CM2031 Datasheet, PDF (6/14 Pages) California Micro Devices Corp – HDMI Receiver Port Protection and Interface Device

they may be daisy chained out through other physical
connectors including other HDMI ports or other
dedicated CEC links. The high level protocol of CEC
can be implemented in a simple microcontroller or
other interface with any I/OD (input/open-drain) GPIO.
Issue X-1
CM2031
CEC
CEC
RX
I/OD
GPIO
TX
Figure 3. Typical μC I/OD Driver
To limit possible EMI and ringing in this potentially
complex connection topology, the rise- and fall-time of
this line are limited by the specification. However,
meeting the slew-rate limiting requirements with
additional discrete circuitry in this bi-directional block is
not trivial without an additional RX/TX control line to
limit the output slew-rate without affecting the input
sensing (See Figure 4).
CEC I/F
μP
CM2031
Figure 5. Integrated CM2031 Solution
The CM2031 also includes an internal backdrive
protected static pullup 120μA current source from the
CE_SUPPLY rail in addition to the dynamic slew rate
control circuitry.
Figure 6 shows a typical shaped CM2031 CEC output
(bottom) against a ringing uncontrolled discrete solu­
tion (top).
RX
TX
TX_EN
CEC
Slew Rate
Limited
3-State Buffer
Figure 4. Three-Pin External Buffer Control
Simple CMOS buffers cannot be used in this
application since the load can vary so much (total
pullup of 27kΩ to less than 2kΩ, and up to 7.3nF total
capacitance.) The CM2031 targets an output drive
slew-rate of less than 100mV/μs regardless of static
load for the CEC line. Additionally, the same internal
circuitry will perform active termination, thus reducing
ringing and overshoot in entertainment systems
connected to legacy or poorly designed CEC nodes.
The CM2031’s bi-directional slew rate limiting is
integrated into the CEC level-shifter functionality thus
allowing the designer to directly interface a simple low
voltage CMOS GPIO directly to the CEC bus and
simultaneously guarantee meeting all CEC output logic
levels and HDMI slew-rate and isolation specifications
(See Figure 5).
Figure 6. CM2031 CEC Output
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