CM2031 Datasheet, PDF (5/18 Pages) California Micro Devices Corp – HDMI Receiver Port Protection and Interface Device

CM2031
For this reason, the CM2031 was designed with an internal I²C accelerator to meet the AC timing specification
even with very long and non-compliant cables.
The internal accelerator works with the source pull-up and the local 47kΩ pullup to increase the positive slew rate
of the DDC_CLK_OUT and DDC_DAT_OUT lines whenever the sensed voltage level exceeds 0.3*5V_SUPPLY
(approximately 1.5V). This provides faster overall risetime in heavily loaded situations without overloading the
mutli-drop open drain I²C outputs elsewhere.
DYNAMIC PULLUPS (CONT’D)
Figure 2. Dynamic DDC Pullups (Discrete - Top, CM2031 - Bottom; 3.3V ASIC - Left, 5V Cable - Right.)
Figure 2 demonstrates the “worst case” operation of the dynamic CM2031 DDC level shifting circuitry (bottom)
against a discrete NFET common-gate level shifter circuit with a typical 1.5kΩ pullup at the source (top.) Both are
shown driving an off-spec, but unfortunately readily available 31m HDMI cable which exceeds the 700pF HDMI
specification. Some widely available HDMI cables have been measured at over 4nF.
When the standard I/OD cell releases the NFET discrete shifter, the risetime is limited by the pullup and the
parasitics of the cable, source and sink. For long cables, this can extend the risetime and reduce the margin for
reading a valid “high” level on the data line. In this case, an HDMI source may not be able to read uncorrupted
data and will not be able to initiate a link.
With the CM2031’s dynamic pullups, when the ASIC driver releases its DDC line and the “OUT” line reaches at
least 0.3*VDD (of 5V_SUPPLY), then the “OUT” active pullups are enabled and the CM2031 takes over driving
the cable until the “OUT” voltage approaches the 5V_SUPPLY rail.
The internal pass element and the dynamic pullups also work together to damp reflections on the longer cables
and keep them from glitching the local ASIC.
I²C LOW LEVEL SHIFTING
In addition to the Dynamic Pullups described in the previous section, then CM2031 also incorporates improved
I²C low-level shifting on the DDC_CLK_IN and DDC_DAT_IN lines for enhanced compatibility.
Typical discrete NFETs level shifters can advertise specifications for low RDS[on], but usually state relatively high
V[GS] test parameters, requiring a 'switch' signal (gate voltage) as high as 10V or more. At a sink current of
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