ACS8946 datasheet(13/40 Pages) SEMTECH

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ADVANCED COMMUNICATIONS

FINAL

DATASHEET

Revision 3/November 2006 © Semtech Corp.

Page 13

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ACS8946 JAM PLL

Source Switching - State Diagram

Figure 11 Simplified State Diagram of Source Switching

The state diagram in Figure 11 shows a simplified view of

the automatic switching behavior in the presence of

activity alarms. The ALARMC_CO3 signal from the PFD is

used to disqualify a clock, and the signals ALARM1_CO0

and ALARM2_CO1 representing no activity on input

clocks CLK1 and CLK2 respectively, are used to

determine whether or not to select the remaining clock.

Switching between CLK1 and CLK2 is non-revertive.

With ALARMC_CO3 providing a view of the currently

selected clock that is independent to ALARM1_CO0 and

ALARM2_CO1 signals, source selection behavior can be

more complex when these alarm signals disagree, and so

the state machine is necessarily more complex than the

one shown here in order to accommodate such behavior

e.g. when a clock signal is disconnected for a very short

period of time, or when an input clock is running at the

wrong frequency. If further details are required contact

Semtech Sales Support.

Configuration

A higher degree of flexibility and programmability is

possible via the use of configuration pins on the device.

Permanent connections made externally from

CFG_IN[7:0] pins to the configuration output pins

ALARM1_CO0, ALARM2_CO1, CFG_OUT2, ALARMC_CO3

or to ground or VDD set up the device.

The ACS8946 GUI software presents the configuration

information in the most user-friendly manner, though the

following tables can be used instead to work out the

connectivity required for a particular configuration. For

example, the last five columns in Table 10 give the results

of the wired connections shown in the second and third

columns. E.g., taking the row 7, connecting pin CFG_IN2

to VDD and CFG_IN3 to ALARM2_C01, gives an input

frequency of 19.44 MHz, a highest output frequency of

622.08 MHz and configures the outputs as LVPECL.

Output Configuration

The output spot frequency selection for OUT1 is

asynchronously controlled by the RATE1A/B select pins

(pins 47 and 48), which select one from a set of four

“Available Rates” that have been pre-selected at power-

up by the wiring configuration of pins 18 and 19

(CFG_IN[1:0]). The wiring configuration of these two pins

preselects a set of any four out of seven rates:

19.44 MHz, 38.88 MHz, 77.76 MHz, 155.52 MHz,

311.04 MHz, 622.08 MHz and disabled, which means

that each of the four outputs can run independently at any

one of the four pre-selected rates - chosen by the AB value

in Table 11 and odd divisions thereof as defined by the

start-up configuration of CFG_IN2/CFG_IN3 and/or

CFG_IN6/CFG-IN7 respectively).

OUT2 is asynchronously controlled by the RATE2A/B

select pins (pins 45 and 46) in the same way as OUT1.

Outputs OUT3 and OUT4 cannot be controlled

asynchronously; the output frequency selection is

controlled at power-up or on reset by a combination of the

connections of CFG_IN[1:0] and CFG_IN[5:4] to either

VSS, VDD, ALARM1_CO0 (pin 13) or ALARM2_CO1 (pin

14). Given the four Available Rates have been configured

as described previously, which one of these four rates is

available on OUT3 is then dependent on the connections

of the CFG_IN4 and CFG_IN5 pins to either VSS, VDD,

ALARM1_CO0 (pin 13) or ALARM2_CO1 (pin 14)—see

Table 12.

The method to configure the device is summarized as

follows:

Select the required “Available Rates” that will be

made available for selection at all four outputs using

CFG_IN[1:0] (See Table 11).

Define the frequencies of the fixed outputs

OUT3/OUT4 using CFG_IN[5:4] (See Table 12) and

the required RESYNC Edge result.

Table 9 156.25 MHz Input Frequency

Bandwidth

R1 & R2/

Ω

C2 & C4/

µF

C1 & C3

µF

Closed Loop

2000

100

4000

150

4.7

8000

270

0.68

7.5

CLK 2

CLK 1

ALARMC_CO3 = 1

ALARM2_CO1 = 0

F8946D_012SimpStateDiag_01

ALARM SIGNALS:

ALARMC_CO3 -- Activity alarm for the currently selected clock (from PFD)

ALARM1_CO0 -- Activity alarm for CLK1

ALARM2_CO1 -- Activity alarm for CLK2

ALARMC_CO3 = 1

ALARM1_CO0 = 0

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