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