MCU of the Day: ST7 Family: 68HC05 Reborn
8-bit 8MHz w/ ADC and 16K EPROM
One of the most well known microcontroller families is the Intel MCS-51. It was introduced in 1980 and is still being made in many many forms. It, however, is not the only popular 8-bit microcontroller. Motorola made many microcontroller versions of the famous 6800 CPU. Namely the MC6804 and the still in production 68HC05 series.
ST, which was formed by the merger of SGS of Italy, and Thomson of France, also makes a wide range of 8-bit microcontrollers which are very popular and widely used. The ST7 microcontroller is a 8-bit Von Neumann architecture (shared address/data bus) MCU. It is a serial accumulator design (so all operations occur in the accumulator, rather than in a wide set of registers like the 8051). The ST7 was introduced in the early 1990’s as an upgrade to the ST6 Family of microcontrollers. The ST7 added more high level programming support, and better interrupt handling. The ST7 provides higher performance then many competing architectures and in various performance tests such as IRQ handling, returns, instruction execution times etc, it even beats the venerable 80C51.
Both the ST6 and ST7 families are based on the Motorola MS680x microcontroller family. The ST6 closely resembles the 6804 and the ST7 is upwards code compatible with the MC68HC05 (assembly level translators exist to port the raw code). The ST7 has 63 instructions and the 6805 has 62 (depending on version). It is not an uncommon practice for one MCU to be based off, or even compatible with another. It provides more familiarity for programmers and design engineers. What really sets a MCU apart is the peripherals that surround the core, and its operating parameters. The ST6 and ST7 are both highly respected for their ESD protection and high noise immunity. These features were both carried over into the ST8M family that was introduced in 2008 to replace the aging ST7. The ST7 can be found in many applications such as automotive appliance control, motor control, and other embedded systems that most people forget are run by a processor.
