I'm trying to build a MON08 interface to program a 68HC908QB8, as per page 13 of this PDF:

Low-Cost Programming and Debugging Options for M68HC08 MCUs

Original MON08 interface by NXP (Freescale)

Below is my schematic, from which I have created a PCB:


I cannot get the Freescale Bootload software to connect, irrespective of any combination of the two switches. It attempts to look for a reset ACK signal, but times out:


Granted this is ageing technology now, but do USB to RS-232 converters run on a different voltage to regular RS-232 interfaces? I believe RS-232 signals are +/-12V? This would determine some of the resistor values in the circuit. If this is the case, would it be worth trying a PCIe-based RS-232 interface?

Am I missing anything in my schematic, compared to the schematic in the PDF?

  • 1
    \$\begingroup\$ Is there a missing VDD voltage regulator (like 78L05)? \$\endgroup\$
    – glen_geek
    Nov 12, 2019 at 20:59
  • 1
    \$\begingroup\$ PC serial ports were +/- 12V interfaces. Modern USB serial interfaces use charge pumps to get about +/- 5V. Perhaps a direct 5V TTL level USB UART interface had been easier to interface. \$\endgroup\$
    – Justme
    Nov 12, 2019 at 22:08

1 Answer 1


the AN2295 program is not the one used to connect to the monitor that comes in the HC08 families. You have to use the flash programmer that comes with the CodeWarrior IDE version 6.3 to connect correctly.(hiwave.exe)


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