D E M O 9 0 8 L B 8
M A R C H 2 4 , 2 0 0 5
12
Figure 6: PWR_SEL Jumper Settings
1
• •
2
• •
Board powered from external input connected to J1-1 (
+
) and
J1-3 (-). +5V Maximum input voltage.
1
• •
2
• •
Board powered from external input connected to TB1 Terminal
Block. J1-1 is open or not connected.
1
• •
2
• •
Board powered from external input connected to TB1 Terminal
Block. Board pr5V output at pin J1 for use by external
circuits.
NOTE:
Power output from the board is limited by the 250 mA maximum output
of the on-board voltage regulator.
NOTE:
External circuit connected to J1 should limit current flow to less than
50 mA to prevent current-limiting the power supply (VR1).
RESET SWITCH
The RESET switch provides a method to apply an asynchronous RESET to the MCU and is
connected directly to the RST* input pin. Pressing the switch asserts RESET forcing the MCU
into a start-up condition. A pull-up bias resistor on the RST* input allows normal MCU opera-
tion. Shunt capacitance ensures an adequate input pulse width.
LOW VOLTAGE INHIBIT
The MC68HC908LB8 applies an internal Low Voltage Inhibit (LVI) protect against under-
voltage conditions. The LVI is enabled out of RESET. Consult the MC68HC908LB8 reference
manual for details on configuring LVI operation.
TIMING
The MCU provides an internal 16MHz timing source out of RESET. By default, the internal
timing source exhibits a 25% tolerance. The OSCTRIM register allows trimming the internal
timing source to ±5% of nominal. The default frequency supports an internal bus frequency of
4MHz. The user must configure the MCU to use the external clock input by writing the CON-
FIG2 configuration register. The MCU may be configured to output the internal oscillator fre-
quency on the OSC2 pin. See MCU datasheet for configuration details.
A 9.8304 MHz clock oscillator at X1 provides alternate timing to the DEMO908LB8. The clock
oscillator exhibits a 100-ppm frequency tolerance. The external clock oscillator produces a
2.46 MHz internal operating frequency. An option jumper at OSC_EN allows the user to en-
able or disable the external clock oscillator input.
