Freescale 68HC08

The 68HC08 is an 8 -bit microcontroller (MCU ) from Freescale Semiconductor. Freescale Semiconductor is the semiconductor division of Motorola, it was spun off in 2004. In addition to the original 68HC08 microcontroller further variants have arisen, so that one now speaks of a HC08 family.

The program code of its predecessor the 68HC05 family is upward compatible with the HC08, which facilitates migration.

  • 3.1 Index Registers
  • 3.2 Status of CCR
  • 3.3 another CPU register

Architecture

The 68HC08 is constructed according to the von Neumann architecture. In the Von Neumann architecture, the memory area is different from the Harvard architecture, are not separated on the program code and data. There are therefore no separate buses needed, but the Von Neumann architecture is slower compared to Harvard architecture, it can not be accessed at the same program code and data.

The CPU

The HC08 has as its core the 6800 microprocessor ( CPU), not to be confused with the 68000 CPU, which forms the core of the 32-bit microcontroller family from Freescale.

Arithmetic instructions

  • Addition with (ADC ) and without transfer ( ADD)
  • Subtraction (SBC ) and without carry (SUB )
  • Fast 16 * 8 unsigned division (DIV )
  • Fast 8 * 8 unsigned multiplication (MUL )

BCD arithmetic

To perform Binary Coded Decimal arithmetic applications, it is the Decimal Adjust Accumulator (DAA ) and the Nible Swap Accumulator (NSA ) command.

High-level language support

The index registers, the stack pointer, the 8-bit jump instructions and the associated commands allow the use of Hochsprachenkompilern. One is no longer forced to program the microcontroller assembly language.

Low-power support

To reduce the power consumption of the CPU, the WAIT and the STOP command may be used. With the WAIT command, the clock generator to the CPU is stopped with the STOP command additional peripheral clock generators can be stopped.

The CPU registers

Index register

The index register is 16 bits wide and allows the addressing of up to 64 KB memory, the composite register is designated H: X, wherein the lower byte is labeled X, and the upper byte, the term H. The H: X only identical to the I: X-register the M6805 family when H = O, and affect any commands to the upper byte. 16, various methods can be used for addressing. The index register can also be used short term for data storage.

Status register CCR

The 8 -bit status register CCR. In the status register, the status of the interrupt mask and the overflow, half -carry, Negative, Zero and Carry- Borrow flags are stored.

More CPU registers

  • 8- bit accumulator A
  • 16- bit program counter PC
  • 16-bit stack pointer SP

Operating voltages

The HC08 microcontroller can be operated with different operating voltages, but decreases with the voltage, the bus clocking. At an operating voltage of 5 V, the bus operates at a clock rate of 8 MHz, with a 3 V supply voltage with only 4 MHz. The microcontroller HCS08 subfamily working with an operating voltage of 1.8 V.

Versions of the 68HC08

HC08 microcontroller family are available in a wide range. In brackets, the choices range from an 8-pin DIP packages up to a 64-pin LQFP package, the user can choose between different types and next number of analog / digital converter or different ways of communication peripherals. The table below shows some properties of currently available HC08 subgroups (September 2010).

Then there are the RS08 family based in principle on a stripped- HC08. RS08 the MCU can not process address stack and the status register has only carry and Zeroflagbits. Its memory space is limited to 16 KB. The development HCS08 already mentioned has focused its low power consumption, it has several modes to reduce consumption.

Availability

From the microcontrollers of the HC08 family, despite their age, many variants are still available because they especially can be used cost in industrial applications. Some variants have been discontinued but the part of the manufacturer and it is not recommended to use these variants in large-scale production.

Hitachi HD6301

Hitachi possessed with the HD6301 and HD6303 on the microcontroller based on the design of the 6800 CPU and could be used as a second source products. The instruction set of the Hitachi microcontroller has been extended by a number of commands which the functionality enlarged. Meanwhile, these microcontrollers are available but neither Hitachi nor in the now outsourced semiconductor division Renesas Electronics.

Software Development

For software development stands next to the CodeWarrior IDE and the proprietary Small Device C Compiler and the SDCC UCSIM simulator available. Meanwhile, with the helium - OS, a real-time operating system for use in RS08.

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