according to embodiment 2. 
     FIG. 7A is a block diagram showing a structure of a FIFO memory control circuit according to embodiment 3 of the present invention. 
     FIG. 7B is a block diagram showing a structure of another FIFO memory control circuit according to embodiment 3 of the present invention. 
     FIG. 8 shows an example of a count-up enable signal generation circuit in the FIFO memory control circuit according to embodiment 3. 
     FIG. 9 shows an example of a count-down enable signal generation circuit in the FIFO memory control circuit according to embodiment 3. 
     FIG. 10 is a timing chart showing signals in the FIFO memory control circuit according to embodiment 3. 
     FIG. 11 is a block diagram showing a structure of a conventional FIFO memory control circuit. 
     FIG. 12 is a block diagram showing a structure of a conventional FIFO memory control circuit. 
     FIG. 13 is a timing chart showing signals in the conventional FIFO memory control circuit of FIG.  11 . 
     FIG. 14 is a timing chart showing signals in the conventional FIFO memory control circuit of FIG.  12 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. 
     (Embodiment 1) 
     FIG. 1 is a block diagram showing a structure of a FIFO memory control circuit  100 . 
     The FIFO memory control circuit  100  includes a memory  10 , a write control section  11 , a read control section  12 , a write address circuit  13 , a read address circuit  14 , and an up-down counter  15 , a memory capacity monitoring section  16 , and a count-down enable signal generation circuit  17 . 
     The memory  10 , the write control section  11 , the read control section  12 , the write address circuit  13 , and the read address circuit  14  respectively have the same structures as those of the memory  101 , the write control section  102 , the read control section  103 , the write address circuit  104 , and the read address circuit  105  in the conventional FIFO memory control circuit  1100  shown in FIG. 11, and therefore, further descriptions thereof are omitted. In embodiment 1, the memory  10  has a memory capacity of 5 words, and a write clock signal (WCLK) has a higher frequency than that of a read clock signal (RCLK). 
     The memory  10  is a dual-port RAM (Random Access Memory) in which reading and writing of data can be performed simultaneously, and which has a memory capacity of N words. In memory  10 , while a write permission signal (WE) from the write control section  11  is asserted, data (WDATA) is written in an address designated by a write address (WADR) on the word-by-word basis at a clock timing of a write clock signal (WCLK). On the other hand, while a read permission signal (RE) is asserted, data (RDATA) is read from an address designated by a read address (RADR) on the word-by-word basis at a clock timing of a read clock signal (RCLK). 
     The write address circuit  13  receives the write clock signal (WCLK) and the write permission signal (WE). While the write permission signal (WE) is asserted, the write address circuit  13  increments the write address (WADR) by one at a clock timing of the write clock signal (WCLK). 
     The read address circuit  14  receives the read clock signal (RCLK) and the read permission signal (RE). While the read permission signal (RE) is asserted, the read address circuit  14  increments the read address (RADR) by one at a clock timing of the read clock signal (RCLK). 
     The up-down counter  15  receives the write permission signal (WE) from the write control section  11  as a count-up enable signal (UPEN) which permits a count-up operation and a count-down enable signal (DNEN) which permits a count-down operation from the count-down enable signal generation circuit  17 . The up-down counter  15  further receives the write clock signal (WCLK) which has a frequency higher than that of the read clock signal (RCLK). The write clock signal (WCLK) is used as a count clock signal (FASTCLK). While the count-up enable signal (UPEN) is asserted, the count value of the up-down counter  15  is incremented by 1 at a clock timing of the write clock signal (WCLK) in response to one writing operation. While the count-down enable signal (DNEN) is asserted, the count value of the up-down counter  15  is decremented by 1 at a clock timing of the write clock signal (WCLK) in response to one reading operation. The count value (CNT) of the up-down counter  15 , which indicates the number of effective data words, is output to the memory capacity monitoring section  16 . 
     Now, a count operation performed by the up-down counter  15  is described in detail with reference to FIGS. 2 and 3. FIG. 2 shows an example of the count-down enable signal generation circuit  17 . FIG. 3 is a timing chart showing signals in the FIFO memory control circuit  100 : (A) the read clock signal RCLK; (B) the least significant bit of the read address (RADR 0 ); (C) an output of a flip-flop  201 ; (D) an output of a flip-flop  202 ; (E) an output of EX-OR  203  (=count-down enable signal DNEN); (F) the read permission signal RE; (G) the write clock signal WCLK (=count clock signal FASTCLK); (H) the write permission signal WE (=count-up enable signal UPEN); and (I) an output of the up-down counter  15 . 
     In the read address circuit  14 , while the read permission signal RE (segment (F) of FIG. 3) is asserted, the read address RADR is output to the count-down enable signal generation circuit  17  at a clock timing of the read clock signal RCLK (segment (A)). Every time the read address RADR varies, the value of the least significant bit of the read address (RADR 0 ) alternately changes between 0 and 1 as shown in segment (B) of FIG.  3 . It should be noted that in the present specification, the “write address” and the “read address” are generically referred to as “operation addresses”. 
     Furthermore, in a writing operation, a clock signal, a permission signal, an address signal, and the LSB value of the address signal correspond to a write clock signal WCLK, a write permission signal WE, a write address signal WADR, and the LSB value of the write address signal (WADR 0 ), respectively. In a reading operation, a clock signal, a permission signal, an address signal, and the LSB value of the address signal correspond to a read clock signal RCLK, a read permission signal RE, a read address signal RADR, and the LSB value of the read address signal (RADR 0 ), respectively. 
     The count-down enable signal generation circuit  17  shown in FIG. 2 is formed by two flip-flops  201  and  202  and an EX-OR  203 . The flip-flop  201  receives the value of LSB of the read address (RADR 0 ) shown in segment (B) of FIG. 3 at the timing of the write clock signal WCLK (=count clock signal FASTCLK) shown in segment (G), and then outputs a signal as shown in segment (C). The flip-flop  202  receives the output of the flip-flop  201  (segment (C)) at the timing of the write clock signal WCLK (=FASTCLK) shown in segment (G), and then outputs a signal as shown in segment (D). The EX-OR  203  receives the output signal of the flip-flop  201  (segment (C)) and the output signal of the flip-flop  202  (segment (D)), and then outputs the exclusive-OR of these signals as a count-down enable signal DNEN (segment (E)). This count-down enable signal DNEN is synchronized with the write clock signal WCLK, and is asserted in the up-down counter  15  for one cycle of the write clock signal WCLK in response to one reading operation. Accordingly, the up-down counter  15  decrements the count value (CNT) once in response to one reading operation. As a result, count operations (incrementation/decrementation) of the up-down counter  15  based on the same clock signal (in embodiment 1, the write clock signal WCLK) can be accurately carried out without causing an error in conjunction with reading and writing operations in the memory  10 . 
     The count-down enable signal DNEN (segment (E)) rises in response to a rising edge of the write clock signal WCLK (segment (G)). In embodiment 1, rising edges of the count-down enable signal DNEN (segment (E)) are delayed with respect to rising edges of the write clock signal WCLK (segment (G)) as shown in FIG.  3 . That is, a time delay is provided from the rising edge of the write clock signal WCLK until the count-down enable signal DNEN (segment (E)) is asserted. Such a time delay assures that the up-down counter  15  performs a count operation at a next rising edge of the write clock signal WCLK (segment (G)). If the count-down enable signal DNEN (segment (E)) and the write clock signal WCLK (segment (G)) were to change at the same time, it may be uncertain in response to which rising edge of the write clock signal WCLK (segment (G)) the up-down counter  15  would perform a count operation. This will be also considered in embodiments 2 and 3. 
     The memory capacity monitoring section  16  receives the count value (CNT) from the up-down counter  15 . When the count value (CNT) is 0, the memory capacity monitoring section  16  outputs to the read control section  12  an empty signal EMP which indicates that the memory  10  has no data to be read. When the count value (CNT) is equal to the number of words storable in the memory  10  (in embodiment 1, “5”), the memory capacity monitoring section  16  outputs to the write control section  11  a full signal FLL which indicates that the memory  10  has no more capacity to store data. 
     With the above structure, the amount of effective data (i.e., the number of effective data words) in the memory  10  can be correctly counted. Thus, when the full signal FLL is asserted in the write control section  11 , the write control section  11  prohibits writing data in the memory  10 , thereby preventing data from being lost by being overwritten. Furthermore, when the empty signal EMP is asserted in the read control section  12 , the read control section  12  prohibits reading data from the memory  10 , thereby preventing data from being read out twice. 
     (Embodiment 2) 
     FIG. 4 is a block diagram showing a structure of a FIFO memory control circuit  400 . 
     The FIFO memory control circuit  400  includes a memory  20 , a write control section  21 , a read control section  22 , a write address circuit  23 , a read address circuit  24 , and an up-down counter  25 , a memory capacity monitoring section  26 , and a count-up enable signal generation circuit  27 . 
     The memory  20 , the write control section  21 , the read control section  22 , the write address circuit  23 , and the read address circuit  24  respectively have the same structures as those of the memory  101 , the write control section  102 , the read control section  103 , the write address circuit  104 , and the read address circuit  105  in the conventional FIFO memory control circuit  1100  shown in FIG. 11, and therefore, further descriptions thereof are omitted. In embodiment 2, the memory  20  has a memory capacity of 5 words, and a read clock signal (RCLK) has a higher frequency than that of a write clock signal (WCLK). 
     The up-down counter  25  receives the read permission signal (RE) from the read control section  22  as a count-down enable signal (DNEN) which permits a count-down operation and a count-up enable signal (UPEN) which permits a count-up operation from the count-up enable signal generation circuit  27 . The up-down counter  25  further receives the read clock signal (RCLK) which has a frequency higher than that of the write clock signal (WCLK). The read clock signal (RCLK) is used as a count clock signal (FASTCLK). While the count-up enable signal (UPEN) is asserted, the count value of the up-down counter  25  is incremented by 1 at a clock timing of the read clock signal (RCLK) in response to one writing operation. While the count-down enable signal (DNEN) is asserted, the count value of the up-down counter  25  is decremented by 1 at a clock timing of the write clock signal (WCLK) in response to one reading operation. The count value (CNT) of the up-down counter  25 , which indicates the number of effective data words, is output to the memory capacity monitoring section  26 . 
     Now, a count operation performed by the up-down counter  25  is described in detail with reference to FIGS. 5 and 6. FIG. 5 shows an example of the count-up enable signal generation circuit  27 . FIG. 6 is a timing chart showing signals in the FIFO memory control circuit  400 : (A) the write clock signal WCLK; (B) the least significant bit of the write address (WADR 0 ); (C) an output of a flip-flop  301 ; (D) an output of a flip-flop  302 ; (E) an output of EX-OR  303  (=count-up enable signal UPEN); (F) the write permission signal WE; (G) the read clock signal RCLK (=count clock signal FASTCLK); (H) the read permission signal RE; and (I) an output of the up-down counter  25  (CNT). 
     In the write address circuit  23 , while the write permission signal WE (segment (F) of FIG. 6) is asserted, the write address WADR is output to the count-up enable signal generation circuit  27  at a clock timing of the write clock signal WCLK (segment (A)). Every time the write address WADR varies, the value of the least significant bit of the write address (WADR 0 ) alternately changes between 0 and 1 as shown in segment (B) of FIG.  6 . 
     The count-up enable signal generation circuit  27  shown in FIG. 5 is formed by two flip-flops  301  and  302  and an EX-OR  303 . The flip-flop  301  receives the value of LSB of the write address (WADR 0 ) shown in segment (B) of FIG. 6 at the timing of the read clock signal RCLK (count clock signal FASTCLK) shown in segment (G), and then outputs a signal as shown in segment (C). The flip-flop  302  receives the output of the flip-flop  301  (segment (C)) at the timing of the read clock signal RCLK (=FASTCLK) shown in segment (G), and then outputs a signal as shown in segment (D). The EX-OR  303  receives the output signal of the flip-flop  301  (segment (C)) and the output signal of the flip-flop  302  (segment (D)), and then outputs the exclusive-OR of these signals as a count-up enable signal UPEN (segment (E)). This count-up enable signal UPEN is in synchronization with the read clock signal RCLK, and is asserted in the up-down counter  25  for one cycle of the read clock signal RCLK in response to one writing operation. Accordingly, the up-down counter  25  increments the count value (CNT) once in response to one writing operation. As a result, count operations (incrementation/decrementation) of the up-down counter  15  based on the same clock signal (in embodiment  2 , the read clock signal RCLK) can be accurately carried out without causing an error in conjunction with reading and writing operations in the memory  20 . 
     The memory capacity monitoring section  26  receives the count value (CNT) from the up-down counter  25 . When the count value (CNT) is 0, the memory capacity monitoring section  26  outputs to the read control section  22  an empty signal EMP which indicates that the memory  20  has no data to be read. When the count value (CNT) is equal to the number of words storable in the memory  20  (in embodiment 2, “5”), the memory capacity monitoring section  26  outputs to the write control section  21  a full signal FLL which indicates that the memory  20  has no more capacity to store data. 
     With the above structure, the amount of effective data (i.e., the number of effective data words) in the memory  20  can be correctly counted. Thus, when the full signal FLL is asserted in the write control section  21 , the write control section  21  prohibits writing data in the memory  20 , thereby preventing data from being lost by being overwritten. Furthermore, when the empty signal EMP is asserted in the read control section  22 , the read control section  22  prohibits reading data from the memory  20 , thereby preventing data from being read out twice. 
     (Embodiment 3) 
     FIG. 7A is a block diagram showing a structure of a FIFO memory control circuit  700 . 
     The FIFO memory control circuit  700  includes a memory  30 , a write control section  31 , a read control section  32 , a write address circuit  33 , a read address circuit  34 , and an up-down counter  35 , a memory capacity monitoring section  36 , a count-up enable signal generation circuit  37 , and a count-down enable signal generation circuit  38 . 
     The memory  30 , the write control section  31 , the read control section  32 , the write address circuit  33 , and the read address circuit  34  respectively have the same structures as those of the memory  101 , the write control section  102 , the read control section  103 , the write address circuit  104 , and the read address circuit  105  in the conventional FIFO memory control circuit  1100  shown in FIG. 11, and therefore, further descriptions thereof are omitted. In embodiment 3, the memory  30  has a memory capacity of 5 words, and a write clock signal (WCLK) has a higher frequency than that of a read clock signal (RCLK). 
     The up-down counter  35  receives a count-up enable signal (UPEN) which permits a count-up operation from the count-up enable signal generation circuit  37  and a count-down enable signal (DNEN) which permits a count-down operation from the count-down enable signal generation circuit  38 . The up-down counter  35  further receives the write clock signal (WCLK) which has a frequency higher than that of the read clock signal (RCLK). Herein, the write clock signal (WCLK) is used as a count clock signal (FASTCLK). While the count-up enable signal (UPEN) is asserted, the count value of the up-down counter  35  is incremented by 1 at a clock timing of the write clock signal (WCLK) in response to one writing operation. While the count-down enable signal (DNEN) is asserted, the count value of the up-down counter  35  is decremented by 1 at a clock timing of the write clock signal (WCLK) in response to one reading operation. The count value (CNT) of the up-down counter  35 , which indicates the number of effective data words, is output to the memory capacity monitoring section  36 . 
     Now, a count operation performed by the up-down counter  35  is described in detail with reference to FIGS. 8,  9 , and  10 . FIG. 8 shows an example of the count-up enable signal generation circuit  37 . FIG. 9 shows an example of the count-down enable signal generation circuit  38 . FIG. 10 is a timing chart showing signals in the FIFO memory control circuit  700 : (A) the read clock signal RCLK: (B) the least significant bit of the read address (RADR 0 ); (C) an output of a flip-flop  501 : (D) an output of a flip-flop  502 : (E) an output of EX-OR  503  (=count-down enable signal DNEN); (F) the read permission signal RE: (G) the write clock signal WCLK(=count clock signal FASTCLK); (H) the least significant bit of the write address (WADR 0 ); (I) an output of a flip-flop  401 ; (J) an output of a flip-flop  402 ; (K) an output of EX-OR  403  (=count-up enable signal UPEN); (L) the write permission signal WE: and (M) an output of the up-down counter  35 . 
     In the write address circuit  33 , while the write permission signal WE (segment (L) of FIG. 10) is asserted, the write address WADR is output to the count-up enable signal generation circuit  37  at a clock timing of the write clock signal RCLK (segment (G)). Every time the write address WADR varies, the value of the least significant bit of the write address (WADR 0 ) alternately changes between 0 and 1 as shown in segment (H) of FIG.  10 . 
     In the read address circuit  34 , while the read permission signal RE (segment (F) of FIG. 10) is asserted, the read address RADR is output to the count-down enable signal generation circuit  38  at a clock timing of the read clock signal RCLK (segment (A)). Every time the read address RADR varies, the value of the least significant bit of the read address (RADR 0 ) alternately changes between 0 and 1 as shown in segment (B) of FIG.  10 . 
     The count-up enable signal generation circuit  37  shown in FIG. 8 is formed by two flip-flops  401  and  402  and an EX-OR  403 . The flip-flop  401  receives the value of LSB of the write address (WADR 0 ) shown in segment (H) of FIG. 10 at the timing of the write clock signal WCLK (=count clock signal FASTCLK) shown in segment (G), and then outputs a signal as shown in segment (I). The flip-flop  402  receives the output of the flip-flop  401  (segment (I)) at the timing of the read clock signal RCLK (=FASTCLK) shown in segment (G), and then outputs a signal as shown in segment (J). The EX-OR  403  receives the output signal of the flip-flop  401  (segment (I)) and the output signal of the flip-flop  402  (segment (J)), and then outputs the exclusive-OR of these signals as a count-up enable signal UPEN (segment (K)). This count-up enable signal UPEN is in synchronization with the write clock signal WCLK, and is asserted in the up-down counter  35  for one cycle of the write clock signal WCLK in response to one writing operation. Accordingly, the up-down counter  35  increments the count value (CNT) once in response to one writing operation. 
     The count-down enable signal generation circuit  38  shown in FIG. 9 is formed by two flip-flops  501  and  502  and an EX-OR  503 . The flip-flop  501  receives the value of LSB of the read address (RADR 0 ) shown in segment (B) of FIG. 10 at the timing of the write clock signal WCLK (=count clock signal FASTCLK) shown in segment (G), and then outputs a signal as shown in segment (C). The flip-flop  502  receives the output of the flip-flop  501  (segment (C)) at the timing of the write clock signal WCLK (=FASTCLK) shown in segment (G), and then outputs a signal as shown in segment (D). The EX-OR  503  receives the output signal of the flip-flop  501  (segment (C)) and the output signal of the flip-flop  502  (segment (D)), and then outputs the exclusive-OR of these signals as a count-down enable signal DNEN (segment (E)). This count-down enable signal DNEN is synchronized with the write clock signal WCLK, and is asserted in the up-down counter  35  for one cycle of the write clock signal WCLK in response to one reading operation. Accordingly, the up-down counter  35  decrements the count value (CNT) once in response to one reading operation. As a result, count operations (incrementation/decrementation) of the up-down counter  35  based on the same clock signal (in embodiment 3, the write clock signal WCLK) can be accurately carried out without causing an error in conjunction with reading and writing operations in the memory  30 . 
     The memory capacity monitoring section  36  receives the count value (CNT) from the up-down counter  35 . When the count value (CNT) is 0, the memory capacity monitoring section  36  outputs to the read control section  32  an empty signal EMP which indicates that the memory  30  has no data to be read. When the count value (CNT) is equal to the number of words storable in the memory  30  (in embodiment 3, “5”), the memory capacity monitoring section  36  outputs to the write control section  31  a full signal FLL which indicates that the memory  30  has no more capacity to store data. 
     With the above structure, the amount of effective data (i.e., the number of effective data words) in the memory  30  can be correctly counted. Thus, when the full signal FLL is asserted in the write control section  31 , the write control section  31  prohibits writing data in the memory  30 , thereby preventing data from being lost by being overwritten. Furthermore, when the empty signal EMP is asserted in the read control section  32 , the read control section  32  prohibits reading data from the memory  30 , thereby preventing data from being read out twice. 
     In embodiments 1-3, the memories  10 ,  20 , and  30  each have a memory capacity of 5 words. However, the present invention is not limited to such a memory capacity but applicable to any memory capacity. Moreover, in embodiment 3, the write clock signal (WCLK) has a higher frequency than that of the read clock signal (RCLK). However, even in the case where the read clock signal (RCLK) has a higher frequency than that of the write clock signal (WCLK), the present invention can be carried out by using the read clock signal (RCLK) as a count clock signal (FASTCLK) in the up-down counter  35 . That is, according to the present invention, one of the write clock signal (WCLK) and the read clock signal (RCLK) which has the higher frequency is used as the count clock signal (FASTCLK). 
     According to embodiment 3, as shown in a FIFO memory control circuit  750  of FIG. 7B, both a signal line for the write clock signal (WCLK) and a signal line for the read clock signal (RCLK) may be connected to a terminal FASTCLK of each of the up-down counter  35 , the count-up enable signal generation circuit  37 , and the count-down enable signal generation circuit  38 . After the frequencies of the write clock signal (WCLK) and the read clock signal (RCLK) are measured by any method, one of these clock signals which has the higher frequency is supplied as a count clock signal (FASTCLK) to each of the up-down counter  35 , the count-up enable signal generation circuit  37 , and the count-down enable signal generation circuit  38 . In such a structure, even when a read clock signal (RCLK) and a write clock signal (WCLK) each have any frequency, count operations of the up-down counter  35  can be accurately carried out without causing an error in conjunction with reading and writing operations in the memory  30 . 
     As described hereinabove, in a FIFO memory control circuit according to the present invention, in the case where the frequencies of a read clock signal and a write clock signal are different, a permission signal which permits a count operation of an up-down counter is synchronized with one of the write clock signal and the read clock signal which has the higher frequency, and the permission signal is asserted for a time period which is equal to one cycle of the clock signal which has the higher frequency, whereby the amount of effective data in the memory can be correctly counted. As a result, data in the memory is prevented from being lost by being overwritten, and data in the memory is prevented from being read out twice. 
     Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.