Freescale Semiconductor MPC5748G EVB, Руководство пользователя, Страница: 15 / 88

Initial Configuration
MPC5748G EVB User Guide, User Guide, Rev. 0, 08/2015
Freescale Semiconductor, Inc. 15
5.3.1. External Clock Input (P7)
The external clock input on the EVB is applied via SMA connector P7. When driving an external clock
into the SMA connector, the jumpers on the daughtercard must be reconfigured to route the external
clock to the MCU.
Note that the following conditions must be met when supplying an external clock:

The clock frequency must be between 8MHz and 40MHz

The amplitude of the clock input should not exceed the voltage being driven into the
VDD_HV_A pins. This is selectable between 3.3V and 5.0V on the daughtercard.
5.3.2. MCU Clock Configuration (J1, J2 on Daughtercard)
There are two external clock crystals on the MPC5748G daughtercards:

40MHz fast external crystal for clocking the main system clock

32KHz slow external crystal for accurate time of day keeping
The 40MHz crystal is connected to the MCU XTAL and EXTAL pins via 3-way jumper headers as
shown in the diagram above. These jumpers allow an external clock to be routed from the SMA
connector (P7) on the main board if desired. The default configuration is with both daughtercard
jumpers (J1 and J2) set to position 1-2 which routes the external 40MHz crystal to the MCU pins. If you
wish to supply a clock via the SMA connector on the main EVB, move the daughtercard jumpers J1 and
J2 to position 2-3.
The 32 KHz external crystal is permanently connected to the MCU EXTAL32 and XTAL32 pins and
has no configuration options.
Table 8. EXTAL Clock Source Selection (J1, J2 Daughtercard)
Jumper Position PCB Legend Description
J1 (XTAL)
1-2 (D)
Y1 MCU XTAL signal is routed to crystal Y1
2-3
GND
MCU XTAL signal is Grounded (for ext clock mode)
4
J2 (EXTAL)
1-2 (D)
Y1 MCU EXTAL signal is routed to crystal Y1
2-3
EXT MCU EXTAL signal is routed from EVB SMA P7
4
Note that the XTAL pin is left open by default with J1 in position 2-3. Resistor R34 must be populated with a zero
ohm resistor in order to ground the XTAL pin.
The external SMA
clock connector is
located in the top left
corner of the EVB