MC68HC05L28 Datasheet, PDF (136/156 Pages) Motorola, Inc – Flexible general-purpose microcomputer

A.2.1 Single chip mode
This is the normal operating mode of the MC68HC705L28. In this mode the device functions as a
self-contained microcomputer (MCU) with all on-board peripherals, including two 8-bit I/O ports
and one 6-bit I/O port, available to the user. All address and data activity occurs within the MCU.
Single chip mode is entered on the rising edge of RESET if the voltage level on the IRQ0 pin is
within the normal operating range.
Warning: In the MC68HC705L28, all vectors are fetched from EPROM in single chip mode;
therefore, the EPROM must be programmed (via the bootloader mode) before the
device is powered up in single chip mode.
A.2.2 EPROM bootloader mode
This mode is used for programming the on-board EPROM. In bootloader mode the operation of
the device is the same as in single chip mode, except that the vectors are fetched from a reserved
area of ROM at locations $3FE4 to $3FEF, instead of the EPROM. The pin assignments are
identical to that of single chip mode shown earlier. A recommended programming circuit is shown
in Figure A-1.
Because the addresses in the MC68HC705L28 and the EPROM containing the user code are
incremented independently, it is essential that the data layout in the 27128 EPROM conforms
exactly to the MC68HC705L28 memory map. The bootloader uses an external 14-bit counter to
address the memory device containing the code to be copied. This counter requires a clock and a
reset function which are provided by the MC68HC705L28.
In this mode all interrupt vectors are mapped to pseudo-vectors in RAM (see Table A-2). This
allows programmers to use their own service routine addresses. Each pseudo-vector is allowed
three bytes of space, rather than the two bytes for normal vectors, because an explicit jump (JMP)
opcode is needed to cause the desired jump to the user’s service routine address.
The bootloader code deals with the copying of user code from an external EPROM into the on-chip
EPROM. The bootloader function can only be used with an external EPROM. The bootloader
performs a programming pass followed by a verify pass.
Pins PB2 and PB3 are used to select various bootloader functions, including the programming
mode (see Figure A-1). Two other pins, PB1 and PB6 are used to drive the PROG and VERF LED
outputs. While the EPROM is being programmed the PROG LED lights up; when programming is
complete the internal EPROM contents are compared to that of the external EPROM and, if they
match exactly, the VERF LED lights up. When finished programming, the PROG LEG turns off. If
the MC68HC705L28 memory contents are the same as the EPROM the VERF LED lights up,
otherwise no LEDs are turned on.
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MOTOROLA
A-2
MC68HC705L28
TPG
MC68HC05L28