JPH0375956A - Single chip microcomputer - Google Patents

Abstract

PURPOSE:To reduce the amount of hardware to reduce the circuit scale by using an output control register and an output data register as a program memory or data memory bank designating register at the time of extending the memory on the outside. CONSTITUTION:When the memory extension mode is designated by a memory extension designating flag 13, a switch 12 always outputs '1', and an output buffer 3 is always in the turn-on state. Consequently, the output signal of a selector 9 is always outputted to an external terminal 2. An output control register 5 functions as the program memory bank register at the time of designating the memory extension mode, and write due to execution of the instruction of a CPU 14 is inhibited. At the time of external program data fetch, the CPU 14 outputs a program fetch signal to a control line 11, and the selector 9 outputs the value of the register 5 to an output buffer 3. An output data holding register 6 functions as the data memory bank register, and the CPU 14 outputs the access command of the external data memory to the selector 9, and the selector 9 outputs the value of the register 6 to the buffer 3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a single-chip microcomputer having an external memory expansion function, and particularly to a memory expansion means for expanding a large capacity of memory.

[Conventional technology]

BACKGROUND ART Many microcomputers whose memory can be expanded externally have been developed and used in various fields. As application systems using these devices become more multi-functional and become cheaper, the amount of data handled by microcomputers tends to increase, and how to process this large amount of data quickly and efficiently has become a major problem. It's coming.

Here, a conventional large-capacity memory expansion method for a microcomputer will be explained with reference to FIGS. 3, 4, and 5.

FIG. 3 is a block diagram of an example of a system configuration when expanding a large capacity memory of a microcomputer. An address bus 3 extending outside the microcomputer 1. Each program memory A21 . Program memory B22. Data memory A23. Data memory B2
4 is connected. The decoder 20 receives address data (A16 to A1) output from the microcomputer 1.
9) and store it in one of the four memories mentioned above.
Select one. In the case of FIG. 3, since 4 bits AI6 to A19 are decoded, a maximum of 16 memory banks can be set, but an even larger number of memory banks can be set by increasing the number of bits to be decoded.

That is, each memory can be associated with each memory bank as shown in FIG. 4, and which memory bank is selected is determined by the address output data of AI6 to A19 (or higher order) of the microcomputer 1. Ru. To specify the bank of the externally connected memory in this way, address output data of A16 or higher is used in the case of bytes for Panchromatic 4, but there are various methods for outputting this address data (memory bank specification means). be.

In single-chip microcomputers,
In order to ensure a large number of valid terminals, address output terminals for specifying memory banks and general-purpose port terminals are often shared.

The memory bank designation method when the address output terminal and the general-purpose port terminal are shared will be described below using FIG.

FIG. 5 is a detailed block diagram of one bit of an address output terminal for specifying a memory bank of a microcomputer that shares an address output terminal and a 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 that controls whether or not the value stored in the output data register 6 is output to the terminal 2.

The program memory bank register 7 is a register for specifying a bank of external program memory. C
The program counter in PU14 increments,
When a carry occurs, the contents of this register are automatically incremented by 1. In addition, a branch instruction is executed in CPU], 4,
When a carry or borrow occurs when the offset value is added to or subtracted from the contents of the program counter, the contents of this register are automatically incremented by +1 or -1.

Data memory bank register 8 is a register for specifying a bank of external data memory. Data writing to this register is performed by the CPU 14.

When the CPU 14 increments the program counter and fetches program data, the control line 11 is
A program fetch signal is output to the selector 9 via the selector 9. When an external memory access command is issued, a memory access signal is output to the selector 9 via the control line 10.

Memory expansion designation flag] 3 is a sink that specifies whether or not to expand the memory externally, and is controlled by the CPU 14.

When memory expansion mode is specified by the memory expansion specification sink 13, the output buffer 3 is set by the switch 12.
is always ONL, and the value of either the program bank register 7 or the data memory bank register 8 is output to the external terminal 2.

Furthermore, when memory expansion is not specified in the memory expansion mode specification register 3 (port mode specified), the output buffer 3 is turned on only when output mode is specified in the output control register 5.

Selector 9 is output data register 6. Select one of the values stored in the program memory bank register 7 and data memory bank register 8 and output it to the output buffer 3.
This is what is output to. When memory expansion mode is specified by the memory expansion designation flag 13, when a program fetch signal from the CPU 14 is input, the value stored in the program memory bank register 7 is
When a memory access signal from the PU 14 is input, the value stored in the data memory bank register 8 is output to the output buffer 3.

As mentioned above, in the conventional microcomputer, the output control register 5. It has independent hardware that functions as a general-purpose port for the output data register 6 and hardware that functions during memory bank expansion of the program memory bank register 7 and data memory bank register 8.

[Problem to be solved by the invention]

The conventional microcomputers with memory expansion functions mentioned above had hardware for general-purpose ports and hardware for memory bank expansion independently built-in.
There was a drawback that the circuit scale became large.

[Means to solve the problem]

The present invention includes an output data holding register, an external terminal, an output control register that controls output of data in the output data holding register to the external terminal, and a central processing unit, and the operation of the general-purpose input/output port is controlled. In a single-chip microcomputer, the single-chip microcomputer has a selection signal for selecting and operating one of the operations for externally extending memory, and further uses a set of external terminals for the two operations, the selection means When an operation to perform external memory expansion is selected, the output data holding register and the output control register exclusively receive the program fetch signal and program counter carry and borrow signals generated by the central processing unit, respectively. , specifying banks of external program memory, and
It is controlled by a memory access signal generated by the central processing unit and specifies a bank of external data memory.

〔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 a first embodiment of the present invention.

The microcomputer in this embodiment also shares a general-purpose port function terminal and an address output terminal for specifying a memory bank, as in the conventional example. The constituent elements will be explained below.

The memory expansion designation flag 13 is a flag that is turned ON when expanding the memory externally, as in the conventional example, and a value is written by the CPU 14.

When the memory expansion specification flag 13 is "'l", the external terminal 2 functions as an address data output terminal for specifying a memory bank, and when the memory expansion specification flag 13 is "0'", the external terminal 2 functions as a general-purpose Functions as a port terminal. Output control register 5 and output data register 6 can also function as general-purpose ports.
This register also operates as an address output function for specifying a memory bank.

Data is written to the output data register 6 by the CPU 14.
is carried out via the internal bus 4. Also, output control register 5
Writing of data to the memory is automatically performed by an instruction included in the CPU 14 or by the hardware of the CPU 14, depending on the designation of the memory expansion designation flag 13.

The switch 12 is set according to the value of the memory expansion designation flag 13.
It controls on/off of the output buffer 3. When the memory expansion mode is designated by the memory expansion designation flag 13, the switch 12 always continues to output '1'' and turns on the output buffer 3. Further, when the port 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.

Selector 9 selects the value of memory expansion designation flag 13 and CP
Memory access signal output by U14.

Each control signal line 10 for the program fetch signal.
11, a value is stored in the output control register 5 or output data register 6, and a certain value is selected and output.

Note that the CPU 14 sends the memory access signal and the program fetch signal to the control line 10. only when the memory expansion mode is specified by the memory expansion designation flag 13.
It is output to the selector 9 via the control line 11.

A case where the external terminal 2 is made to function as a general-purpose port function and a case where it is made to function as an address output terminal for specifying a memory bank will be explained below using the above-mentioned components.

First, a 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, output buffer 3 is turned on or off depending on the value of output control register 5. Further, when the selector 9 receives a port mode signal from the memory expansion designation flag 13, it always outputs the value of the output data register 6 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 performs on/off control of the output buffer 3. The above is a case where the external terminal 2 operates as a general-purpose port terminal.

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

When memory expansion mode is specified by the memory expansion designation flag 13, the switch 12 always outputs 1111+.
The output buffer 3 is always on.

Therefore, the output signal of selector 9 is always output to terminal 2.

The output control register 5 functions as a program memory bank register when memory expansion mode is specified. That is,
Writing by executing instructions of the CPU 14 is prohibited, and the value of this register increases or decreases due to carry or borrow of the program counter in the CPU 14. When external program data is fetched, the CPU 14 outputs a program fetch signal to the control line 11, so the selector 9 outputs the value of the output control register 5 to the output buffer 3 to specify the bank of the program memory.

Further, the output data register 6 functions as a data memory bank register in the memory expansion mode. This register specifies the bank of the data memory by the CPU 14. Set the values for this in advance. When the CPU 14 executes an external data memory access command,
The CPU 14 outputs a memory access signal to the control line 10. When the selector 9 receives a memory access signal via the control line 10, it 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 can specify the banks of the program memory and data memory. Do this.

As described above, in the embodiment of Mizui 1, the output control register 5 and the output data register 6, which conventionally operated only as a general-purpose port function, are given the function as a register for specifying a bank for memory expansion. .

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

Similarly to the first embodiment, the microcomputer in the second embodiment also shares a general-purpose port function terminal and an address output terminal for specifying a memory bank.

Further, the constituent elements are also the same as those in the first embodiment.

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

In the memory expansion mode, the output control register 5 serves as a pointer that specifies a bank of data memory.

Further, the output data register 6 serves as a register for specifying a bank of the program memory. That is, the two registers function in the opposite way to the first embodiment.

In the second embodiment, the CPU 4 outputs only a program fetch signal to the selector 9 and does not output a 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 specifying the bank of the data memory, to the terminal 2, and inputs the program fetch signal via the control line 11. Only then is the value of the output data register 6, that is, the value for specifying the bank of the program memory, outputted to the terminal 2.

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

〔Effect of the invention〕

As explained above, when the microcomputer of the present invention expands memory externally, the output control register and the output data register, which conventionally function only as general-purpose ports, are used to specify a program memory bank or a data memory bank. It is also used as a register.

Therefore, since there is no need to separately provide a program memory bank register and a data memory bank register, there is an effect that the amount of hardware can be significantly reduced.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a microcomputer according to a first embodiment of the present invention, FIG. 2 is a block diagram of a microcomputer according to a second embodiment of the present invention, and FIG. 3 is an external memory expansion of a conventional microcomputer. FIG. 4 is a conceptual diagram of the memory banks 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, 1] ...Program fetch signal, 12...Switch, 13...
Memory expansion specification flag, 14...CPU, 20...
...decoder, 21. ．． 22...Program memory, 23゜24...Data memory.

Claims (1)

[Claims] It has an output data holding register, an external terminal, an output control register that controls the output of the data in the output data holding register to the external terminal, and a central processing unit, and has memory expansion to the outside depending on the operation of the general-purpose input/output port. In a single-chip microcomputer, the single-chip microcomputer has a selection means for selecting and operating one of the operations, and further uses a set of external terminals for the two operations,
When the operation of external memory expansion is selected by the selection means, the output data holding register and the output control register exclusively use the program fetch signal and program counter carry generated by the central processing unit, respectively. , a single-chip microcomputer that is controlled by a borrow signal to specify a bank of an external program memory, and is controlled by a memory access signal generated by the central processing unit to specify a bank of an external data memory.