ETC APPLICATIONNOTE28

Application Note 28
Micrel
Application Note 28
Data Squelch Using the MICRF002
By Sean Montgomery
Introduction
Digital Squelch
There are many new applications being created every day
which use low power radio as a link for simple remote
actuation such as garage doors, keyless entry, remote controls etc... With the release of the Micrel MICRF001 RF
receiver and data demodulator IC, the design of such a link
has never been easier. Applications which implement this
Micrel receiver IC often require both low price and low power
consumption, with the digital functions, including data decoding, often performed by low cost 4-bit microcontrollers.
As is common with all superheterodyne AM receivers, the
output will contain noise when there is no RF carrier present.
The operation of the AGC (automatic gain control) and the
demodulator in the MICRF001 converts very low level noise
into a corresponding logic level output noise. This output
noise combined with the relatively low processing power of
the 4-bit microcontroller consequently takes up a great deal
of the processing time which could be better spent on other
functions. There are two ways in which this problem can be
addressed: introducing analog squelch and introducing digital squelch.
Introduction to MICRF002 Features
The MICRF002 includes two important features that differentiate it from the MICRF001 and MICRF011:
Shutdown (SHUT) allows duty-cycled operation to extend
battery life in battery operated systems.
Wake up (WAKEB) operates as a simple data-preamble
detector output and can be used to interrupt a microcontroller.
If only one microcontroller input is available, it can be used as
a control signal for a digital data squelch circuit.
RSC
CTH
“Probe B”
Signal Level
128-Bit
Counter
To utilize the wake-up function, an uninterrupted 5ms must be
transmitted at the start of each data word, or a single 5ms
carrier at the start of the data pattern. (Sending carrier at the
start of each data word is recommended as it improves
communication reliability). When uninterrupted carrier is detected for 128 clock cycles of the nominal 26.4kHz clock,
WAKEB will transition low and stay low until data begins.
This output can be used directly by the microcontroller if there
is an available I/O pin. Alternatively, we can use some
discrete circuitry to effectively ‘filter’ the data output. The
following discusses a possible solution for this function.
Signal from
peak detector
Unmodified
Output
256
Figure 2. Simplified Wake-Up Block Diagram
Adding a small offset to the CTH pin can prevent noise from
producing logic-level transitions at the data output. Since we
have now added a signal path attenuation, the range will be
somewhat reduced.
Background Noise
(Probe A)
MICRF002
modified
slicing level
Whip
Antenna
slicing level
SEL0
Time
Demodulated
Background Noise (Probe B)
logic-high
5V
Time
Modified
Output
Demodulated
Data
WAKEB
(active low)
Analog Squelch
“Probe A”
~6.75MHz
Reference
Oscillator
C3
C2
10nF
4.7µF
C4
33nF
Demodulated
Background Quiet (Probe B)
logic-high
SWEN
VSSRF
REFOSC
VSSRF
SEL1
ANT
VDDRF
CAGC
WAKEB
VDDBB
SHUT
CTH
NC
CR1
6.75MHz
C1
4.7µF
Interrupt
DO
VSSBB
Data Output
Figure 3. Receiver with Data and Interrupt Outputs
WAKEB can be connected directly to a microcontroller input
pin and used as an enable signal (interrupt) for the
microprocessor’s data input as shown in Figure 3.
Time
Figure 1. Adding Offset (Analog Squelch)
QwikRadio is a trademark of Micrel, Inc. The QwikRadio ICs were developed under a partnership agreement with AIT of Orlando, Florida
Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com
August 1999
1
Application Note 28
Application Note 28
Micrel
MICRF002
Whip
Antenna
5V
SEL0
C3
C2
10nF
4.7µF
C4
33nF
SWEN
VSSRF
REFOSC
VSSRF
SEL1
ANT
VDDRF
CAGC
WAKEB
VDDBB
SHUT
CTH
NC
MIC1555
CR1
6.75MHz
TRG OUT
GND
C1
4.7µF
THR
VS
R1 1.8MΩ
C4
100nF
SquelchControlled
Output
D1 1N4148
DO
VSSBB
Figure 4. Receiver with Digital Data Squelch
VDO
MICRF002
Wake-Up Output
VOUT(AND) VOUT(1555)
MICRF002
Digital Output
VWAKEB
Digital Data Squelch
Figure 4 shows the MICRF002 controlling an add-on digital
data squelch. The WAKEB output triggers an MIC1555
monostable which allows the digital output to pass through
the AND gate.
Transmit
Data
MICRF002
Digital
Output
SquelchControlled
Output
Holtek
Code Good
Output
Figure 6. Circuit Applied to Evaluation Board
MIC1555
Output
VTX
MICRF002
Digital Output
VOUT(1555) VOUT(AND)
Transmit
Data
VDO
SquelchControlled
Output
Figure 5. Digital Output Behavior with Carrier
Figure 5 demonstrates that a continuous carrier of 4.8ms is
required to trigger the WAKEB output of the MICRF002.
Trace 4 represents the output of the MIC1555 monostable
going high as a result of the DO pin (trace 1) detecting a
continuous tone for approximately 4.8ms. This “Carrier Detect” signal remains high for approximately 120ms (set by R1
and C4) before returning to zero.
Figure 6 is an example using the Micrel RF evaluation board
which uses a Holtek decoder chip connected to the squelchcontrolled output.
The Holtek decoder must receive three successful codes
before it indicates a successful decode. As expected, the
code following the 10ms preamble is successfully decoded
and the decoder output is asserted to indicate this.
Figure 7 shows a comparison of unfiltered output (trace 2)
and filtered output. Note that prior to the 4.8ms continuous
signal, the filtered output (trace 3) remains low.
SquelchControlled
Output
MIC1555
Output
Figure 7. DO-Pin Noise Prior to Squelch
MICRF002 Advantages
The MICRF002 is ideal for systems that need to reduce
power consumption and unnecessary processing when there
is no RF signal present. The MICRF002 shutdown current is
approximately 1µA and start-up time is approximately 8ms. It
can give a significant reduction in power consumption if used
in a duty-cycled mode which can be easily achieved by
clocking the SHUT pin at some user defined duty ratio.
MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131
TEL
+ 1 (408) 944-0800
FAX
+ 1 (408) 944-0970
WEB
USA
http://www.micrel.com
This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or
other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc.
© 1999 Micrel Incorporated
Application Note 28
2
August 1999