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7
Configuring the Board
7.1
Programming and Testing Configuration (USB Cable Attached)—Default Configuration
7.2
Programming Configuration (USB Cable Attached)
AUX
Using external
3.3V supply
Using only USB
power supply
7.3
Testing Configuration from a Saved Configuration (with USB Cable Removed After
Configuring the Board
The CDCE421EVM can be powered from the USB power supply or from an external source. The
CDCE421EVM only needs a USB cable attached for programming purposes; however, for test
measurements it is recommended to also use an external 3.3V power supply. Test measurements can
also be taken with only the USB supplied power. Note that because of USB power variance, results may
degrade. It is also possible to program the CDCE421 and then disconnect the USB cable with minor board
configuration changes. To enable power and programming of any of the four blocks on the
CDCE421EVM, the respective switch must be turned on
as explained earlier.
By default, the CDCE421EVM is configured to operate with the USB cable attached and a 3.3V power
supply added to AUX VDD and GND. In this configuration, the USB microcontroller is powered by the USB
port 5V supply while the CDCE421 is powered by the 3.3V external supply. This setup is optimal for
programming the CDCE421 device while also taking measurements. This configuration removes the
power variations inherent with USB power supplies by isolating the CDCE421 from the USB supply.
The CDCE421EVM can also use power supplied through the USB cable as its sole power source.
However, because of power-supply variance in the USB supply, this configuration is not recommended for
most measurements. Users are advised to enable this setup for saving configuration settings to the
CDCE421 and later powering the device from its internal memory. (This option is useful if there is not a
USB port available in a lab or test area.) In this configuration, JP1 must be moved from its default position
to the new position shown in
Figure 8
. The Enable onboard power checkbox must be selected on the
EVM software, followed by pushing the Apply button (see
Figure 5
).
Figure 8. JP1 Setting for USB Programming Configuration
Programming)
When operating the CDCE421 without the USB programming cable (useful in lab settings where a PC is
not immediately available at a lab station), the CDCE421EVM must be preprogrammed in one of the two
configurations discussed above, and then reconfigured for external power supply usage.
Before making these board modifications, the CDCE421 settings must be saved with one of the two USB
cable attached configurations. The Write Chronos Settings to EEPROM NO LOCKING software button
must be used to save the CDCE421 settings to its internal EEPROM. After the settings are saved, the
USB cable can be removed. Once the cable has been disconnected, JP1 should be in the Using External
3.3V Supply position (as shown in
Figure 8
). The EVM is now ready for use without the USB cable
connected. The CDCE421 will always start from its saved configuration state in this mode.
If the CDCE421 must be isolated from the microcontroller, the switch corresponding to the block in use
should be set to OFF for CE_x, SDATA_x, and LVPECL_TERM_x where x represents the block name
(see
Figure 9
). The power line switch for the respective block, however, should remain on. This sequence
also allows the user to remove the USB cable without affecting performance while the CDCE421 is
powered on.
SCAU020 – March 2007
10.9MHz–1175MHz Low Phase Noise Clock Evaluation Board
13
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