8.5.1 Output Driver State When Disabled
The disabled state of an output driver is configurable as disable low or disable high. When the output driver is disabled, the outputs will
drive either logic high or logic low, selectable by the user. The output common mode voltage is maintained while the driver is disabled,
reducing enable/disable transients. By contrast, powering down the driver rather than disabling it increases output impedance and shuts
off the output common mode voltage. For all output drivers connected in the system, it is recommended to use Disable rather than
Powerdown to reduce enable/disable common mode transients. Unused outputs may be left unconnected, powered down to reduce
current draw, and, with the corresponding VDDOx, left unconnected.
8.5.2 Synchronous Output Enable/Disable Feature
The output drivers provide a selectable synchronous enable/disable feature when OUTx_SYNC_EN = 1. Output drivers with this feature
turned on will wait until a clock period has completed before the driver is disabled or enabled. This prevents unwanted runt pulses from
occurring when disabling an output. When this feature is turned off OUTx_SYNC_EN = 0, the output clock will disable immediately
without waiting for the period to complete and will enable immediately without waiting a period to complete. The default state is for the
synchronous output disable/enable to be turned on OUTx_SYNC_EN = 1 .
Table 8.14. Synchronous Disable Control Registers
Setting Name
Hex Address
[Bit Field]
Function
Si5391/Si5391P
OUT0A_SYNC_EN
0104[3]
When this bit is high, the output will turn on/off (enable/disable) without
generating runt pulses or glitches. The default for this bit is high. When this
bit is low, the outputs will turn on/off asynchronously. In this case, there may
be glitches on the output when it turns on/off.
OUT0_SYNC_EN
0109[3]
OUT1_SYNC_EN
010E[3]
OUT2_SYNC_EN
0113[3]
OUT3_SYNC_EN
0118[3]
OUT4_SYNC_EN
011D[3]
OUT5_SYNC_EN
0122[3]
OUT6_SYNC_EN
0127[3]
OUT7_SYNC_EN
012C[3]
OUT8_SYNC_EN
0131[3]
OUT9_SYNC_EN
0136[3]
OUT9A_SYNC_EN
013B[3]
8.6 Output Buffer Supply Voltage Selection
These power supply settings must match the actual VDDOx voltage so that the output driver operates properly.
Table 8.15. OUTx VDD Settings
Setting Name
Description
OUTx_VDD_SEL_EN
These bits are set to 1 and should not be changed
OUTx_VDD_SEL
These bits are set by CBPro to match the expected VDDOx voltage. 0: 3.3 V; 1:
1.8 V; 2: 2.5 V; 3: Reserved
Si5391 Reference Manual • Outputs
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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Rev. 0.5 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • January 11, 2022
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