PRELIMINARY
CM2031
DYNAMIC PULLUPS (CONT’D)
Figure 2. Dynamic DDC Pullups (Discrete - Top, CM2031 - Bottom; 3.3V ASIC - Left, 5V Cable - Right.)
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Figure 2 demonstrates the "worst case" operation of
I C/CMOS LEVEL SHIFTING
the dynamic CM2031 DDC level shifting circuitry (bot•
Using a standard CMOS input buffer for the DDC inter•
tom) against a discrete NFET common-gate level
face on the local HDMI ASIC can create a failure to
shifter circuit with a typical 1.5kΩ pullup at the source
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2
communicate at the specified I C levels. With an I C
VIL of approximately 1.5V (0.3*VDD), and typical
CMOS/TTL VIL input levels of 0.8V or less, an incom•
patibility can arise where valid HDMI specified logic
lows on the cable are not recognized within the ASIC.
(top.) Both are shown driving an off-spec, but unfortu•
nately readily available 31m HDMI cable which
exceeds the 700pF HDMI specification. Some widely
available HDMI cables have been measured at over
4nF.
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When the standard I/OD cell releases the NFET dis•
crete 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.
The CM2031 incorporates automatic CMOS/I C logic
low
translation
on
the
DDC_CLK_IN
and
DDC_DAT_IN lines.
As highlighted in Figure 2 above, when the cable-side
level ("OUT" side) of each DDC line is below 0.3*VDD,
the ASIC-side ("IN" side) of the DDC line is forced to a
CMOS low-level while the "OUT" side is allowed to
slew normally.
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 CM2031 dynamic pullups, low level shifters, and
pass element shifters are all matched to allow trans•
parent operation. This bi-directional circuit can be used
as easily as any discrete common-gate NFET level
shifter, but with the added functionality.
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.
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.
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8/8/2006 430 N. McCarthy Blvd., Milpitas, CA 95035-5112
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Tel: 408.263.3214
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Fax: 408.263.7846
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CALMIRCO [ CALIFORNIA MICRO DEVICES CORP ]
