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Texas Instruments SID'01 WhisperBus‚äó Application notes
SID'01 WhisperBus?ao
Literature Number: SNLA172
9.3 / R. McCartney
9.3: WhisperBus™: An Advanced Interconnect Link For TFT
Column Driver Data
Richard McCartney, James Kozisek and Marshall Bell
National Semiconductor, Inc., Chandler, Arizona
Abstract
Increasing video data rates to liquid crystal displays brought
about by increasing display format sizes and increased grayscale
content are placing very high demands on digital data busses.
These higher data rates coupled with constraints on EMI levels,
size, power and component count are rendering CMOS data
busses inadequate for the connection between the timing
controller and the column drivers. The operation and
performance of a comprehensive solution using a current
signaling method (WhisperBus™) is presented.
1.
Introduction and Background
As display formats become larger, video rates to the display
correspondingly increase. For example, UXGA (1200 x 1600
pixels) is the equivalent of four SVGA panels (600 x 800 pixels)
tiled together and requires a corresponding four-fold increase in
the video data rate compared with an SVGA display for the same
refresh frequency. In addition, there is a trend toward 24-bit
grayscale rather than 18-bit grayscale in the larger formats
especially when they are used for monitor applications. This
increased grayscale represents an additional 33 percent increase in
data rates. At the same time, there is a strong market demand to
minimize bezel width and panel thickness, reduce costs through
reduced PCB layers and components and to do all this while
improving the robustness of the electrical and mechanical design.
These factors combine to put demands on the traditional CMOS
data bus connecting the timing controller and the column driver
that it cannot meet. The most significant problem is EMI. Even
the present day, mainstream panel, the XGA display (768 x 1024
pixels) requires substantial design effort to manage the EMI to
acceptable limits. This paper describes a new interconnect
signaling method, WhisperBus™, used in a point-to-point
interconnect topology to eliminate the EMI problem for today's
XGA designs as well as the larger formats and is extensible well
into the future. The new interconnect method uses fewer PCB
wires than the CMOS bus interconnect facilitating lower cost and
thinner displays through the use of fewer layer PCBs. In addition,
a number of other significant benefits arise from the use of this
topology that will be discussed here.
Electro-magnetic interference (EMI) from the radio signals
launched by the digital video data signaling to the display panel
has been a serious problem for the LCD display industry for some
time. The introduction of the XGA format into mass production,
circa 1997, brought with it the need for new technology to solve
the EMI and wire density issue at the hinge of the LCD notebook
PC (NBPC) application. A number of technologies were
experimented with but in the end, LVDS, low voltage differential
signaling, developed by National Semiconductor, emerged as the
defacto standard for this interconnect, particularly in NBPCs.
Within the display module, the video data link between the timing
controller ASIC and the column drivers is a major source of EMI.
In the SVGA display, with its 600 x 800 pixels format, the 18-bit
color gray-scale video data is bussed directly as an 18-bit word to
each column driver. This approach was impractical in the XGA
display, with its 60 percent larger 768 x 1024 format and
corresponding 60 percent higher digital video data rate compared
with the SVA display. The EMI generated from the link between
the timing controller and the column drivers is managed in part
through the adoption of a dual bus system. In this convention, 18bit video data is split into two 18-bit data paths, one for the even
numbered columns and one for the odd numbered columns. Of
course 24-bit color gray-scale is sent on dual 24 bit busses. This
technique, while adding wires and thereby space to the link
between the TCON and the column drivers, made the
management of EMI practical for the XGA display.
The dual bus technique together with good PCB layout
techniques, shielding and various other methods are the methods
used to this point manage the EMI through the SXGA format
(1024 x 1280 pixels) and it's bigger cousin the SXGA+ format
(1050 x 1400). The UXGA format (1200 x 1600) forces a new
level of response from the industry. The generally agreed upon
solution is to replace the bus between the TCON and the column
driver with a new, low EMI link and several companies have
introduced technology to do this. National has two technologies,
RSDS and WhisperBus™. RSDS is named for its description,
Reduced Swing Differential Signaling (compared with LVDS). It
replaces the CMOS data path with a differential pair operated at
higher frequencies which allows fewer data lines than CMOS but
due to its differential signaling reduces EMI to very low levels.
The other solution, WhisperBus™ will be described in detail here.
2.
WhisperBus™ Link Operation
WhisperBus™ uses singled-ended, low current signaling rather
than differential signaling to transfer the digital video data without
the EMI generated by conventional CMOS signaling. The key
advantage of single-end drive over differential is of course the
number of wires required for a channel of data. An 18-channel
link for example requires 36 wires in differential mode
transmission but only 18 wires plus ground in a single-ended
mode transmission, CMOS TTL signaling being a common
example of single-ended transmission.
2.1
Receiver Operation
The WhisperBus™ ability to signal in single-ended, low level
currents is due to the nature of the receiver. Conventional
differential signaling, also signals in current but receives in
voltage. The differential current develops a voltage across a
terminating resistor, which the receiver differentially amplifies in
the voltage domain to convert it back to a CMOS level voltage
signal. In this approach, the voltage across the terminating resistor
must be well above the voltage noise floor to reliably recover the
signal. The proper termination resistance of a typical PCB
transmission line is approximately 50 Ohms per line or 100 Ohms
differential. Since the resistance is quite low, the differential
current must be on the order of milli-amps in order to produce the
hundreds of milli-volts needed for a practical system. In RSDS
for example, 2mA is typical (200mv differential).
SID 01 DIGEST •
1
9.3 / R. McCartney
RZ0
2RZ0
IC
Differential Receiver
IC
WhisperBus™ Receiver
Figure 1. Comparison of WhisperBus™ Receiver to a conventional, differential
mode receiver.
In the WhisperBus™ system, binary digital data is signaled as
either a lower current or a higher current, on a single wire. Both
current states flow in the same direction, always from the receiver
to the transmitter. The transmitter transmits its binary information
to the receiver by sinking one or another current from the receiver
based on an incoming CMOS voltage level. The two currents can
be considered to be an AC current riding on a DC bias current.
Typically, the two currents would be 50µA and 150µA and
considered to be +/- 50µA riding on a constant 100µA bias
current. This 100µA is more than an order of magnitude smaller
than the 2000µA typical of RSDS for example.
The voltage at the summing junction of the amplifier in the
WhisperBus™ receiver shown in Figure 1 is maintained at
approximately 1 volt by design. The actual value of the voltage is
not critical to the operation of WhisperBus™ and is dependent on
transistor thresholds in the receiver. From an operational
standpoint, its purpose is simply to allow the transmitter to be able
to sink current from the receiver without taking the node to a
voltage below ground. From an AC perspective, the input to the
WhisperBus™ receiver looks like a resistance of R Z0, which is set
to approximately 50 ohms to match the characteristic impedance
of a typical PCB trace.
The DC component of the WhisperBus™ signal (100µA for
example) flows through R Z0 an produces a DC voltage of 5mv
across it which when subtracted from the approximate 1 volt bias
at the summing junction (current flows from the receiver to the
transmitter) is a DC voltage of approximately 0.995 volts on the
WhisperBus™ line. The AC signal of +/- 50µA across the 50
ohm termination resistor produces a +/- 2.5mV signal riding on
the 1 volt bias. This +/- 2.5mV AC component is 660 times
smaller than a 3.3 volt CMOS TTL signal (+/- 1.65 volts). This
reduction in signal swing over TTL is in large part responsible for
the more than 225 times reduction in EMI achieved by
WhisperBus™ over CMOS TTL.
2.2
Transmitter Operation
The WhisperBus™ transmitter functions as a two-state current
sink, sinking one or another current from the receiver depending
2
• SID 00 DIGEST
on the input CMOS TTL state. See Figure 2. Notice that the
circuit doesn’t determine the voltage on the output of the
WhisperBus™
Vlogic supply
transmitter but rather the
receiver sets the voltage.
The transmitter can
accommodate a wide
range
of
receiver
IC
CMOSlogic
voltages and in practice
can operate to as low as
a few hundred milli-volts
2 x I min
I min
above ground. This is
important because it
allows the transmitter
Figure 2. WhisperBus™ Transmitter
and receiver to be
equivalent circuit.
fabricated in different
processes and even to be
powered by different supply voltages. Notice also, that the
current into ground is constant and not data dependent. This is
important to other circuitry, especially analog circuits, because
steady substrate currents allow the ground reference voltages to
remain stable.
2.3
The WhisperBus™ Link
Figure 3 illustrates two WhisperBus™ channels communicating
between two ICs as an example. The actual number of channels
in the application will depend on the total data transfer rate
required. Note the signal ground return path, common to all the
channels. This ground is isolated from the power ground points
on the substrate by several ohms. This feature assures that the
receiver and transmitter are referenced to the same electrical node
and that high currents do not pass through the ground path
between the transmitter and the receiver creating a voltage
difference between the two ends of the link.
Vlogic supply 1
L
H
IN 1
L
IN 2
Vlogic supply 2
WhisperBus™
The current to voltage conversion in the WhisperBus™ receiver is
fundamentally different from conventional differential receivers.
The WhisperBus™ receiver converts the signal current in the link
directly to voltage using active circuitry that intrinsically provides
the termination resistance. Figure 1 shows an equivalent circuit
comparing both the differential mode receiver and the
WhisperBus™ receiver. Notice that there are no external
components required with WhisperBus™.
LS
OUT1
H
LS
OUT2
Figure 3. Two channel, chip-to-chip, WhisperBus™ link with potentially
independent logic supplies and IC process geometries.
The bandwidth of the link is not limited by the WhisperBus™
transmitter or receiver circuit but by the speed of the CMOS logic
on either end. The WhisperBus™ link itself is a very high
bandwidth circuit with a relatively small signal amplitude
compared with the rail-to-rail CMOS logic. The limiting speed of
the link then outside of the CMOS logic speed constraints is the
signal to noise ratio on the link itself. The signal current can be
9.3 / R. McCartney
programmed higher or lower with the +/- 50µA working well in
the typical column driver environment.
3.
Point-To-Point Topology
WhisperBus™ forms an important base on which the topology of
the TCON and column driver system is built. There are a number
of significant features this topology/link provides including: ultra
low EMI, reduced power, reduced component and signal trace
count, reduced column driver die size, 6 or 8 bit grayscale with no
additional cost and support for independent RGB gamma for color
temperature control without graylevel reduction. Each of these
features will be discussed here.
3.1
Benefits to Point-to-Point
Figure 4 shows an XGA example using WhisperBus™. There are
several important features illustrated in this example. First is the
point-to-point topology, which allows each column driver to
receive data simultaneously throughout a line time. The multidrop bus topology requires a column driver to receive its data in
LVDS
Row Driver Signals
2
CD
2
2
CD
CD
TCON
2
2
CD
CD
2
CD
2
2
CD
CD
Figure 4. 18-bit or 24-bit XGA example showing point-to-point topology in “T”
configuration, differential clock, and 2 wire WhisperBus™ data link per driver.
short bursts since the bus is shared among other column drivers.
In the point-to-point topology each column driver is receiving
data simultaneously, which allows the data rate to each driver to
be reduced by a factor of at least eight (in the XGA case with its
eight drivers). In addition, the TCON buffers the incoming data at
the dot clock rate but re-transmits it to the column driver at a rate
that uses the whole line time and thus can further minimize the
data rate to column driver. While the TCON does require a full
line buffer in the point-to-point topology, we have designed a
special memory cell that makes the memory size so small as to not
be a significant size and cost factor. Any small size increase in
the TCON is more than offset by the reduction in CD die size
multiplied across the 8 or 10 or more column drivers per system.
3.1.1
Few Data Lines, Small TCON
The 2 wire WhisperBus™ data link between each column driver
(CD) and the timing controller (TCON) is sufficient for either 18bit or 24-bit color data rates. The XGA solution only requires a
total of 16 data pins from the TCON (and just 2 per column
driver). This small data pin count in the TCON coupled with an
LVDS input for example, allows the TCON to be packaged in an
80 pin TQFP.
3.1.2
“T” or “L” Configuration Supported
Only one differential clock is needed to drive all the column
drivers. The differential clock allows multi-drop bussing to all the
drivers and clocking is on both edges to minimize frequency.
Dual clock sources on the TCON facilitate the “T” configuration
layout with having to drive a transmission line from the center
with a long stub. The “L” configuration is also supported using
just one of the two clock sources. Outside of any power supply
decoupling capacitors, the column driver PCB is free of any
additional components except for the termination resistor(s) for
the differential clock. This is in contrast to differential busses,
which require external termination of each data channel.
(WhisperBus™ is self-terminating).
3.1.3
Data & Control Supported on WhisperBus™
The WhisperBus™ link is used to support both data transfers and
column driver control protocols to the column drivers. One of the
features of this protocol is the clock de-skew function. Each
driver has a slightly different data path length between it and the
TCON. In addition, the clock path length also varies from driver
to driver. At the start of each row, the WhisperBus™ data link
sends a special sync pulse, which the receivers use to adjust the
clock delay to provide the best clock to recover the data. This
dynamic adjustment assures robustness of the design.
3.1.4
Minimal PCB Signals, CD I/O
The control protocol also provides the transmission of the polarity
signal to the column driver. The data shift direction is determined
by the TCON and not by the column driver so that no shift
direction signal is needed, eliminating a column driver input and a
column driver PCB signal trace. The point-to-point topology
eliminates the need for a data shift in pulse and a data shift out
pulse between drivers.
Each driver receives its data
simultaneously and so two input signals to the column driver and
a PCB trace between column drivers is eliminated. A data invert
signal is not needed. In fact, aside from the WhisperBus™ data
lines and clock, the only other control signal from the TCON sent
to the column driver is the load data signal which determines
when the outputs transition to new data on each row. Even the
column driver output current can be programmed through this
control interface across WhisperBus™ and additional features can
be added in the future.
3.1.5 Small CD Die Size
By far the greatest advantage of the point-to-point topology with
its slow data rate to the column driver is the ability to take a new
approach to the column driver architecture. Virtually all column
drivers in use today use the RDAC architecture in which a single
resistor ladder produces a set of internal reference busses, each
one being a different gray level. Each output stores a digital word
that is applied to an analog switch decoder to select the correct
graylevel to drive from that output. Each output requires the
decoder and the voltage reference bus. These two features
typically account for about 50% of the column driver die area.
In the point-to-point topology, the slower data rate to each column
driver allows for a flash D-to-A conversion. In principal, only
one decoder is required and the incoming digital word is
converted and stored in the analog domain immediately. This
saves a large amount of die space. It should be pointed out that
the industry experimented with a sample and hold architecture in
the past with poor results and efforts were quickly abandoned. In
that case the topology was a capacitive-DAC (C-DAC) that was
prone to capturing noise from digital signals in the sample and
hold voltage. Here the WhisperBus™ plays an important and
groundbreaking role. Because the WhisperBus™ switching is
very quiet from a substrate noise perspective, the sample and hold
(S&H) architecture is made practical. The S&H architecture
reduces die size significantly. Moreover, there is virtually no die
size difference between a 6-bit grayscale driver and an 8-bit
grayscale driver. In other words, one die design serves both
applications and is smaller than a conventional 6-bit die.
SID 01 DIGEST •
3
9.3 / R. McCartney
3.1.6
Color Temperature Control
The key reason that there is virtually no die size penalty for 8-bit
over 6-bit grayscale using the S&H architecture is that there is in
principal, one decoder and no busing of the RDAC reference
voltages. In practice, the flash DAC rate can be made more
practical by having a few DACs operating simultaneously. Still
this area is small compared with the 384, 402, 420, 480 or 516
decoders for example required in the RDAC approach and is not a
barrier to the architecture. On the contrary, having at least three
separate RDACs allows an unprecedented feature in a column
driver, independent RGB gamma control. A separate RDAC and
flash decoder can be applied to each the red, green and blue
channels allowing independent gamma correction of each color.
This facilitates color temperature control of the LCD without a
loss of grayscale or the addition of other complex backlighting
methods.
It is impractical to have independent RGB using the RDAC
architecture. The die size would be prohibitively large. Even if
this weren’t true, there isn’t enough room on the input side of a
conventional RDAC to support separate RGB gamma reference
supplies. The ability to support independent RGB gamma across
the driver interface in the point-to-point architecture is made
practical by the reduction in the number of inputs to the column
driver through WhisperBus™.
3.1.7
3.1.8. Growth Path
Perhaps the most important feature of WhisperBus™ and the
point-to-point topology is its upward growth path. The UXGA
and QXGA formats require only four data lines per CD (480
outputs) for a total of 40 data pins from the TCON. In addition
the platform provides the base for additional features, including
several gamma control features, which will become baseline
requirements in the near future.
Evaluation Results
Several companies have evaluated WhisperBus™ and the pointto-point architecture successfully, in both XGA and SXGA
applications.
Production designs are now well underway.
Naturally, one of the larger purposes for the on panel evaluation
was to verify the system’s EMI performance. Figure 5 is a sideby-side comparison of EMI from a conventional 6-bit XGA
NBPC panel (without shielding for EMI) and the EMI signature
from a 15-inch, XGA monitor panel using WhisperBus™ and the
point-to-point topology with 24-bit color.
The reduced EMI is obvious when comparing these graphs. The
EMI limits are shown as the thick horizontal lines in the above
4
Conventional
TTL Panel
frequency [MHz]
WhisperBus™
WhisperBus™
Panel
Panel
Dual Function 18- or 24-Bit Color Grayscale
The fact that the column driver is in reality an 8-bit column driver
means the same chipset illustrated in Figure 4 can be used in both
18-bit and 24-bit color applications. The TCON is programmed
to accept 18-bit or 24 bit data input with a pin selection and when
operating in 18-bit mode the 6-bit data is scaled to 8-bits before
being sent to the column driver. This scaling assures the column
driver extends across the full dynamic range.
6.
graphs and are well above the WhisperBus™ panel signature.
The EMI from the conventional panel is significantly over the
limit. Without substantial EMI filtering and shielding the
conventional panel could not be brought into tolerance. However,
without any additional effort, the WhisperBus™ panel performed
as shown above, the first time. The baseline signature is primarily
due to the LVDS residual. Further testing showed that the spikes
riding on the baseline were from the clock, not the WhisperBus™
data lines. Furthermore, the clock was broadcasted not from the
PCB but rather from inside the TCON. Additional effort is being
made in the subsequent TCON designs to reduce clock radiation.
• SID 00 DIGEST
frequency [MHz]
Figure 5. Comparison of EMI signature from two XGA panels. The upper graph
shows conventional TTL data busses with untreated EMI and the lo wer graph
shows the results from a panel using WhisperBus™.
The evaluations also confirmed the reduced power using the
WhisperBus™ link verses a conventional, dual TTL data bus. It
is a little difficult to compare power in that a TTL data bus draws
no power when the image is a solid white or black field for
example since power is only consumed when video data bits are
transitioning. However, for practical images typical of a display,
power reductions on the order of 5 to 10 times were observed. A
typical TTL bus will consume about 700mW of power so a 5 to
10 times reduction in power is significant.
7.
Summary
WhisperBus™ is an enabling technology to the point-to-point
topology that features many improvements for the TFT display
panel including reduced column driver die size, smaller PCBs,
low EMI, low power and advanced features such as independent
RGB gamma correction. WhisperBus™ is a unique signaling
solution that allows both data and control features and
demonstrates many advantages over the differential bus
topologies.
8.
References
[1] Kim, E. G. and Martin, R. A compact LCD Driver and
Timing Controller System. SID 00 Digest, 6.3, pp. 46-49
(May 2000).
[2] Amemiya, T. Characterization of Column Driver for TFT
LCD, SID 98 Digest, 45.5, pp. 1161-1164
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