august 1999 1 application note 28 application note 28 micrel application note 28 data squelch using the micrf002 by sean montgomery introduction 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 con- trols 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 decod- ing, 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 digi- tal squelch. analog squelch adding a small offset to the c th 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. signal from peak detector c th r sc slicing leve l logic-high modified slicing leve l time time signal level unmodified output demodulated background noise (probe b) logic-high time modified output demodulated background quiet (probe b) background noise (probe a) ?robe b ?robe a figure 1. adding offset (analog squelch) micrel, inc. ?1849 fortune drive ?san jose, ca 95131 ?usa ?tel + 1 (408) 944-0800 ?fax + 1 (408) 944-0970 ?http://www.mic rel.com digital squelch introduction to micrf002 features the micrf002 includes two important features that differen- tiate 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. ~6.75mhz reference oscillator 256 demodulated data 128-bit counter wakeb (active low) figure 2. simplified wake-up block diagram 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 de- tected 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. sel0 swen vssrf refosc vssrf sel1 ant cagc vddrf wakeb vddbb shut cth do c4 33nf cr1 6.75mhz mi c rf002 c1 4.7f 5v c2 4.7f c3 10nf whip antenna nc vssbb interrupt 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. qwikradio is a trademark of micrel, inc. the qwikradio ics were developed under a partnership agreement with ait of orlando, fl orida
application note 28 micrel application note 28 2 august 1999 sel0 swen vssrf refosc vssrf sel1 ant cagc vddrf wakeb vddbb shut cth do c4 33nf cr1 6.75mhz micrf002 c1 4.7f 5v c2 4.7f c3 10nf whip antenna nc vssbb out gnd vs trg thr d1 1n4148 r1 1.8m ? mic1555 c4 100nf squelch- controlled output figure 4. receiver with digital data squelch 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. v out(1555) v out(and) v wakeb v do micrf002 digital output micrf002 wake-up output mic1555 output squelch- controlled 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 de- tect 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 squelch- controlled 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. squelch- controlled output holtek code good output transmit data micrf002 digital output figure 6. circuit applied to evaluation board v out(and) v out(1555) v do v tx squelch- controlled output mic1555 output transmit data micrf002 digital 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 1 a 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 usa tel + 1 (408) 944-0800 fax + 1 (408) 944-0970 web 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 an y 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 pat ent right of micrel inc. ? 1999 micrel incorporated
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