TWR-56F8400 User’s Manual
Page 16 of 35
2.4
OSBDM/OSJTAG
2.4.1
Debug Interface
An on-board MC9S08JM60 based Open Source BDM (OSBDM/OSJTAG) circuit provides a debug
interface to the MC56F84789. A standard USB A male to Mini-B male cable (supplied) can be used for
debugging via the USB connector, J18.
2.4.2
USB Serial Bridge Interface
The on-board MC9S08JM60 can also be used as a USB to Serial bridge interface for the UART signals
from the MC56F84789 DSC. This bridge circuit is described in detail in section 2.3.2 Serial I/O Source
Select Headers.
The RXD_SEL signal goes to the MC56F84789 DSC. The USB bridge chip is powered by 5V so its output
is a 5V output. The buffer (U505) is able to accept the 5V signal from the USB bridge chip (T_TXD1) and
converts it to the 3.3V signal (RXD_SEL) for the DSC. The buffer output is enabled by an inverted RTS
signal (TXD_RXD_EN_B) from the USB bridge chip. If there is no USB connection to the TWR board the
RTS signal is not driven and the 3.3V powered inverter (U502C) input is biased low disabling the output
of the buffer.
In a similar way TXD_SEL is a 3.3V signal from the MC56F84789 DSC. The USB bridge chip is expecting a
5V input on T_RXD1. The buffer between these two signals (U504C) is powered by P3_3V/5V. It will
accept the 3.3V input from the DSC and convert it to the 5V signal needed by the USB bridge chip. The
buffer output is enabled by the same inverted RTS signal (TXD_RXD_EN_B) discussed above. If there is
no USB connection to the TWR board, the RTS signal is not driven and the 5V powered buffer disabled
so nothing is driving the powered down USB bridge chip.
The serial interface signals from the MC56F84789 DSC may be routed to the MC9S08JM60 serial
interface via header and Berg straps. Using the USB serial bridge the MC9S08JM60 will convert the
serial interface data into USB packets and send them to the host PC where they may be handled by a
PC application normally conversant with a serial port.
2.4.3
Clocking the OSBDM/OSJTAG MCU (MC9S08JM60)
The MC9S08JM60 MCU uses an on board 4 MHz external crystal circuit (Y2, R16, C7, and C9) for its
clock. There are no user options for clocking the MC9S08JM60.
2.4.4
Reserved Function Select Header
Header J20 selects whether the on-board MC9S08JM60 MCU operates as an OSBDM/OSJTAG debug
interface or as a USB Serial Bridge interface on
older
versions of S08 firmware such as may have existed
