LMKDB1104FS85 datasheet(27/69 Pages) TI2

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8.3.4.3 Programmable Output Slew Rate

The LMKDB family offers slew rate control options through SMBus and pin modes. The pin mode option is

global slew rate control for all the outputs. SMBus slew rate control supports programmable output slew rate for

each individual output. The slew rate is heavily dependent on trace characteristics including trace width, copper

thickness, substrate height, dielectric constant, and loss tangent.

LMKDB slew rate control settings are tested with PCIe test load shown in Figure 7-1.

8.3.4.3.1 Slew Rate Control Through SMBus

The LMKDB12xx has 16 different slew rates options that can be assigned to the outputs. 0x0 is the fastest slew

rate setting and 0xF is the slowest slew rate setting. To set the slew rate of each output, follow these steps:

1. There are four different registers, SLEWRATE_OPT#, that can store up to four different slew rates. Select

your desired slew rates by assigning a value from 0x0 (fastest) to 0xF (slowest) to each SLEWRATE_OPT#

a. For example, if you wanted the fastest, second fastest, and the slowest slew rate, assign 0x0, 0x1,

and 0xF to registers SLEWRATE_OPT#. SLEWRATE_OPT1 = 0x0 (fastest), SLEWRATE_OPT2 = 0x1

(second fastest), and SLEWRATE_OPT3 = 0xF (slowest). SLEWRATE_OPT4 does not have to be

assigned, but if you want more than one register set to a slew rate, then SLEWRATE_OPT4 can be

assigned to any of the three previous settings.

2. Set a slew rate option for each output by using the SLEWRATE_SEL_CLKX_LSB and

SLEWRATE_SEL_CLKX_MSB as shown in Table 8-4 or drop-down menus under the Output Slew

Rate Control Section in TICSPro. The default SLEWRATE_OPT# register assignment for all outputs is

SLEWRATE_OPT2, which has a default slew rate of 0x6.

The corresponding ranges for the four default slew rates can be found in Section 6 under CLOCK OUTPUT

CHARACTERISTICS - 100MHz 85Ω PCIe or CLOCK OUTPUT CHARACTERISTICS - 100MHz 100Ω PCIe for

the specification Output slew rate.

Table 8-4. LMKDB Default SLEWRATE_OPT_# Values

Default Value

Default Slew Rate

SLEWRATE_OPT_1

0x0

Highest

SLEWRATE_OPT_2

0x6

High (default for all outputs)

SLEWRATE_OPT_3

0xA

Low

SLEWRATE_OPT_4

0xF

Lowest

Table 8-5. SLEWRATE_SEL_CLKX_LSB & SLEWRATE_SEL_CLKX_MSB Slew Rate Selection

SLEWRATE_SEL_CLKX_LSB

SLEWRATE_SEL_CLKX_MSB

Slew Rate Option Selection

SLEWRATE_OPT_4

SLEWRATE_OPT_3

SLEWRATE_OPT_2

SLEWRATE_OPT_1

To program the slew rate to the desired slew rate, the following sequence needs to be followed:

1. [Optional]: if the default assignments shown in Table 8-4 for each slew rate speed is not as desired, one of

the slew rate options value can be changed to another slew rate.

2. [LMKDB1108 and 1104 only]: Program SLEWRATE_CTRL_MODE register to 1 to select SMBus

programming mode for slew rate control. Refer to Section 9 section for LMKDB1108 and LMKDB1104

3. Program SLEWRATE_SEL_CLKX_MSB and SLEWRATE_SEL_CLKX_LSB to assign clock output X to

desired slew rate speed option, as shown in Table 8-5. The default assignments for each option can be

found in Table 8-4.

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LMKDB1202, LMKDB1204, LMKDB1208

SNAS927A – AUGUST 2025 – REVISED APRIL 2026

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