PRELIMINARY
CM20LT-C
Consumer Electronics Control (CEC)
The Consumer Electronics Control (CEC) line is a high
level command and control protocol, based on a single
wire multidrop open drain communication bus running
at approximately 1kHz (See Figure 3). While the HDMI
link provides only a single point-to-point connection, up
to ten (10) CEC devices may reside on the bus, and
they may be daisy chained out through other physical
connectors including other HDMI ports or other dedi-
cated CEC links. The high level protocol of CEC can be
implemented in a simple microcontroller or other inter-
face with any I/OD (input/open-drain) GPIO.
and overshoot in entertainment systems connected to
legacy or poorly designed CEC nodes.
The CM20LT-C’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).
CEC
RX
TX
CEC I/F
µP
I/OD
GPIO
CEC
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 addi-
tional 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).
CM20LT-C
Figure 5. Integrated CM20LT-C Solution
The CM20LT-C 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 CM20LT-C CEC out-
put.
CEC
RX
TX
TX_EN
Slew Rate
Limited
3-State Buffer
X
Figure 4. Three-Pin External Buffer Control
Simple CMOS buffers cannot be used in this applica-
tion since the load can vary so much (total pullup of
27kΩ to less than 2kΩ, and up to 7.3nF total capaci-
tance.) The CM20LT-C 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
Figure 6. CM20LT-C CEC Output
© 2005, 2006California Micro Devices Corp. All rights reserved.
4/3/2006
430 N. McCarthy Blvd., Milpitas, CA 95035-5112
-
Tel: 408.263.3214
-
Fax: 408.263.7846
-
www.calmicro.com
5
CALMIRCO [ CALIFORNIA MICRO DEVICES CORP ]
