CM2031 Datasheet, PDF (5/14 Pages) California Micro Devices Corp – HDMI Receiver Port Protection and Interface Device

DYNAMIC PULLUPS (CONT’D)
Issue X-1
CM2031
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.
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 4mA for the ASIC on DDC_XX_IN,
the CM2031 guarantees no more than 140mV increase
to DDC_XX_OUT, even with a switching control of
2.5V on LV_SUPPLY.
Additionally, when I²C devices are driving the external
cable, an internal pulldown on DDC_XX_IN guarantees
that the VOL seen by the ASIC on DDC_XX_IN is
equal to or lower than DDC_XX_OUT.
Multiport DDC Multiplexing
Additionally, by switching LV_SUPPLY, the DDC/HPD
blocks can be independently disabled by engaging
their inherent “backdrive” protection. This allows N:1
multiplexing of the low-speed HDMI signals without
any additional FET switches.
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
Consumer Electronics Control (CEC)
The Consumer Electronics Control (CEC) line is a high
level command and control protocol, based on a single
wire multidrop open drain communication bus running
at approximately 1kHz (See Figure 3). While the HDMI
link provides only a single point-to-point connection, up
to ten (10) CEC devices may reside on the bus, and
© 2007 California Micro Devices Corp. All rights reserved.
07/10/07
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