CMX910 datasheet(30/61 Pages) CMLMICRO

30 / 61 page

AIS Baseband Processor

CMX910

D/910/6

events must be loaded in chronological order, with a minimum delay of 1 sample period between

consecutive events (maximum delay = 32767). All fourteen of these event slots must be filled; if

certain transmit events are not required, they should be replaced with the DUMMY event.

4. Apply special command “setup_tx_sequence” ($0F). This causes the CMX910 to process the

table of Tx events and configure its internal transmit logic accordingly.

Note that when the start sample (Tx_Start) is reached within Tx_Slot, the CMX910 inserts a fixed delay of

four samples – this is in addition to the delay specified for the first event in the sequence table. Also note

that the last six events specified in the sequence table are timed from the end of the last message bit

being fed to the internal modulator.

The transmit events and their code numbers are as follows:

RF_ON

Pin ENAB0 is asserted.

TX_ON

Pin ENAB4 is asserted.

PA_ON

Pin ENAB5 is asserted.

DAC0_RAMPUP

DAC0 begins ramping up (used for PA ramp up control).

CSTDMA_START

Defines the start of the CSTDMA sensing window.

CSTDMA_END

Defines the end of the CSTDMA sensing window.

MODULATE_START

Data modulation begins. (Tx data fed to internal filters).

MODULATE_END

Data modulation ends; reference for power down events.

DAC0_RAMPDOWN

DAC0 begins ramping down.

PA_OFF

Pin ENAB5 is negated.

TX_OFF

Pin ENAB4 is negated.

RF_OFF

Pin ENAB0 is negated.

DUMMY

No action.

CHAINED_MESSAGE_START 14

The start of the sub-sequence repeated for chained messages.

CHAINED_MESSAGE_END

The end of the sub-sequence repeated for chained messages.

For the duration that ENAB5 is asserted (i.e. between the PA_ON and PA_OFF events) the CMX910

turns off the three Rx control lines (ENAB1-3), and disables its internal Rx1/Rx2 circuits. It is intended that

during this period the external Tx/Rx switch for the antenna switches to select Tx.

When calculating the MODULATE_START timing value, the delay through the CMX910’s internal transmit

filters and any external components must be taken into account to ensure that data bits appear on-air at

the correct time (the filter delays are specified in section 7). The MODULATE_END event has an in-built

delay of 46 sample times to allow the last bit to make its way out of the transmit filter. Allowance must be

made for this built-in delay, as well as for the delay through any external components, when calculating

the timing of the transmit power down events.

Because all actions subsequent to Tx_Start effectively use relative timings, they will not be disturbed by

any sample counter “nudge” that may occur during transmission. It is important, however, to ensure that

any “nudge” that occurs does not cause the sample counter to skip past the Tx_Start point, which would

cause the transmission to be missed.

A working example of how to set up a transmit event sequence is shown in Table 2 (the order of events

and delay timings shown are for illustrative purposes only):

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