Calling Line Identifier

FX602

© 1998 Consumer Microcircuits Limited

9

D/602/7

In FSK Receive mode the FX602 DET output will be set high when the level has exceeded the threshold for

sufficient time. Amplitude and time hysteresis are used to reduce chattering of the DET output in marginal

conditions.

Note that in FSK Receive mode this circuit may also respond to non-FSK signals such as speech.

See section 1.7.1 for definitions of Teon and Teoff

Figure 4 : FSK Level Detector operation

1.5.5

FSK Demodulator

This block converts the 1200 baud FSK input signal to a logic level received data signal which is output via the

RXD pin as long as the Data Retiming function is not enabled (see section 1.5.6). This output does not depend

on the state of the FSK Level Detector output.

Note that in the absence of a valid FSK signal, the demodulator may falsely interpret speech or other

extraneous signals as data.

1.5.6

FSK Data Retiming

The Data Retiming block extracts the 8 data bits of each character from the received asynchronous data

stream, and presents them to the

µC under the control of strobe pulses applied to the RXCK input. The timing

of these pulses is not critical and they may easily be generated by a simple software loop. This facility removes

the need for a UART in the

µC without incurring an excessive software overhead.

The block operates on a character by character basis by first looking for the mark to space transition which

signals the beginning of the start bit, then, using this as a timing reference, sampling the output of the FSK

Demodulator in the middle of each of the following 8 received data bits, storing the results in an internal 8-bit

shift register.

When the eighth data bit has been clocked into the internal shift register, the FX602 examines the RXCK input.

If this is low then the IRQN output will be pulled low and the first of the stored data bits put onto the RXD output

pin. On detecting that the IRQN output has gone low, the

µC should pulse the RXCK pin high 8 times. The high

to low transition at the end of the first 7 of these pulses will be used by the FX602 to shift the next data bit from

the shift register onto the RXD output. At the end of the eighth pulse the FSK Demodulator output will be

reconnected to the RXD output pin. The IRQN output will be cleared the first time the RXCK input goes high.

Thus to use the Data Retiming function, the RXCK input should be kept low until the IRQN output goes low; if

the Data Retiming function is not required the RXCK input should be kept high.

The only restrictions on the timing of the RXCK waveform are those shown in Figure 5a and the need to

complete the transfer of all eight bits into the

µC within 8.3mSec (the time of a complete character at 1200

baud).