Could you program a microcontroller without a computer?

Originally, in programmable machines, the programs were manually punched into paper tape or cards. Later they used a kind of typewriter to do the punching, and even later still the computers (as they had come to be known by then) could punch their own paper tape.

It was, literally, sitting there with a card and manually making holes in it.

Each column is an instruction or bit of data that is interpreted by the CPU's state machine to perform tasks.

Now if course it's all flash memory, and yes, getting the data in there in the first place normally needs a computer to do it. But that's not strictly 100% true.

After all, what does a computer and hardware programmer do but set the state of IO pins at the right times? So, if you are really masochistic enough you can build a system which allows you to send the right instructions to the microcontroller's programming pins in the right way to install some instructions into the flash memory for execution.

Maybe something involving lots of switches, some shift registers, a clock generator, etc.

I remember at college we had a new-fangled Z80 based programming system - a unit the size of a suitcase with a Z80, some small amount of RAM, a bunch of 7-segment LED displays, and a big row of switches. Program it by manually loading instructions into RAM using the switches.

You said "small microcontroller and had no interfacing, no computer, no debugger, compiler, or assembler.". Well you can get by without the computer, debugger, compiler, or assembler, but you have to have some sort of interface to connect with the microcontroller in order to load the program.

First of all, forget about any high level languages like C. Then the easiest way is to write your program in assembly language, but don't use the assembler to translate it to machine code. Instead, look up each of the assembly instructions in the programming manual, find their equivalent hex coding, and write that down.

You will have to allocate your own variables, so you can fill in the appropriate addresses in the operand portion of the instructions. Also forget about using the stack and heap, too much work. When done, you'll have a machine program that can be loaded directly into the flash memory of the microcontroller -- no linker needed either.

Because of the lack of external address and data busses on almost all microcontrollers (since they would take up too many pins which are needed by I/O ports and peripherals), virtually all microcontrollers are programmed via a special interface like this:

Normally the "programmer" on the left would be connected to a PC, which would download a machine file generated by a compiler/linker.

However if the programmer has a keyboard, like the one below:

then one can enter the hex codes for a manually generated program right into the programmer, and program the device without needing a compiler, linker, or PC which was what you were inquiring about. (The keyboard is also used for editing hex files, and generating checksums.)

Granted very few programmers have keyboards like this. This one would not be suitable for a hobbyist as it probably costs a couple thousand dollars.

This particular programmer programs parts before they are put onto a board; interfaces and sockets are available for several PICs and also Atmel AVRs among others.

As a practical matter, something like toggling JTAG pins manually would be very error-prone. To input a program of any decent size would take thousands of toggles. But theoretically, it's possible.

If you're allowed to prepare the microcontroller in advance (or write a boot ROM for it), you could easily implement something like the external toggle switches used on early mainframes. If you allow external logic, you could even use a hex keypad to speed things up.

Finally, although it's not as common these days, you can buy a ROM-based microcontroller with custom code in it. You'd have to offer some money up front and guarantee a certain volume, but I've seen customers do it to save money on high-volume production. In that case, there would be no external signals needed to program the microcontroller at all. The code would be built into the physical layout of the chip.

Of course, you can also use a microcontroller that runs code out of an external memory interface, but that's cheating. :-)

Absolutely. In fact, the first microprocessor course I took (circa 87) used a Motorola 68000 board. We would work out the code in assembly, look up the hex for the assembly, and type the hex into a terminal to program the board. If you edited, you had to make sure the code you put in was shorter than the code you were replacing, and then buffer the rest with NOPs. If the coDE were longer, you would have to retype everything! We learned to add plenty of NOPs.

Absolutely. Once you have a binary format of the actual instructions (easy to do for well-specified instruction sets), all you need is to implement the programming protocol.

For example, take something small like an AVR Tiny 4. Section 14 details the programming interface including the protocol and physical layer. It's relatively simple and you just need a few buttons and pullup/down resistors to send/recieve a 1 or a 0. If you're feeling ambitious, add some LED's to read back the output.

The last remaining issue is if you can do it fast enough, which luckily* for the AVR Tiny 4 (section 16), it doesn't have any minimum frequency when it comes to programming it.

*note: as far as the datasheet specifies. This may differ in reality... I doubt anyone has tried programming it at mHz clock speeds.

Feel free to automate it as much/little of the process as desired. This is what most programming devices do anyways: automate the process.

By whatever means you input the program to the uC, you are "interfacing" with it, because that's what interfacing means. Even a bunch of switches you flip manually are an "interface".

So I will treat this question as "how do I interface to a uC with largely -mechanical- (as opposed to electronic) interfacing hardware, and in as cool a way as possible?"

To which my answer is, input the program as a piano roll. The piano roll "tracker bar" from a player piano basically gives you at least 65 (depending on format) bits that you can turn on and off independently based on perforations in the paper. The rest is exacting work with an exacto knife. But at least your program, once running, is (somewhat) permanently stored (unlike a flip-the-switches-manually approach).

Yes. The main issue may be interacting with the programming interface. If you have a plain address and data bus, things are a bit easier. Have you ever seen some of the really early computers? The programs are entered one instruction at a time into RAM with switches on the front.

Digital Equipment's PDP-8 mincomputers of 40+ years ago had no automatic booting ability. Programs were usually loaded from punched paper tapes but the loader to load the paper tapes had to be manually entered on the front panel switches.

At Digital and PDP-8 customers' sites such as OEMs who used them as the embedded-controllers of the day, it wasn't uncommon for users to memorize the boot loaders and compete with one another for the fastest time taken to enter the boot loader into the machine. The machines used magnetic-core memories (hand strung in far-flung parts of the world, by ladies with tiny fingers, using needles and hair-fine wire).

Since magnetic memory contents is non-volatile the boot loaders survived a power-down and manual entry was only needed after initial assembly, the loader was wiped out by a program bug, or a program needed to use the loader's space.

The answer is yes, you could program a micro controller without a computer,debugger, etc. What you would need would be to provide the various voltages, data, clock, and programming sequences specified by the uC manufacturer (not an easy job).

Not only is it possible to program a microcontroller without a computer, but you can actually purchase a computer today that is specifically designed to be programmed with hardware toggle switches. The kit is called a Membership Card, and it is a re-creation of the COSMAC Elf computer from the 1970's. The kit was described in a recent article in IEEE Spectrum magazine. It is also possible to program more recent versions of the Membership Card from a PC.