Silicon Laboratories SI5375 - Manual (Page 59)

Si53xx-RM
Rev. 1.2 59
5.6.5. Disabling FS_OUT (Si5366)
The FS_OUT maybe disabled via the DBLFS pin, see Table 20. The additional state (M) provided allows for
FS_OUT to drive a CMOS load while the other clock outputs use a different signal format as specified by the
SFOUT[1:0] pins.
5.7. Output Clock Drivers
The devices include a flexible output driver structure that can drive a variety of loads, including LVPECL, LVDS,
CML, and CMOS formats. The signal format is selected jointly for all outputs using the SFOUT [1:0] pins, which
modify the output common mode and differential signal swing. See the appropriate data sheet for output driver
specifications. The SFOUT [1:0] pins are three-level input pins, with the states designated as L (ground), M (V
DD
/
2), and H (V
DD
).
Table 21 shows the signal formats based on the supply voltage and the type of load being driven. For the CMOS
setting (SFOUT = LH), both output pins drive single-ended in-phase signals and should be externally shorted
together to obtain the drive strength specified in the data sheet.
The SFOUT [1:0] pins can also be used to disable the output. Disabling the output puts the CKOUT+ and CKOUT–
pins in a high-impedance state relative to V
DD
(common mode tri-state) while the two outputs remain connected to
each other through a 200
 on-chip resistance (differential impedance of 200 ). The maximum amount of internal
circuitry is powered down, minimizing power consumption and noise generation. Changing SFOUT without a reset
causes the output to output skew to become random. When SFOUT = LH for CMOS, PLL bypass mode is not
supported.
5.7.1. LVPECL and CMOS TQFP Output Signal Format Restrictions at 3.3 V (Si5365, Si5366)
The LVPECL and CMOS output formats draw more current than either LVDS or CML. However, the allowed output
format pin settings are restricted so that the maximum power dissipation for the TQFP devices is limited when they
are operated at 3.3 V. When SFOUT[1:0] = MH or LH (for either LVPECL or CMOS), either DBL5 must be H or
DBL34 must be high.
Table 20. FS_OUT Disable Control (DBLFS)
DBLFS FS_OUT State
H Tri-State/Powerdown
M Active/CMOS Format
L Active/SFOUT[1:0] Format
Table 21. Output Signal Format Selection (SFOUT)
SFOUT[1:0] Signal Format
HL CML
HM LVDS
LH CMOS
LM Disabled
MH LVPECL
ML Low-swing LVDS
All Others Reserved