CMX469A

-1200/2400/4800 Baud FFSK Modem

Modems

CMX469A

-1200/2400/4800 Baud FFSK Modem

The CMX469A is a CMOS LSI circuit which operates as a full-duplex 1200, 2400 or 4800 baud FFSK Modem.

The mark and space frequencies are 1200/1800, 1200/2400 and 2400/4800Hz respectively. Tone frequencies are phase continuous; transitions occur at the zero crossing point.

A common Xtal oscillator is employed with a choice of two clock frequencies (1.008MHz or 4.032MHz) to provide baud-rate, transmit frequencies, and Rx and Tx synchronisation.

The modem includes circuitry for Carrier Detect and Rx Clock recovery, both of which are made available as output pins. Rx, Tx and Carrier Detect paths contain a bandpass filter to ensure the provision of optimum signal conditions.

The CMX469A demonstrates a high sensitivity and good bit-error-rate under adverse signal conditions.

This low-power device requires few external components and is available in small outline plastic SOIC and TSSOP packages.

Devices are available through our Distributor Network.

Download Datasheet

Block diagram of the CMX469A chip, showing Tx generator/filter, clock oscillator and divider, Rx filter, limiter, digital filter chain, digital PLL, data latch and S/N comparator with carrier detect output

Block Diagram – CMX469A

Need more information? Contact us here

CMX469A – Simple Data Communications

Simple Data Communications with the CMX469A

View

CMX469A Firmware

Firmware listing (text file) for the above CMX469A Application Note

View

MX469 to CMX469A

Design Migration from MX469 to CMX469A

View

I currently have a 10kohm pullup resistor on each of the RX SYNC and CARRIER DETECT O/P pins. Will this cause a problem?

A. Yes it will. The digital output impedance of those pins is typically 4000 ohms. When a 10kohm pullup resistor is attached to each of those pins, a voltage divider is created that prevents the voltage on those pins from approaching the expected “low” value of near 0V. This can cause problems for a microcontroller that is monitoring the signal on those pins. For example, the higher-than-expected voltage on the CMX469A digital output pin (with an attached pullup resistor) may not reach the microcontroller’s digital low “threshold” value, and this can cause lots of problems.

What is the difference between the FX469 and the CMX469A?

A. There are no major differences between the FX469 and the CMX469A. The CMX469A is the preferred device for new designs and it posesses the advantage of operation down to 2.7V DC. For those customers contemplating migration to the CMX469A there is a difference in the operation of the Carrier Detect Output that may be important. When powersaved the FX469 Carrier Detect always returns to Vss (low).

Why does the CMX469A generate interrupts for received data when there is no data present?

A. The FFSK receiver of the CMX469A has a very high sensitivity and in the absence of valid signals will decode noise as data. This is not a fault but is due to the wideband properties of noise where sufficient energy may be present within the FFSK decoder’s bandwidth. To prevent spurious reception a baseband carrier detect output (CARRIER DETECT O/P) is used to determine the validity of the received signal.

Does it matter which edge of the “Tx Sync” signal I use to strobe Tx data in on?

A. Yes it does. The rising edge of the Tx Sync (data clock) signal determines when the data is latched into the CMX469A. The data must be valid for 2.0us before and after the rising edge; otherwise erroneous operation will result.