Advances in microcontrollers — a potted viewpoint

I wrote this piece to explain to myself the advances in microcontroller technology before I restarted my electronics making hobby with a project using a retro MCU, the 8042, from the 8048 family. It was originally embedded in the writeup of the project but I think it stands on its own well enough.

Semiconductor logic technology

MCUs of the 8048 era used NMOS logic. This was already an improvement over PMOS logic which required higher voltages so interfacing with TTL had become easier. However NMOS logic has now been supplanted by CMOS logic which contributes to some of the advantages below.

Power consumption

MCUs of that era drew tens of milliamps. This obviously limited battery operation. Nowadays MCUs can draw microamps in quiescent mode. So battery and solar cell operation become possible. Cue the rise of IoT devices. This partly offsets concerns about the power consumption of lots of these devices in consumer hands.

Power handing

The MCUs of the past could only drive a small number of TTL unit loads, so interface chips were needed if you wanted to drive loads. Now MCUs are capable of driving higher currents of the order of tens of milliamps per pin and can be connected directly to displays, saving chips.

Clock rate

Clock rates of that era were around 1-10 MHz. Now MCUs can easily be clocked 5-10 times faster.

Clock drive

Clock drive inputs were already getting simpler. The 8080 microprocessor required a complicated set of clock signals, usually provided by an auxiliary chip, the 8224. The 8085 simplified that by taking that clock circuit on chip.

Later on, for non-timing critical applications, the quartz crystal could be replaced by a lower cost ceramic resonator, or even an RC circuit. Today there are chips that have the RC components on chip which saves another pin or two and the designer doesn't even have to think about the clock.

Program storage

In that era, programs were stored in mask ROM or EPROM. Mask ROM was only for mass production, so testing and small runs had to be done with EPROM. So either one had to connect an external EPROM (see Pin count below) or the MCU had to have a quartz window for UV erasure, increasing chip price. If you were quite sure the program worked, you could use an OTP version that was programmed in the same way, but only once.

The first advance was moving to EEPROM. This meant that the program memory could be reprogrammed electrically, eliminating the quartz window. Then the next step, combining the EEPROM with the MCU contributed to advantages below.

Greater integration

One advantage MCUs had over microprocessors was that peripherals were integrated on the same chip. However this ran up against the number of package pins. It was not unusual to multiplex the buses to the external memory and peripherals to conserve pins. Or use expansion chips like the 8243. With EEPROM and peripherals on chip, the buses no longer needed to be exposed to the outside world. This means that more pins can be dedicated to peripherals. Or you could put MCUs in packages with fewer pins, even as low as 8. At the other end of the spectrum, smaller pin footprints of SMD ICs allow more pins to the outside world if desired.

CPU program and data bit width

Now that the EEPROM is on chip, there no longer needs to be a relationship between the width of the program memory and the data memory. For instance the PIC program words are 12 bits and other weird sizes. This assumes a Harvard computer architecture where the program and data stores are separate.

Glue technologies

These days instead of using discrete logic ICs to interface MCUs to other devices, one can use CPLDs or an even older technology, GALs. These are reprogrammable, sometimes even in situ, allowing you to correct logic errors without circuit board rework.

However, CPLDs cater more to professional products which are transitioning to 3.3V or lower so there are less options for connecting with 5V logic and also the packaging won't be DIP, but PLCC or SMD.

Some hobbyists have cunningly used MCUs like AVRs as glue logic. One recent Z80 SBC used an AVR to provide program memory and interfaces. Yes, it does feel strange to use a more powerful micro as a slave to an old one.

Rise of serial protocols

Another way to conserve pins and interconnections is to transfer data serially. Nowadays many peripherals are interfaced with serial protocols reducing lines compared to parallel protocols. Protocols like I2C, SPI, and custom ones are used. This is assisted by the increased clock rate of MCUs so transfer time isn't an issue. Sometimes the chip hardware or instruction set has assists for these protocols.

Rise of modules

With standard protocols, manufacturers could make modules to simplify design. Instead of, for example, designing drive circuitry for an array of seven-segment LEDs, one can get a module which incorporates the LEDs, multiplexing, current limiting, brightness control, and even keyboard input. One example is the TM1637 chip found on many cheap 4 digit LED displays. There are only 4 pins to connect, 2 power, clock and data.

You see usage of this in this clock where I have reduced the hassle of interfacing seven-segment LEDs with all the attendant drive transistors and resistors by using a cheap off-the-shelf serial driven display board. But this requires writing bit-banging code and ensuring that the MCU has enough processor cycles.

The whole concept of modules and specific implementations like Arduino shields makes life easy for the experimenter. Less soldering is required.

Toolchains

As MCU architectures became more sophisticated, they became better targets for higher level languages like C. The architectural conventions of C mean that you could not support C on an 8048 or 8042. On the 8051 family, you can. The increase in sizes of program memory and clock speed mean that it's no longer as crucial to program in assembler and one can afford to "waste" memory and CPU power in return for being able to write clearer code and larger programs. The Arduino ecosystem allows experimenters to connect their computer to the board, then edit a sketch, compile and test in a tight loop.

Improvements in software tools means that it has become easier to write compilers; it's no longer an arcane art. The open source movement makes more candidates to choose from and community participation makes the software better.

Construction techniques

While this is not directly related to MCUs, construction choices are different now. The breadboard is still useful, but for production, you design PCBs with CAD tools and send the plot files over the Internet for fabrication. No need for the old-school resist pens, or photoresist and exposure masks, and noxious chemicals to prepare, etch, and clean the board. Other steps like silk screen printing, solder masks, and drilling are thrown in. The factory can even presolder standard SMD components, or custom components you provide, for a price of course.