JP2522063B2 - Single-chip micro computer - Google Patents

Description

The present invention relates to a single chip microcomputer having a function of expanding a memory to the outside, and more particularly to a memory expanding means for expanding a large capacity memory.

[Conventional technology]

Conventionally, many microcomputers whose memories can be expanded to the outside have been developed and used in various fields. Multi-functionalization of application system using these,
As the price becomes lower, the amount of data handled by the microcomputer tends to increase, and how to process such a large amount of data at high speed and efficiently becomes a big problem.

Here, a conventional large-capacity memory expansion method for a microcomputer will be described with reference to FIG. 3, FIG. 4, and FIG.

FIG. 3 is a block diagram of a system configuration example when a large capacity memory of a microcomputer is expanded. A program memory A21, a program memory B22, a data memory A23, and a data memory B24 are connected to an address bus 3 and a data bus 4, which are extended outside the microcomputer 1, respectively. The decoder 20 decodes the value of the address data (A16 to A19) output from the microcomputer 1 and selects any one of the above four memories. In the case of Fig. 3, since 4 bits of A16 to A19 are decoded, the maximum
16 memory banks can be set, but more memory banks can be set by increasing the number of decoding bits.

That is, as shown in FIG. 4, each memory can be associated with each memory bank, and which memory bank is selected is determined by the address output data of A16 to A19 (or higher) of the microcomputer 1. It In this way, for the bank designation of the externally connected memory, address output data of A16 or more is used for one bank of 64 Kbytes, but there are various methods for outputting this address data (memory bank designation means). .

In a single-chip microcomputer or the like, in order to secure a large number of effective terminals, the address output terminal for specifying a memory bank and the general-purpose port terminal are usually shared.

Hereinafter, the memory bank designation method when the address output terminal and the general-purpose port terminal are shared will be described with reference to FIG.

FIG. 5 is a detailed block diagram of one bit of the address output terminal for designating the memory bank of the microcomputer which shares the address output terminal and the general-purpose port terminal.

The output data register 6 in the microcomputer 1 is an output data latch when the external terminal 2 functions as a general-purpose output port. The output control register 5 is a register for controlling whether to output the value stored in the output data register 6 to the terminal 2.

The program memory bank register 7 is a register for designating a bank of an external program memory.
When the program counter in the CPU 14 increments and a carry occurs, the content of this register is automatically incremented by 1. If a carry or borrow occurs when a branch instruction is executed in the CPU 14 and the offset value is added to or subtracted from the content of the program counter, the content of this register is automatically incremented by +1 or -1.

The data memory bank register 8 is a register for designating a bank of an external data memory. CPU14 writes data to this register.

CPU14 increments the program counter,
When fetching program data, a program fetch signal is output to the selector 9 via the control line 11.
When an external memory access instruction is issued, the control line
The memory access signal is output to the selector 9 via 10.

The memory expansion designation flag 13 is a flag that specifies whether or not to externally expand the memory, and is controlled by the CPU 14. When the memory expansion mode is specified by the memory expansion specification flag 13, the output buffer 3 is always turned ON by the switch 12.
Then, the value of either the program bank register 7 or the data memory bank register 8 is output to the external terminal 2. When the memory expansion mode designation register 13 does not specify the memory expansion (when the port mode is specified), the output buffer 3 is turned on only when the output control register 5 specifies the output mode.

The selector 9 selects one of the values stored in the output data register 6, the program memory bank register 7, and the data memory bank register 8 and outputs it to the output buffer 3. When the memory expansion mode is specified by the memory expansion specification flag 13, when the program fetch signal from the CPU 14 is input, the value stored in the program memory bank register 7 is input, and when the memory access signal from the CPU 14 is input. , The value stored in the data memory register 8 is output to the output buffer 3.

As described above, in the conventional microcomputer,
It independently has hardware that functions as a general-purpose port of the output control register 5 and the output data register 6, and hardware that functions when the memory bank of the program memory bank register 7 and the data memory bank register 8 is expanded.

[Problems to be Solved by the Invention]

In the above-described conventional microcomputer having the memory expansion function, the general-purpose port hardware and the memory bank expansion hardware are independently incorporated, so that the circuit scale becomes large.

[Means for solving the problem]

The present invention provides an output data holding register, an external terminal,
It has an output control register for controlling the external terminal output of the data in the output data holding register, and a central processing unit, and the operation of the general-purpose input / output port selects one of the operations for externally expanding the memory. In a single-chip microcomputer that has a selection signal to be operated in addition, and further uses a pair of external terminals for the two operations, when the operation for expanding the memory to the outside is selected by the selecting means. The output data holding register and the output control register are controlled exclusively by the program fetch signal and the carry and borrow signals of the program counter generated by the central processing unit, respectively, and specify the bank of the external program memory. Controlled by a memory access signal generated by the central processing unit, It is that to specify the bank of Tamemori.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of a microcomputer having a memory expansion function according to the first embodiment of the present invention.

In the microcomputer of this embodiment, the general-purpose port function terminal and the address output terminal for specifying the memory bank are shared as in the conventional example. The components will be described below.

The memory expansion designation flag 13 is a flag that is turned on when the memory is expanded to the outside as in the conventional example, and a value is written by the CPU 14.

When the memory expansion specifying flag 13 is "1", the external terminal 2 functions as a memory bank specifying address data output terminal, and when the memory expansion specifying flag 13 is "0", the external terminal 2 functions as a general-purpose port terminal. Function. The output control register 5 and the output data register 6 are registers that operate as both a general-purpose port function and an address output function for specifying a memory bank. Data writing to the output data register 6 is performed by the CPU 14 via the internal bus 4. Further, the data writing to the output control register 5 is either an instruction included in the CPU 14 or the CPU 14 according to the designation of the memory expansion designation flag 13.
Hardware will do it automatically.

The switch 12 controls ON / OFF of the output buffer 3 according to the value of the memory expansion designation flag 13. When the memory expansion mode is designated by the memory expansion designation flag 13, the switch 12 always outputs "1" and turns on the output buffer 3. When the port expansion mode is designated by the memory expansion designation flag 13, the output buffer 3 is turned on according to the value of the output control register 5.

The selector 9 has the value of the memory expansion designation flag 13 and the CPU 1
The value stored in the output control register 5 or the output data register 6 is selected and output by the control signal lines 10 and 11 of the memory access signal and the program fetch signal output from the memory 4.

The CPU 14 outputs the memory access signal and the program fetch signal to the selector 9 via the control line 10 and the control line 11, respectively, only when the memory expansion mode is designated by the memory expansion designation flag 13.

Hereinafter, a case where the external terminal 2 operates as a general-purpose port function and a case where the external terminal 2 functions as an address output terminal for designating a memory bank will be described using the above-described components.

First, the case where the external terminal 2 operates as a general-purpose port terminal will be described. When the port mode is designated by the memory expansion designation flag 13, the switch 12 connects the output control register 5 and the output buffer 3. Therefore, on / off of the output buffer 3 is selected according to the value of the output control register 5. The selector 9 is a memory expansion designation flag.
When the port mode signal is received from 13, the value of the output data register 6 is always output to the output buffer 3.

As described above, in the port mode, the value of the output data register 6 is always output to the output buffer 3, and the output control register 5 controls ON / OFF of the output buffer 3. The above is the case where the external terminal 2 operates as a general-purpose port terminal.

Next, the operation when external terminal 2 functions as an address output terminal for specifying a memory bank will be described.

When the memory expansion mode is designated by the memory expansion designation flag 13, the switch 12 always outputs "1", and the output buffer 3 is always on. Therefore, the output signal of the selector 9 is always output to the terminal 2.

The output control register 5 functions as a program memory bank register when the memory expansion mode is designated. That is, writing by executing an instruction of the CPU 14 is prohibited, and the value of this register is increased or decreased by the carry or borrow of the program counter in the CPU 14. At the time of external program data fetch, the CPU 14 outputs a program fetch signal to the control line 11, so that the selector 9 outputs the value of the output control register 5 to the output buffer 3 so as to specify the bank of the program memory.

In the memory expansion mode, the output data register 6
Functions as a data memory bank register. A value for designating the bank of the data memory is preset in this register by the CPU 14. When the CPU 14 executes an external data memory access instruction, the CPU 14 outputs a memory access signal to the control line 10. Selector 9
When receiving the memory access signal via the control line 10, outputs the value of the output data register 6 to the output buffer 3.

Therefore, in the memory expansion mode, the output control register 5 functions as a program memory bank register and the output data register 6 functions as a data memory bank register, so that the output data of the terminal 2 designates the bank of the program memory and the data memory. Do.

As described above, in the first embodiment, the output control register 5 and the output data register 6, which conventionally operate only as a function of a general-purpose port, have a function as a bank designation register for memory expansion. .

FIG. 2 is a block diagram of a microcomputer having a memory expansion function according to the second embodiment of the present invention.

The microcomputer according to the second embodiment also shares the general-purpose port function terminal and the address output terminal for designating the memory bank, as in the first embodiment. The constituent elements are also the same as those in the first embodiment.

However, in the second embodiment, the operations of the output control register 5, output data register 6, and selector 9 in the memory expansion mode are different.

In the memory expansion mode, the output control register 5 serves as a pointer designating a bank of data memory. The output data register 6 serves as a register for designating a bank of the program memory. That is, the two registers function oppositely to those in the first embodiment.

Further, in the second embodiment, the CPU 14 outputs only the program fetch signal to the selector 9 and does not output the memory access signal. Therefore, in the memory expansion mode, the selector 9 normally outputs the value of the output control register 5, that is, the value for designating the bank of the data memory to the terminal 2.
Only when the program fetch signal is input through the control line 11, the value of the output data register 6, that is, the value for designating the bank of the program memory and the value are output to the terminal 2.

In the second embodiment described above, the output control register 5 serves as a bank designation register of the data memory, and the output data register 6 serves as a bank designation register of the program memory.

〔The invention's effect〕

As described above, in the microcomputer of the present invention, when the memory is expanded to the outside, the output control register and the output data register, which conventionally function only as a general-purpose port, are used to specify the bank of the program memory or the bank of the data memory. It is used as a register at the same time.

Therefore, it is not necessary to separately provide the program memory bank register and the data memory bank register, which has the effect of significantly reducing the amount of hardware.

[Brief description of drawings]

FIG. 1 is a block diagram of a microcomputer of a first embodiment of the present invention, FIG. 2 is a block diagram of a microcomputer of a second embodiment of the present invention, and FIG. 3 is an external memory expansion of a conventional microcomputer. FIG. 4 is a system configuration diagram at that time, FIG. 4 is a conceptual diagram of a memory bank of each external memory in FIG. 3, and FIG. 5 is a block diagram of a conventional microcomputer. 1 ... Microcomputer, 2 ... External terminal, 3 ...
Output buffer, 4 ... Internal bus, 5 ... Output control register, 6 ... Output data register, 7 ... Program memory bank register, 8 ... Data memory bank register, 9 ... Selector, 10 ... Memory access signal , 11 ...
… Program fetch signal, 12 …… Switch, 13 …… Memory expansion designation flag, 14 …… CPU, 20 …… Decoder, 21,
22 …… Program memory, 23,24 …… Data memory.

Claims (1)

(57) [Claims] 1. An output data holding register, an external terminal,
It has an output control register for controlling the external terminal output of the data in the output data holding register, and a central processing unit, and selects either the operation of the general-purpose input / output port or the operation of expanding the memory to the outside. In a single-chip microcomputer which has a selecting means for operating the same and further uses a pair of external terminals for the two operations, when the selecting means selects an operation for external memory expansion. In addition, the output data holding register and the output control register are controlled exclusively by the program fetch signal and the carry and borrow signals of the program counter generated by the central processing unit, respectively, and specify the bank of the external program memory. Controlled by a memory access signal generated by the central processing unit, Single-chip micro-computer, characterized in that to specify the bank of Tamemori.