Cirrus Logic CS61584A - Manual

1
Copyright  Cirrus Logic, Inc. 1999
(All Rights Reserved)
P.O. Box 17847, Austin, Texas 78760
(512) 445 7222 FAX: (512) 445 7581
http://www.cirrus.com
CS61584A
Product Technical Brief
Reading and Writing Waveforms on the CS61584A
The Arbitrary Waveform Generator (AWG) is a
unique feature of Crystal LIU’s that allows the cus-
tomization of the output waveform for non-stan-
dard terminations. The function of the AWG on the
CS61584A is described in the CS61584A datasheet
(DS261PP4). The following note describes the pro-
cedure for accessing the waveform memory. Note
that the memory access function is only available
in Host mode.
1. The first step is to set up the pulse shape measur-
ing equipment according to the applications
note, “Measurement and Evaluation of Pulse
Shapes in T1/E1 Systems” (AN07REV2). Only
after this has been done can the user get a picture
of the true pulse shape.
2. The fastest way to arrive at an ideal pulse shape
is to start with one of the pulse shapes that
already exist in ROM. To read the ROM data,
first set the CON[3:0] bits to match the imped-
ance and line length conditions that are closest to
the given application. This is done with a write
to the Control B register for Channel 1 (address
0x16 using the serial port, or 0x06 with the par-
allel port). The four least significant bits of the
data are set to the desired line configuration (see
Table 1 in the data sheet). For a more complete
description of the input timing, please refer to
timing diagrams in the beginning of the
datasheet.
3. Reading a byte from the ROM takes two
accesses; the first is a memory read that tells the
device that the ROM is being accessed, and the
next one fetches the data. The ROM address is
0x1A on the serial port, and 0x0A on the parallel
port. This is followed by another read, which has
the desired cell number (0x00 to 0x29) for its
address. The data read back is the value of that
cell. It’s best to read and save the contents of all
42 cells for a given pulse shape before making
any modifications.
4. Now the shape of the output pulse can be modi-
fied. The conversion from the value of a cell to a
voltage depends on the setting of the CON[3:0]
bits. The conversion factors are given in the sec-
tion titled, "Arbitrary Waveform Registers" in
the data sheet. The pulse shape values are 7 bit,
two’s complement numbers. The device uses the
same stored pulse for both positive and negative
pulses. The entire stored pulse shape is inverted
for a negative pulse. Negative values in the
memory are for negative overshoot on a positive
pulse.
When adjusting the values, note that the output
amplifier is slew rate limited. It will take two
cells for the output pulse to go from 0V to posi-
tive or negative full scale. Making large changes
in the pre-stored waveform should be avoided,
since the results may be hard to predict. If large
changes are necessary, it’s better to start with a
different waveform.
5. Write the new data into the AWG RAM. This is
a single port RAM, so it can't be used to drive the
output pulse while it is being modified. To
ensure that there are no memory conflicts, the
LIU must be configured to use one of the ROM
waveforms while the AWG RAM is being
accessed (e.g., CON[3:0] = 0000).
TB261BPP1
SEP ‘99