Calling Line Identifier
CMX602A
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
µ
C 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
µ
C 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 CMX602A 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
µ
C 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 CMX602A 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
µ
C within 8.3ms (the time of a complete character at 1200 baud).
©
1998 Consumer Microcircuits Limited
9
D/602A/4
CMLMICRO [ CML MICROCIRCUITS ]
