Gary Boone, a Texas Instruments (TI) engineer, developed the first microcontroller due to boredom and family issues while working on calculator chips. The first calculator chips drastically reduced the number of integrated circuits needed to build a working calculator. Initially, customers were resistant to the idea of buying similar calculator chips differentiated only by some bits in an on-chip ROM. The first microcontroller, the TMS1802NC, combined a CPU, RAM, ROM, and I/O circuits on one chip. TI announced the TMS1802NC in 1971, predating Intel’s first microprocessor. The TMS1802 was used in calculators like the Sinclair Executive and paved the way for microcontroller development.
https://www.eejournal.com/article/a-history-of-early-microcontrollers-part-1-calculator-chips-came-first/