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The 68HC12 (6812 or HC12 for short) is a microcontroller family from Freescale Semiconductor. Originally introduced in the mid-1990s, the architecture is an enhancement of the Freescale 68HC11. Programs written for the HC11 are usually compatible with the HC12, which has a few extra instructions. The first 68HC12 derivatives had a maximum bus speed of 8 MHz and flash memory sizes up to 128 kB.

Architecture of the 68HC12

Like the 68HC11, the 68HC12 has 2 8-bit accumulators A and B (referred to as a single 16-bit accumulator, D, when A & B are cascaded so as to allow for operations involving 16 bits), 2 16-bit registers X and Y, a 16-bit program counter, a 16-bit stack pointer and an 8 bit Condition Code Register.

HCS12/MC9S12 derivatives

Beginning in 2000 the family was extended with the introduction of the MC9S12 derivatives which have bus speeds of up to 25 MHz and flash sizes up to 512 kB.

The Mbit/s controller.

MC9S12X derivatives

The MC9S12XDP512 which was introduced in 2004 has a bus speed of 40 MHz and a peripheral co-processor known as the XGATE which allows for some tasks to be offloaded from the CPU. The CPU of the S12X derivative also features several new instructions to increase performance.

Freescale announced the EEPROM. HCS12 products contain a single processor, the HCS12X feature the additional XGATE peripheral processor.

The S12X family offer two main methods to address more than 64KBytes.

  • Paged memory regions in the 64KByte local map: PPAGE for paged program data, RPAGE for paged RAM, EPAGE for paged EEPROM/flash
  • Global addressing permits access to any addresses in the 8 MByte address space. GPAGE is used in conjunction with special opcodes. (gldaa etc.)


The XGATE co-processor is a 16-bit RISC processor operating at twice the main bus clock. It offloads work from the S12X core by handling interrupts only and does not run a background loop. The first versions of the XGATE do not allow for higher priority interrupts to pre-empt a currently handled interrupt, but the "XGATEV3" as featured in the 9S12XEP100 (and others) does allow this. The S12X can trigger software interrupts on the XGATE core and vice-versa. A semaphore system is implemented to allow the S12X and XGATE cores to synchronize access to peripherals.

Typically the XGATE code is copied to RAM at device startup and then executed from RAM for a speed benefit. The XGATE has a partial 64KByte address space with no paging. The registers share addresses, but the flash and ram appear at different addresses between the cores. (See the datasheet for more details.)

External links

  • Free real-time kernel source code for HCS12 with sample apps
  • Freescale's official OpenTCP project on SourceForge for MC9S12NE family
  • DRAGON12-Plus HCS12/9S12 Trainer, EVB and Development Board
  • MiniIDE development environment that works with both 68HC11 and 68HC12
  • 68HC12 Discussion Group
  • 68HC12 Development and training system
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