CS61884 datasheet(43/72 Pages) CIRRUS

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CS61884

DS485PP4

15. ARBITRARY WAVEFORM

GENERATOR

Using the Arbitrary Waveform Generator (AWG)

allows the user to customize the transmit pulse

shapes to compensate for nonstandard cables,

transformers, protection circuitry, or to reduce

power consumption by reducing the output pulse

amplitude. A channel is configured for a custom

pulse shape by storing data representing the pulse

shape into the 24/26/28 phase sample locations and

then enabling the AWG for that channel. Each

channel has a separate AWG, so all eight channels

can have a different customized pulse shape. The

microprocessor interface, is used to read from or

write to the AWG, while the device is in host mode.

In the AWG RAM, the pulse shape is divided into

two unit intervals (UI). For E1 mode, there are 12

sample phases in each UI, while in T1/J1 mode, the

number of sample phases per UI are either 13 or 14.

The first UI is for the main part of the pulse and the

second UI is for the “tail” of the pulse (Refer to

Figure 14). A complete pulse-shape is represented

by 24 phase samples in E1 mode or 26/28 phase

samples in T1/J1 mode. In E1 mode, data written in

the first UI represents a valid pulse shape, while

data in the second UI is ignored and should be set

to zero.

The mode of operation is selected using the Line

Length Channel ID Register (10h) (See Section

14.17 on page 38) and the Line Length Data Reg-

ister (11h) (See Section 14.18 on page 39). A

phase sample, or cell, is accessed by first loading

the channel address and the phase sample address

into the AWG Phase Address Register (17h) (See

Section 14.24 on page 40), and then reading or

writing the AWG Phase Data Register (18h) (See

Section 14.25 on page 40). The upper locations in

each channel’s address space are not used; reading

and writing to these registers produces undefined

results.

The data in each phase sample is a 7-bit two’s com-

plement number with a maximum positive value of

0x3f, and a maximum negative value of 0x40. The

terms “positive” and “negative” are defined for a

positive going pulse only. The pulse generation cir-

cuitry automatically inverts the pulse for negative

going pulses. The data stored in the lowest phase

address corresponds to the first phase sample that

will be transmitted in time. When the mode of op-

eration calls for only 24/26 phase samples if the

phase samples that are not used (25 through 28) are

written to, they are ignored and don’t effect the

shape of the customized pulse shape.

The following procedure describes how to enable

and write data into the AWG to produce custom-

ized pulse shapes to be transmitted for a specific

E1 AWG Example

DSX-1 (54% duty cycle) AWG Example

DSX-1 (50% duty cycle) AWG Example

Figure 14. Arbitrary Waveform UI

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