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MT8940

ISO-CMOS ST-BUS FAMILY T1/CEPT Digital Trunk PLL

Mitel Networks 

ISO-CMOS MT8940

Applications

The following figures illustrate how the MT8940 can

be used in a minimum component count approach to

providing the timing and synchronization signals for

the Mitel T1 and CEPT interfaces, and the ST-BUS.

The hardware selectable modes and the

independent control over each PLL adds flexibility to

the interface circuits. It can be easily reconfigured to

provide the timing and control signals for both at the

master and slave ends of the link.

Synchronization and Timing Signals for the T1

Transmission Link

Figures 4 and 5 show examples of how to generate

the timing signals for the master and slave ends of a

T1 link.

At the master end of the link (Figure 4), DPLL #2 is

the source of the ST-BUS signals derived from the

4.096 MHz system clock. The frame pulse output is

looped back to DPLL #1 (in NORMAL mode), which

locks to it to generate the T1 line clock. The timing

relationship between the 1.544 MHz T1 clock and

the 2.048 MHz ST-BUS clock meets the

requirements of the MH89760/760B. The crystal

clock at 12.355 MHz is used by DPLL #1 to generate

the 1.544 MHz clock, while DPLL #2 uses the 4.096

MHz system clock to provide the ST-BUS timing

signals. The ST-BUS signals can also be obtained

from DPLL #2 in FREE-RUN mode, using a crystal

clock at 16.388 MHz instead of 4.096 MHz system

clock. The uncommitted NAND gate converts the

received signals, RxA and RxB of the MH89760 to a

single Return to Zero (RZ) input for the clock

extraction circuits of the MH89760. This is not

required for the MH89760B. The generated ST-BUS

signals can be used to synchronize the system and

the switching equipment at the master end.

At the slave end of the link (Figure 5) both the DPLLs

are in NORMAL mode with DPLL #2 providing the

ST-BUS timing signals locked to the 8 kHz frame

pulse (E8Ko) extracted from the received signal on

the T1 line. The regenerated frame pulse is looped

back to DPLL #1 to provide the T1 line clock as at

the master end. The 12.355 MHz and 16.388 MHz

crystal clock sources are necessary for DPLL #1 and

#2.

Synchronization and Timing Signals for the

CEPT Transmission Link

The MT8940 can be used to provide the timing and

synchronization signals for the MH89790/790B,

MITEL’s CEPT(30+2) digital trunk interface hybrid.

Since the operational frequencies of the ST-BUS and

the CEPT primary multiplex digital trunk are same,

only DPLL #2 is required to achieve synchronization

between the two.

Figures 6 and 7 show how the MT8940 can be used

to synchronize the ST-BUS and the CEPT

transmission link at the master and slave ends,

respectively.

Crystal Clock

(12.355 MHz

±100 ppm)

4.096 MHz

System Clock

(ST-BUS

compatible)

MT8940

MS0

MS1

MS2

MS3

F0i

C12i

ENCV

C8Kb

C16i

ENC4o

ENC2o

Ai

Bi

VSS RST

VDD

CV

C4b

C2o

F0b

Yo

MH89760

C1.5i

C2i

F0i

RxA

RxB

RxD

DSTi

DSTo

CSTi

CSTo

TxT

TxR

RxT

RxR

TRANSMIT

RECEIVE

MT8980/81

ST-BUS

SWITCH

T1

LINK

(1.544 Mbps)

MODE OF OPERATION FOR THE MT8940

DPLL #1 - NORMAL (MS0 = X; MS1 = 0)

DPLL #2 - OVERRIDE THE MAJOR MODES (MS2 = 1; MS3 = 0)

Figure 4 - Synchronization at the Master End of the T1 Transmission Link

3-33

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