Electronic – ROM Output order

A counter is just a state machine that advances from one state to the next on each clock edge. Normally, you use logic to determine the next state from the current state to create, for example, binary counters, BCD counters or Gray-code counters.

But there's no reason that the next-state logic couldn't be replaced by a ROM. The ROM would be addressed by the state flip-flops, and the output of the ROM would be the next state of the flip-flops. This allows you to create a sequence of states of any length, up to 2^N, where N is the number of flip-flops. The states can occur in any order, as long as each state has a unique next state.

The way you describe is a valid one, at least it's one we use in our projects. First we flash the whole program which contains an initial set of values, so the program will run fine. During calibration the serial number (UUID) of the device will be set, along with the other calibration factors. For this we reserve a flash page at the end of the flash, so an update of the program is possible without erasing the calibration constants.

For this to work, of course, you need some way of communicating with the controller and a program part which will do the writing to the flash. If your product is not designed to have that, you have to inject that data during the initial flashing. For that you can change the hex file on the fly for each (the format is rather easy) or use capabilities of the flasher to include an automatically incremented serial number or something like that. The capabilities of the flashers are also quite different - for example, Segger produces some with quite good production programming capabilities.

If you just need a way to identify a device without a requirement for what that UUID has to look like, microcontrollers often have some device info in them, among others an ID. Whether that is usable has to be determined for every microcontroller type you use, as there is no standard that I am aware of.

For example the STM32F401 (a device I'm currently working with) has a 96-bit Unique Device ID which can be read by the CPU and with JTAG. (more details in the reference manual)