Abstract:
A device for changing a frequency of an internal control clock for testing a chip, by incorporating a mode selection circuit (30) and a high voltage detection circuit (40) in a serial input/output memory. The mode selection circuit (30) is connected between two selected adjacent circuits C n-2 , C n-1  among a plurality of frequency conversion circuits C 1  . . . C n , for accessing selectively either a clock pulse CP n-2  from the frequency conversion circuit C n-2 , arranged in front thereof or a system clock XSK, in dependence upon an internal voltage sense signal IV, IVB. The high voltage detection circuit (40) transmits the internal voltage sense signal to the mode selection circuit (30) by detecting a level of externally applied voltage XV. The internal control clock ICK provided by this device attains a period of T XSK  ×2 n-M+1 , wherein &#34;M&#34; is a number of the counter receiving the mode selection signals MS, MSB next to the mode selection circuit.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a serial input/output memory, and more particularly, to a device for increasing transmission speed of test bits in a test operation in the serial input/output memory using a serial clock applied from an exterior source as an internal clock, by converting a frequency of the serial clock. 
     Generally, it is well known that a serial transmission method is useful in transmission between a system such as a central processing unit for processing data in parallel and a system such as an auxiliary unit for processing data in series. A transmitter sends each bit of the data separately and a receiver assembles the bits in order to reconstruct the data upon reception. For example, the receiver completes a word after receiving eight bits sequentially, in the case where one word comprises eight bits. 
     An universal asynchronous receiver and transmitter (hereinafter referred to as &#34;UART&#34;) is widely used as such an interface device. A transmission method using the UART is called an UART protocol method. 
     In the UART protocol method, input data is transmitted to a memory core through a plurality of shift registers; and likewise output data is transmitted through a plurality of shift registers to an external side of an accessed memory, to be received as data bits in serial format (hereinafter referred to as a &#34;serial block memory&#34;). Such shift registers for transmitting data are controlled by an internal clock at various frequencies based on a serial clock (called a &#34;system clock XSX  &#34;). 
     In conventional serial input and output memory devices, test time is delayed unnecessarily because the internal control clock (which has a frequency that is 1/512 times the system clock) is passed through nine counters prior to use in testing character and access functions of a chip. 
     SUMMARY OF THE INVENTION 
     It is one object of the current invention to provide an improved serial input/output memory device. 
     It is another object to provide a circuit for reducing the testing time of a chip in a serial input/output memory. 
     It is still another object to provide a circuit enabling selective access to either a clock pulse generated by a frequency conversion circuit, and a system clock signal. 
     It is yet another object to provide a circuit enabling access to selected ones of a plurality of related frequencies, based on a single system clock frequency. 
     In accordance with the present invention, a serial input/output memory comprises a plurality of serially connected frequency conversion circuits using a system clock of a given frequency to generate an internal control clock signal, a mode selection circuit and a high voltage detection circuit. The mode selection circuit is connected between two adjacent frequency conversion circuits among the plurality of frequency conversion circuits, for selectively accessing either a clock pulse from the frequency conversion circuit arranged in front thereof or the system clock signal, in dependence upon an internal voltage sense signal. The high voltage detection circuit is employed to transmit the internal voltage sense signal to the mode selection circuit by detecting a level of an externally applied voltage. 
     The novel features are believed to have the characteristic of reducing the time required for testing a chip in a serial input/output memory as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention itself, as well as other features and advantages thereof, will best be understood by reference to the following detailed description of a particular embodiment, read in connection with the accompanying drawings, wherein: 
     FIG. 1 is a block diagram showing a conventional configuration of a device for data input; 
     FIG. 2 is a graph showing timing of test performance according to the conventional configuration as shown in FIG. 1; 
     FIG. 3 is a block diagram showing a configuration of a device constructed according to the principles of the present invention; 
     FIG. 4A is a schematic of the mode selection circuit of FIG. 3; 
     FIG. 4B is a schematic of the high voltage detection circuit of FIG. 3; 
     FIG. 4C is a schematic of an alternative embodiment of the high voltage detection circuit of FIG. 3; 
     FIG. 5 is a graph of timing data showing test performance of FIG. 3; and 
     FIG. 6 is a block diagram showing an alternative to the configuration shown in FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the FIGS. 1 and 2, a description on how to access a received data bit in a conventional serial input and output memory such as via a shift register stage 1 is given. 
     The system clock XSK received from an external side is transformed through a frequency dividing circuit using a given number &#34;n&#34; of counters C 1  . . . C n  to the internal control clock ICK. For example, nine counters are required for configuring the frequency dividing circuit so as to communicate at 9600 bps (bits per second) while using a system clock of 4.9152 megaHertz. The number nine is a value of &#34;n&#34; in the equation 1/9600=2 n  ÷(4.9152 megaHertz) 
     It takes 1/9600=0.104 milliseconds to transmit one bit, because 9600 bits are transmitted per second. It takes 1.04 milliseconds to transmit a ten bit word comprising eight data bits, one extra stop bit and one extra parity bit. Accordingly, it takes 0.42598 seconds to read data comprising 4096 (512 words×8 bits per word) bits. The internal control clock ICK obtained from the ninth counter represented in FIGS. 1 and 2 has a frequency f XSK  of 1/512 or (1/2 9 ) times that of the system clock XSK, and one data bit is accessed in a trailing edge of the internal control clock at an input shift register stage 1. 
     In the above conventional device of FIG. 1, there is a problem that a test time is delayed unnecessarily because the internal control clock used in testing character and access functions of a chip (which has a frequency f ICK  that is 1/512 times the frequency f XSK  of the system clock XSK) is obtained by passing the system clock XSK through the nine counters C 1  . . . C n . 
     Referring now to FIG. 3, a high speed test device has a mode selection circuit 30 connected between the counters C n-2  and C n-1  among &#34;n&#34; counters C 1  . . . C n  each connected in series. The mode selection circuit 30 divides the frequency f xsk  of a system clock XSK applied thereto from an external source (not shown). A high voltage detection circuit 40 connected to the mode selection circuit 30, supplies an internal sensing voltage IV, IVB (B indicates an inverted signal) to the mode selection circuit 30. 
     The mode selection circuit receives clock pulses CP n-2 , CP n-2B  from a preceding counter C n-2  and system clock signals XSK, XSKB directly from input terminal 20. Mode selection signals MS, MSB are transmitted from the mode selection circuit 30 to the next counter C n-1 . 
     A desirable configuration of the mode selection circuit 30 is illustrated in FIG. 4A. The signals clock system XSK, XSKB are transmitted to NAND gate 31 and, via inverter I 2  to NAND gate 34 respectively. The internal sensing voltage IV, IVB is transmitted to NAND gate 31, and NAND gates 32 and 33 respectively, via inverter I 1 , and the clock pulses CP n-2 , CP n-2B  are transmitted to NAND gate 32, and via inverter I 3 , to NAND gate 33. 
     A mode selection signal MS is output by NAND gate 35 in response to outputs from NAND gates 31 and 32, and a complementary mode selection signal MSB is generated by NAND gate 36 in response to outputs from NAND gates 33, 34. 
     The mode selection circuit 30 is configured so that the mode selection signals MS, MSB correspond to either the system clock XSK, XSKB, or to the clock pulses CP n-2 , CP n-2B  from the preceding counter C n-2  based upon the internal sensing voltages IV, IVB. Accordingly, it is noted that a mode selection circuit satisfying the requirements of this invention can also be configured in similar fashion with respect to selected other counters. 
     Desirable alternative embodiments of the high voltage detection circuit 40 are illustrated in FIGS. 4B and 4C. 
     In FIG. 4B, the high voltage detection circuit 40 comprises NMOS transistors 41, 42, 43 having electrodes connected in source follower type configurations between a terminal 49 for an externally applied voltage source XV and a sensing node 45, and a resistance 44 connected between the sensing node 45 and a terminal for a ground voltage V SS . 
     The internal sensing voltage IV, IVB is derived from the sensing node 45. The internal sensing voltage IV becomes an electric potential of &#34;high&#34; state or &#34;low&#34; state according to the externally applied voltage XV. For example, if an external voltage XV of more than ten volts is supplied, the internal sensing voltage IV has an electric potential, at node 45, of a &#34;high&#34; state owing to the sensing node 45 being connected to the external voltage XV through the NMOS transistors 41, 42, 43. When the external voltage XV has a level of a CMOS operating voltage however, the internal sensing voltage IV has the electric potential of a &#34;low&#34; state owing to the sensing node 45 being connected through the resistance 44 to the ground voltage V SS  at terminal 50. Sensing node 45 is coupled to mode sensing circuit 30 via inverter I 4 . 
     FIG. 4C shows a high voltage detection circuit with the resistance 44 replaced with a MOS transistor 48. PMOS transistor 47 is connected between a source of the NMOS transistor 43 having gate and drain electrodes coupled in a source follower type configuration and the sensing node 45; a NMOS transistor 46 is connectable to a voltage source V CC  so as to maintain an electric potential of V CC  -V TH  at the source of the NMOS transistor 43, thus resulting in a reduction of electric current consumption and stabilization of an electric potential at the sensing node 45. The PMOS transistor 47 controls a current flow to the sensing node 45 according to a signal φ applied to a gate of PMOS transistor 47. The circuit of FIG. 4C is more desirable than the circuit of FIG. 4B, because the electric potential of the sensing node 45 is triggered by the inverters as is shown in FIG. 4C. 
     FIG. 5 more specifically describes operation of the present invention. When the external voltage XV is more than ten volts, the electric potential of the sensing node 45 of the high voltage detection circuit 40 is raised so as to place the internal sensing voltage IV in a logic &#34;high&#34; state. Accordingly, the NAND gates 32, 33 of the mode selection circuit 30 are disabled (output logic states of NAND gates 32, 33 are fixed in a logic &#34;high&#34; state) so as to prevent transmission of clock pulses CP n-2 , CP n-2B  of the counter C n-2 . Therefore, an output signal responding to the system clock XSK is generated from the NAND gates 31, 34, 35 and 36. Conversely, when the electric potential of node 45 is in logic low state, NAND gate 31, 34 are disabled, thereby enabling NAND gates 35, 36 to transmit counter clock pulses CP n-2  and CP n-2B  received via NAND gates 32, 33. 
     These output signals which are the mode selection signals MS, MSB, are applied to the next counter C n-1 . The frequency of the mode selection signals MS, MSB are reduced to 1/128 of the conventional clock pulse frequency of 4/9152 megaHertz, assuming that the mode selection circuit is placed between the 7th and 8th counters, because the mode selection signal MS has the same frequency as the system clock XSK of 4.9152 megaHertz at the time, as is shown in FIG. 4A. Thereafter, the mode selection signal having the same frequency as the system clock passes through the next two counters (the 8th and 9th counters), with the result that the frequency of the internal control clock ICK of f ICK  is equal to [4.9152×1/4] megaHertz results. In other words, the mode selection signals MS, MSB have a frequency of 4.9152 megaHertz, that is, of four times the frequency f ICK  of the internal control clock ICK when NAND gates 32, 33 are disabled (i.e., when node 45 is in a &#34;high&#34;  logic state). Mode selection signals MS, MSB have a period when NAND gates 32, 33 are disabled, that is 128 times greater than the conventional ICK of period of frequency 4.9152×512 megaHertz. 
     In the conventional art, the internal control clock ICK has a frequency of 1/[T XSK  ×2 n  ] (i.e., &#34;T XSK  &#34; is a period of the system clock and &#34;n&#34; is the number of counters C 1  . . . C n ), while the internal control clock according to the present invention has a period of T XSK  ×2 n-M+1 , wherein &#34;M&#34; is a number of the counter receiving the mode selection signal MS next to the mode selection circuit). If the mode selection circuit 30 is between the 7th and 8th counters, the value of M is 8. The transmission time of the data bit is reduced by the associated amount, that is T XSK  ×(2 n  -2 n-M+1 ), because the internal control clock ICK with the period of 1/2 M+1  of the conventional access period is used. It may be seen therefore, that circuits constructed according to the foregoing principles enable the test time of chip to be reduced by the rejection of a period of an internal control clock for controlling a data bit transmission. 
     In FIG. 6, mode selection circuit 30 may be connected between the counters C n-1  and C n  of the serial array of &#34;n&#34; counters C 1  . . . C n . The mode selection circuit 30 divides the frequency f xsk  ÷2 of a system clock XSK applied to counter C 1  from an external source (not shown). High voltage detection circuit 40 connected to the mode selection circuit 30, supplies an internal sensing voltage IV, IVB. Mode selection circuit 30 receives clock pulses CP n-1 , CP n-1B  from a preceding counter C n-1  and clock pulses CP 1 , CP 1B  from first counter C 1  ; clock pulses CP 1 , CP 1B  have frequencies equal to one half of the frequency f XSK  of system clock signals XSK, XSKB. Mode selection signal MS, MSB is transmitted from the mode selection circuit 30 to the next counter C n . Accordingly, circuits constructed according to the foregoing principles illustrated in FIG. 6 also enable the test time of chip to be reduced by the rejection of a period of an internal control clock for controlling a data bit transmission. 
     Although the invention has been described with reference to the specific embodiment, this description is not meant to be construed in a limiting sense, as other embodiments of the invention will become apparent to person skilled in the art upon reference to the foregoing description of the invention. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.