NBC12439

http://onsemi.com

11

Most of the signals available on the TEST output pin are

useful only for performance verification of the NBC12439

itself. However, the PLL bypass mode may be of interest at

the board level for functional debug. When T[2:0] is set to

110, the NBC12439 is placed in PLL bypass mode. In this

mode the S_CLOCK input is fed directly into the M and N

dividers. The N divider drives the FOUT differential pair

and the M counter drives the TEST output pin. In this mode

the S_CLOCK input could be used for low speed board level

functional test or debug. Bypassing the PLL and driving

FOUT directly gives the user more control on the test clocks

sent through the clock tree. Figure 6 shows the functional

setup of the PLL bypass mode. Because the S_CLOCK is a

CMOS level the input frequency is limited to 250 MHz or

less. This means the fastest the FOUT pin can be toggled via

the S_CLOCK is 250 MHz as the minimum divide ratio of

the N counter is 1. Note that the M counter output on the

TEST output will not be a 50% duty cycle due to the way the

divider is implemented.

T2

T1

T0

TEST OUTPUT

0

0

0

0

1

1

1

1

0

0

1

1

0

0

1

1

0

1

0

1

0

1

0

1

SHIFT REGISTER OUT

HIGH

FREF

M COUNTER OUT

FOUT

LOW

PLL BYPASS

FOUT

B 4

Figure 4. Parallel Interface Timing Diagram

M[8:0]

N[1:0]

P_LOAD

M, N

Figure 5. Serial Interface Timing Diagram

S_CLOCK

S_DATA

S_LOAD

T2

T1

T0

N1

N0

M6

M5

M4

M3

M2

M1

M0

First

Bit

Last

Bit

Figure 6. Serial Test Clock Block Diagram

FDIV4

MCNT

LOW

FOUT

MCNT

FREF

HIGH

TEST

MUX

7

0

TEST

FOUT

(VIA ENABLE GATE)

N

B

(1, 2, 4, 8)

0

1

PLL 12439

LATCH

Reset

PLOAD

M COUNTER

SLOAD

T0

T1

T2

VCO_CLK

SHIFT

REG

12- BIT

DECODE

SDATA

SCLOCK

MCNT

FREF_EXT

SEL_CLK

PLOAD acts as reset for test pin latch. When latch reset, T2 data is shifted out TEST pin.