
APPLICATION INFORMATION
Supply Voltage
TMDS Input Fail-Safe
TMDS Outputs
DDC I
2C Function Description
www.ti.com............................................................................................................................................................................................ SLLS953 – DECEMBER 2008
Table 3. TMDS261 vs TMDS251 Pinout
PIN NUMBER
I/O
TMDS251
TMDS261
Pins 32 and 33
I
GPIO mode: S1 and S2 configured as
GPIO mode: S1 and S2 configured as source selector pins (same as
source selector pins
TMDS251)
I2C mode: S1 and S2 configured as SCL and SDA for local slave I2C
communication
Pin 34
I
EQ: TMDS input equalization control
I2C_SEL: GPIO / local I2C control select
select
EQ = Low – HDMI 1.3 compliant cable
I2C_SEL = High – Device is configured by GPIO logic.
EQ = High – 10-m 28 AWG HDMI cable
I2C_SEL= Low – Device is configured by I2C logic.
Pin 49
—
VDD: HPD/DDC power supply
LP: Low-power mode select bar
LP = High – Normal operational mode
LP = Low – Device goes into low-power state.
Based on the differences listed in
Table 3, attention must be given to pin 34, which determines whether the
device uses I2C or GPIO control.
The TMDS261 is powered up with a single power source that is 3.3-V VCC for the TMDS circuitry for HPD, DDC,
and most of the control logic.
The TMDS261 incorporates clock-detect circuitry. If there is no valid TMDS clock from the connected HDMI/DVI
source, the TMDS261 does not switch on the terminations on the source-side data channels. Additionally, the
TMDS outputs are placed in the high-impedance state. This prevents the TMDS261 from turning on its outputs if
there is no valid incoming HDMI/DVI data.
A 10% precision resistor, 4.02-k
, is recommended to control the output swing to the HDMI-compliant 800-mV to
1200-mV range VOD(pp) (1000 mV typical). The TMDS outputs are high-impedance under standby mode
operation, S1 = H and S2 = L.
The TMDS261 provides buffers on the DDC I2C lines on all two input ports. This section explains the operation of
the buffer. For representation, the source side of the TMDS261 is represented by RSCL/RSDA, and the sink side
is represented by TSCL/TSDA. The buffers on the RSCL/RSDA and TSCL/TSDA pins are 5-V tolerant when the
device is powered off and high-impedance under low supply voltage, 1.5 V or below. If the device is powered up,
the driver T (see
Figure 28) is turned on or off depending on the corresponding R-side voltage level.
When the R side is pulled low below 1.5 V, the corresponding T-side driver turns on and pulls the T side down to
a low level output voltage, VOL. The value of VOL and VIL on the T side or the sink side of the TMDS261 switch
depends on the output-voltage select (OVS) control settings. OVS control can be changed by the slave I2C, see
Table 8. When the OVS1 setting is selected, VOL is typically 0.7 V and VIL is typically 0.4 V. When the OVS2 setting is selected, VOL is typically 0.6 V and VIL is typically 0.4 V. When OVS3 setting (default) is selected, VOL is
typically 0.5 V and VIL is typically 0.3 V. VOL is always higher than the driver-R input threshold, VIL on the T side
or the sink side, preventing lockup of the repeater loop. The TMDS261 is targeted primarily as a switch in the
HDTV market and is expected to be a companion chip to an HDMI receiver; thus, the OVS control has been
provided on the sink side, so that the requirement of VIL to be less than 0.4 V can be met. The VOL value can be
selected to improve or optimize noise margins between VOL and VIL of the repeater itself or VIL of some external
device connected on the T side.
When the R side is pulled up, above 1.5 V, the T-side driver turns off and the T-side pin is high-impedance.
Copyright 2008, Texas Instruments Incorporated
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