MC68HC05J3 Datasheet, PDF (18/92 Pages) Motorola, Inc – High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcomputer Unit

2.1.2 RAM bootloader mode
2
The RAM bootloader mode for the MC68HC05J3 allows the user to run a program in RAM. To
make use of this feature a circuit board should be constructed as shown in Figure 2-1. It is then
possible, by correctly configuring TCAP and PA3, to load a user program into RAM and then to
execute it.
1.8 V_DD
4.7 KΩ
10 MΩ
IRQ
OSC1
PA0
20 pF
V_DD
4.0 MHz
20 pF
OSC2
TCAP
VDD
PA3
100 KΩ
RESET
100 nF
VSS
MC68HC05J3
All resistors are 10 kΩ unless otherwise specified
RXD
V_DD
PA3
Function
0 Jump to RAM ($0081)
1 Load RAM & execute ($0081)
Figure 2-1 RAM bootloader circuit
The RAM bootloader is selected when the device is put into bootloader mode with TCAP held high.
If PA3 is low, the program counter is set to $0081 and a previously loaded RAM program can be
executed. If PA3 is high at reset a program is serially loaded from PA0 into the RAM and executed
from $0081, once the last byte has been received.
The first byte to be loaded is the count byte which must contain the total number of bytes to be
transferred, including the count byte itself. Therefore, for a program length of $30, the count should
equal $31. The maximum program size including the count byte is 124 bytes ($7C), since four
bytes must be left for the stack during download.
MOTOROLA
2-2
MODES OF OPERATION AND PIN DESCRIPTIONS
TPG
MC68HC05J3