TFP401, TFP401A
TI PanelBus DIGITAL RECEIVER
SLDS120B - MARCH 2000 – REVISED JUNE 2003
detailed description (continued)
TFP401/401A clocking and data synchronization
The TFP401/401A receives a clock reference from the DVI transmitter that has a period equal to the pixel time,
Tpix. The frequency of this clock is also referred to as the pixel rate. Since the TMDS encoded data on Rx[2:0]
contains 10 bits per 8 bit pixel it follows that the Rx[2:0] serial bit rate is 10 times the pixel rate. For example,
the required pixel rate to support an UXGA resolution with 60 Hz refresh rate is 165 MHz. The TMDS serial bit
rate is 10x the pixel rate or 1.65 Gb/s. Due to the transmission of this high-speed digital bit stream, on three
separate channels (or twisted-pair wires) of long distances (3-5 meters), phase synchronization between the
data steams and the input reference clock is not guaranteed. In addition, skew between the three data channels
is common. The TFP401/401A uses a 4x oversampling scheme of the input data streams to achieve reliable
synchronization with up to 1-Tpix channel-to-channel skew tolerance. Accumulated jitter on the clock and data
lines due to reflections and external noise sources is also typical of high speed serial data transmission, hence
the TFP401/401A’s design for high jitter tolerance.
The input clock to the TFP401/401A is conditioned by a phase-locked-loop (PLL) to remove high frequency jitter
from the clock. The PLL provides four 10x clock outputs of different phase to locate and sync the TMDS data
streams (4x oversampling). During active display the pixel data is encoded to be transition minimized, whereas
in blank, the control data is encoded to be transition maximized. A DVI compliant transmitter is required to
transmit in blank for a minimum period of time, 128-Tpix, to ensure sufficient time for data synchronization when
the receiver sees a transition maximized code. Synchronization during blank, when the data is transition
maximized, ensures reliable data bit boundary detection. Phase synchronization to the data streams is unique
for each of the three input channels and is maintained as long as the link remains active.
TFP401/401A TMDS input levels and input impedance matching
The TMDS inputs to the TFP401/401A receiver have a fixed single-ended termination to AV
The
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TFP401/401A is internally optimized using a laser trim process to precisely fix the impedance at 50 Ω. The
device will function normally with or without a resistor on the EXT_RES pin, so it remains drop-in compatible
with current sockets. The fixed impedance eliminates the need for an external resistor while providing optimum
impedance matching to standard 50-Ω DVI cables.
Figure 14 shows a conceptual schematic of a DVI transmitter and TFP401/401A receiver connection. A
transmitter drives the twisted pair cable via a current source, usually achieved with an open drain type output
driver. The internal resistor, which is matched to the cable impedance, at the TFP401/401A input provides a
pullup to AV
. Naturally, when the transmitter is disconnected and the TFP401/401A DVI inputs are left
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unconnected, the TFP401/401A receiver inputs pullup to AV . The single ended differential signal and full
differential signal is shown in Figure 15. The TFP401/401A is designed to respond to differential signal swings
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ranging from 150 mV to 1.56 V with common mode voltages ranging from (AV -300 mV) to (AV -37 mV).
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