Clocks, Decoupling for power supplies, Miscellaneous – Cypress West Bridge Astoria AN46860 User Manual
Page 3: About the author
AN46860
December 12, 2008
Document No. 001-46860 Rev. *A
3
Clocks
1. Ensure that the XTALSLC[1:0] pin levels correspond to
the frequency of the signal at XTALIN and XTALOUT.
2. Leave the XTALOUT floating if an external clock source
is used.
3. Clock or crystal characteristics must conform to the
requirements specified in the data sheet.
4. The design must adhere to the power supply noise
specifications for the PLL specified in the data sheet.
5. XVDDQ is the select pin for crystal and clock. XVDDQ
must be 3.3V when using a crystal. XVDDQ must be
1.8V when using a clock source as an input.
lists the various clock selection input settings.
Table 5. Clock Selection Input Settings
XTALSLC[1]
XTALSLC[0]
Clock
Frequency
Crystal
Support
0
0
19.2 MHz
Yes
0
1
24 MHz
Yes
1
0
48 MHz
No
1
1
26 MHz
Yes
Decoupling for Power Supplies
1. VDD requires 2.2 µF and 0.1 µF decoupling.
2. Although AVDDQ is tied to the same supply as VDD,
route it separately with 0.01 µF and 0.1 µF capacitors.
3. UVDDQ requires 2.2 µF and 0.1 µF decoupling.
4. GVDDQ, PVDDQ, SSVDDQ, SNVDDQ, and XVDDQ
do not have any specific decoupling requirements.
Combine them with the decoupling for other supplies at
the same level. If in doubt, use 2.2 µF and 0.1 µF.
Miscellaneous
All unused output-only pins may be left floating, but do not
leave unused input-only and input/output pins floating. Tie
the unused input-only and input/output to a valid logic level
using a single 10k pull up resistor. There is a negligible
difference if the unused input-only pins are tied HIGH or
LOW. For lowest leakage, tie unused input/output pins
HIGH.
Ensure that all unused pins handled in this manner are tied
to their corresponding power domain. For example, an
unused GPIO[1] is tied HIGH to GVDDQ through a 10k pull
up, which is shared with other unused signals in the GVDDQ
power domain.
Astoria is not hardware backward compatible to Antioch. So,
if the system is designed in Antioch, it requires PCB change
when replaced by Astoria.
About the Author
Name:
Praveen Kumar
Title:
Applications Engineer
Contact:
5B