MC68HC05L28 Datasheet, PDF (26/156 Pages) Motorola, Inc – Flexible general-purpose microcomputer

2.1.2 MC68HC705L28 modes of operation
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2.1.2.1 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 2-2.
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 2-4). 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 2-2). 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.
Note: The EPROM must be erased before performing a program cycle.
2.1.2.2 RAM bootloader mode
See Section 2.1.1.2.
MOTOROLA
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MODES OF OPERATION AND PIN DESCRIPTIONS
TPG
MC68HC05L28