A.5 Using HC11 in Bootstrap Mode

If the input lines MODA and MODB are grounded during RESET, HC11 enters a special operating mode, called bootstrap. In bootstrap mode, the microcontroller executes a program, called bootloader, located in a small ROM, invisible in the memory map in normal modes.

The bootloader allows user programs to be loaded into the MCU RAM, via the serial communication interface SCI, and, when the transmission completes, the user program is automatically launched.

In principle, the user program loaded this way can use any of the MCU resources, but the most common use of the bootstrap mode is for writing the CONFIG register and EEPROM constants. Some members of the HC11 family have internal EPROM or OTPROM (One Time Programmable ROM), which can also be programmed in bootstrap mode.

The first action of the bootloader is to send a break (a long zero) character on the serial line. If the answer of the host is another break, then the bootloader passes the control to a program located in the EEPROM, by executing an unconditional jump to the first address of the EEPROM. If the host sends a $FF, this is used by the bootloader of 68HC11E9 to determine the baud rate, by selecting one of the two possible baud rates (1200 or 7812 baud). Note that other versions of HC11 allow different baud rates. See the specific data sheets for details.

Once the baud rate is selected, the bootloader starts receiving the binary characters...