CMX910 datasheet(17/61 Pages) CMLMICRO

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AIS Baseband Processor

CMX910

D/910/6

5.4.2

Auto Nudge

Two auto nudge modes are provided which assist the µC with the initial synchronisation of the slot and

sample counters, and allow the CMX910 to subsequently keep the sample counter aligned without further

intervention. This requires an accurate UTC1PPS signal to be applied to the CMX910:

Auto nudge acquire

(Slot_Sample_Control b1-0 = 01). This mode can be used for initial counter

synchronisation after a device reset or in the case where the slot and sample counters have

become grossly misaligned for some reason. Auto nudge acquire should be enabled when the

CMX910 has just received a UTC1PPS rising edge from an “even” UTC second, which means

that the next UTC1PPS rising edge will be an “odd” second. The CMX910 will calculate the error

in the sample counter latched in from the “even” UTC second and will apply the required

correction to the sample counter at the next Nudge_Trigger point. A Nudge_Done interrupt is then

generated, indicating that sample counter alignment has been achieved.

Auto nudge track

(Slot_Sample_Control b1-0 = 10). In this mode, the CMX910 calculates the

correction needed for the sample counter once per second (on the rising edge of UTC1PPS).

“Odd” and “even” UTC seconds are treated differently: if 320

≤ Sample_Count < 960 when the

rising edge of UTC1PPS occurs, the CMX910 assumes it to be an “odd” UTC second and

calculates a sample nudge value of (640 - Sample_Count). Otherwise, the sample nudge value is

calculated as (-1

× Sample_Count). This calculated value (or ±Max_Auto_Nudge, whichever is

smaller in magnitude) is then added to the sample counter at the next Nudge_Trigger point. Note:

if the µC writes a non-zero value to the Slot_Nudge register when in auto nudge track mode, this

will be added to the slot counter at the same time that the sample counter value is updated. The

Slot_Nudge register gets auto-cleared after being used which causes a Nudge_Done interrupt,

otherwise Nudge Done interrupts are not generated in auto nudge track mode.

The Max_Auto_Nudge register is used to limit the magnitude of the allowed nudge in order to avoid

potential timing problems. The Max_Auto_Nudge register is ignored in manual nudge and auto nudge

acquire mode.

The typical sequence of events that the µC must perform to achieve and retain slot and sample counter

synchronisation (using auto nudge) is shown below:

a) If the device has just come out of reset, initialise Nudge_Trigger (see section 5.4.4) and

Max_Auto_Nudge registers.

b) Wait for an even UTC second to occur, then put the CMX910 into auto nudge acquire mode.

c) Wait for a Nudge Done interrupt, then put the CMX910 into auto nudge track mode.

d) Wait for the next UTC1PPS rising edge, then read the Slot_Sample_UTC1PPS register, and use

this to determine the error in the slot counter (the sample counter should be correctly aligned at

this point). Write the necessary correction to the Slot_Nudge register.

e) Wait for a Nudge Done interrupt. Both slot and sample counters will now be correctly aligned, and

the µC can proceed with AIS Rx and Tx operations, as required. No further Nudge Done

interrupts will be generated while in auto nudge track mode unless Slot_Nudge is written to again.

Continue monitoring the UTC time signal. If the CMX910 slot and sample counters become

misaligned for any reason (for instance, when a UTC leap second occurs), the µC must perform

another synchronisation sequence. If a direct UTC time signal becomes lost for any reason, then

the µC must switch the CMX910 to manual nudge mode and maintain synchronisation to a UTC

indirect source or an appropriate base station.

5.4.3

Sleep Mode

The Rx1 and Rx2 channels are individually configurable using bits in the receiver control registers

Rx1_Control and Rx2_Control. When enabled for AIS reception, it is possible to reduce power

consumption significantly by configuring the CMX910 to automatically turn off its internal receiver circuits

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