SI53115 Datasheet, PDF (16/33 Pages) Silicon Laboratories – 15-OUTPUT PCIE GEN3 BUFFER/ ZERO DELAY BUFFER

Si53115
2.5. HBW_BYPASS_LBW
The HBW_BYPASS_LBW pin is a tri-level function input pin (refer to Table 1 for VIL_Tri, VIM_Tri, and VIH_Tri
signal levels). It is used to select between PLL high-bandwidth, PLL bypass mode, or PLL low-bandwidth mode. In
PLL bypass mode, the input clock is passed directly to the output stage, which may result in up to 50 ps of additive
cycle-to-cycle jitter (50 ps + input jitter) on the differential outputs. In the PLL mode, the input clock is passed
through a PLL to reduce high-frequency jitter. The PLL HBW, BYPASS, and PLL LBW modes may be selected by
asserting the HBW_BYPASS_LBW input pin to the appropriate level described in Table 14.
Table 14. PLL Bandwidth and Readback Table
HBW_BYPASS_LBW Pin
L
M
H
Mode
LBW
BYPASS
HBW
Byte 0, Bit 7
0
0
1
Byte 0, Bit 6
0
1
1
The Si53115 has the ability to override the latch value of the PLL operating mode from hardware strap Pin 5 via the
use of Byte 0 and Bits 2 and 1. Byte 0 Bit 3 must be set to 1 to allow the user to change Bits 2 and 1, affecting the
PLL. Bits 7 and 6 will always read back the original latched value. A warm reset of the system will have to be
accomplished if the user changes these bits.
2.6. Miscellaneous Requirements
Data Transfer Rate: 100 kbps (standard mode) is the base functionality required. Fast mode (400 kbps)
functionality is optional.
Logic Levels: SMBus logic levels are based on a percentage of V_DD for the controller and other devices on the
bus. Assume all devices are based on a 3.3 V supply.
Clock Stretching: The clock buffer must not hold/stretch the SCL or SDA lines low for more than 10 ms. Clock
stretching is discouraged and should only be used as a last resort. Stretching the clock/data lines for longer than
this time puts the device in an error/time-out mode and may not be supported in all platforms. It is assumed that all
data transfers can be completed as specified without the use of clock/data stretching.
General Call: It is assumed that the clock buffer will not have to respond to the “general call.”
Electrical Characteristics: All electrical characteristics must meet the standard mode specifications found in
Section 3 of the SMBus 2.0 specification.
Pull-Up Resistors: Any internal resistor pull-ups on the SDATA and SCLK inputs must be stated in the individual
data sheet. The use of internal pull-ups on these pins of below 100 K is discouraged. Assume that the board
designer will use a single external pull-up resistor for each line and that these values are in the 5–6 k range.
Assume one SMBus device per DIMM (serial presence detect), one SMBus controller, one clock buffer, one clock
driver plus one/two more SMBus devices on the platform for capacitive loading purposes.
Input Glitch Filters: Only fast mode SMBus devices require input glitch filters to suppress bus noise. The clock
buffer is specified as a standard mode device and is not required to support this feature. However, it is considered
a good design practice to include the filters.
PWRDN: If a clock buffer is placed in PWRDN mode, the SDATA and SCLK inputs must be Tri-stated and the
device must retain all programming information. I_DD current due to the SMBus circuitry must be characterized and
in the data sheet.
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Rev. 1.1