CMX654 データシートの表示（PDF） - CML Microcircuits

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CMX654

V23 Transmit Modulator

CML Microcircuits 

V.23 Transmit Modulator

CMX654

1.5.4 Tx Data Retiming

The Data Retiming block, when enabled in 1200bits/sec transmit mode, requires the controlling µC to load 1 bit

at a time into the device by a pulse applied to the CLK input. The timing of this pulse is not critical and it 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 Tx re-timing circuit consists of two 1-bit registers in series, the input of the first is connected to the TXD pin

and the output of the second feeds the FSK modulator. The second register is clocked by an internally

generated 1200Hz signal and when this occurs the CLK input is sampled. If the CLK input is high the TXD pin

directly controls the FSK modulator, if the CLK input is low the FSK modulator is controlled by the output of the

second register and the RDYN pin is pulled low. The RDYN output is reset by a high level on the CLK input pin.

A low to high change on the CLK input pin will latch the data from the TXD input pin into the first register ready

for transfer to the second register when the internal 1200Hz signal next occurs.

So to use the retiming option the CLK input should be held low until the RDYN output is pulled low. When the

RDYN pin goes low the next data bit should be applied at the TXD input and the CLK input pulled high and then

low within the time limits set out in Figure 6.

To ensure synchronisation between the controlling device and the CMX654 when entering Tx retiming mode,

the TXD pin must be held at a constant logic level from when the CLK pin is first pulled low to the end of loading

in the second retimed bit. Similarly when exiting Tx retiming mode the TXD pin should be held at the same

logic level as the last retimed bit for at least 2 bit times after the CLK line is pulled high.

If the data retiming facility is not required, the CLK input to the CMX654 should be kept high at all times. The

asynchronous data to the FSK modulator will then be connected directly to the TXD input pin. This is illustrated

in Figure 5.

© 1998 Consumer Microcircuits Limited

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D/654/3

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