The minimalist 6502 system uses a CPU and not much else
A central processing unit, or CPU, is the heart of any computer system. But that's certainly not the only part: you also need RAM, ROM and at least a few peripherals to make it a complete system that can actually do something useful. Modern microcontrollers usually have some or all of these functions built into a single chip, but classic CPUs don't: they were meant to be put on motherboards with dozens of other chips. That's why [c0pperdragon]'s latest project, the SingleBreadboardComputer, is such an amazing design: to assist its 6502 processor, there are only four associated chips.
The whole system only takes one solderless breadboard strip. Next to the CPU we find 32 KB of SRAM, 32 KB of flash and a clock oscillator. The fifth chip is a 74HC00 quad two-input NAND gate, which is used as a very small piece of glue logic to connect everything together. Two of its NAND gates are used for address decoding logic, allowing the ROM or RAM chip to be selected based on the state of the CPU's A15 line as well as blocking RAM during the low clock phase system. This last function is necessary because the address lines are not guaranteed to be stable during the low phase and can cause writes to random memory locations.
The remaining two NAND gates are connected as an RS flip-flop to implement a serial output. This is necessary because the processor cannot keep its outputs in the same state for several clock cycles, which is required for a serial port. Instead, [c0pperdragon] uses the MLB pin, normally used to implement multiprocessor systems, to generate two-clock pulses and stores the state in the flip-flop for as long as needed. A few well-timed software routines can then be used to transmit and receive serial data without additional hardware.
Currently the only software for this system is a simple demo that sends data back to its serial port, but if you fancy a challenge you can write programs to do just about anything. You could probably find inspiration in other minimalist 6502 boards, or projects that emulate a full motherboard in an FPGA.
A central processing unit, or CPU, is the heart of any computer system. But that's certainly not the only part: you also need RAM, ROM and at least a few peripherals to make it a complete system that can actually do something useful. Modern microcontrollers usually have some or all of these functions built into a single chip, but classic CPUs don't: they were meant to be put on motherboards with dozens of other chips. That's why [c0pperdragon]'s latest project, the SingleBreadboardComputer, is such an amazing design: to assist its 6502 processor, there are only four associated chips.
The whole system only takes one solderless breadboard strip. Next to the CPU we find 32 KB of SRAM, 32 KB of flash and a clock oscillator. The fifth chip is a 74HC00 quad two-input NAND gate, which is used as a very small piece of glue logic to connect everything together. Two of its NAND gates are used for address decoding logic, allowing the ROM or RAM chip to be selected based on the state of the CPU's A15 line as well as blocking RAM during the low clock phase system. This last function is necessary because the address lines are not guaranteed to be stable during the low phase and can cause writes to random memory locations.
The remaining two NAND gates are connected as an RS flip-flop to implement a serial output. This is necessary because the processor cannot keep its outputs in the same state for several clock cycles, which is required for a serial port. Instead, [c0pperdragon] uses the MLB pin, normally used to implement multiprocessor systems, to generate two-clock pulses and stores the state in the flip-flop for as long as needed. A few well-timed software routines can then be used to transmit and receive serial data without additional hardware.
Currently the only software for this system is a simple demo that sends data back to its serial port, but if you fancy a challenge you can write programs to do just about anything. You could probably find inspiration in other minimalist 6502 boards, or projects that emulate a full motherboard in an FPGA.
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