Motorola M68EVB912B32, User Manual

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HARDWARE REFERENCE
68EVB912B32UM/D 4-6
4.6 CLOCK CIRCUITRY
The EVB comes with a 16-MHz crystal, Y1, with appropriate startup capacitors. The board
should be able to accommodate most crystals and ceramic resonators.
Header W16 may be used to disconnect Y1 from the MCU’s on-chip oscillator. An external
clock may then be supplied to EXTAL through W16.
4.7 RESET
The reset circuit includes a pull-up resistor, reset switch (S1), and a low-voltage inhibit device
with a toggle voltage of 3.0 Vdc. This reset circuit drives the MCU’s RESET* pin directly.
Note that header W15 may be used to provide an alternate reset input.
4.8 LOW-VOLTAGE INHIBIT
Low voltage inhibition (LVI) uses a Motorola undervoltage sensing device (U1) to automatically
drive the MCU’s RESET* pin low when Vdd falls below U1's threshold. This prevents the
accidental corruption of EEPROM data if the power-supply voltage should drop below the
allowable level.
Depending on the date of manufacture, the sensing device installed on the EVB may have either a
2.7-volt or 4.5-volt threshold. U1 may be identified by part number:
MC34164P-3

2.7 Vdc
MC34164P-5

4.5 Vdc
If operation below U1's threshold (but no less than 2.7 Vdc) is required, one of two methods can
be used:
1. Replace U1 with a device that has the required threshold voltage.
2. Cut the trace on header W15 to disconnect U1 from the RESET* line. If this is done,
an external reset signal should be provided via W15 in case the supply voltage falls
below the acceptable level.
4.9 BACKGROUND DEBUG MODE (BDM) INTERFACE
The MCU’s serial BDM interface can be accessed through two 2x3 headers, BDM IN (W9) and
BDM OUT (W12). The pin assignments are shown in Table 4-3.