TMS320 DSP
DESIGNER’S NOTEBOOK
Initializing the TLC32040
AIC on the TMS320C5x
DSK
APPLICATION BRIEF: SPRA275
Agnès Delaurière and Olivier Grislain
Digital Signal Processing Products
Semiconductor Group
Texas Instruments
July 1996
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Contents
Abstract......................................................................................................................... 7
Design Problem ............................................................................................................ 8
Solution......................................................................................................................... 8
Figures
Figure 1. Possible values of TA and TB according to desired fc and fs................ 11
Figure 2. Shannon criteria satisfied ......................................................................... 13
Figure 3. Shannon criteria unsatisfied: aliasing...................................................... 14
Initializing the TLC32040 AIC on the
TMS320C5x DSK
Abstract
This document discusses the proper way to initialize the registers of
the AIC given certain specifications. Details of the setup criteria,
equations defining the acceptable options, and calculated data are
presented to illustrate the solution.
Initializing the TLC32040 AIC on the TMS320C5x DSK 7
Design Problem
What is the proper way to initialize the registers of the AIC, given a
certain prototype (sampling frequency and lowpass cutoff
frequency)?
Solution
The main problem which is often encountered consists in setting the
correct AIC parameters in order to meet the specifications of a
digital filter in terms of sampling frequency and bandwidth.
Introduction
The purpose of this document is to provide an easy and efficient
way to:
Study the effects of antialiasing according to the sampling
frequency (limited to 19.2 kHz for a TLC32040 device) and the
low bandpass cutoff frequency.
Adapt a given prototype defined by its bandpass to the hardware
specifications of the TMS320C5x DSP Starter Kit (DSK).
Synthesize a digital filter and implement it on a ’C5x DSK.
The programs enclosed in the appendices consist in sampling an
analog input signal issued from a function generator (e.g., a
sinewave) and sending it back to the analog output of the AIC, for
visualization on an oscilloscope.
Overview
The initialization of the system can be divided into three distinct
parts:
Initialization of the DSP (software wait-states, sign extension
mode, global interrupt register, etc.);
Initialization of the timer and serial port (e.g., Master Clock
frequency, receive and transmit registers, etc.);
Initialization of the AIC (sampling frequency, low passband cutoff
frequency, and other parameters, etc.).
The initialization of the ’C50 DSP on the DSK board (status and
control registers for wait-state generation, interrupt flags settings,
etc.) as well as the initialization of the timer and serial port are
explained in details in the TMS320C5x User’s Guide (SPRU056),
specifically chapters 3 and 5.
The initialization of the AIC is based on three parameters:
8 SPRA275
The AIC control register (to set up gain, synchronization,
loopback, etc.);
The Tx counter A register (and Rx counter A if synchronous
mode is disabled);
The Tx counter B register (and Rx counter B if synchronous
mode is disabled).
The above parameters, in turn, are based on the two following
frequencies specified by the system:
The sampling frequency, F s ;
The low passband cutoff frequency, Fc LP .
In this example, Tx counter A and Tx counter B (also named TA and
TB) AIC registers are used in synchronous mode to determine the
D/A (and therefore A/D) conversion timing.
The low bandpass cutoff frequency is determined by the
approximate formula below:
f MCLK (kHZ ) 1
fc = × × 3.6kHz
288kHz 2 × TA
(where f MCLK is supplied by the TOUT pin of the DSP and will be
equal to 10 MHz in this example).
In case a passband filter is used (cf. programmation of the AIC_CTR
register), the high bandpass cutoff frequency is determined by the
approximate formula below:
f MCLK (kHZ ) 1
f cHP = × × 300 Hz
288kHz 2 × TA
Method to Determine TA and TB Values According to Fs and Fc
Note: More detailed information regarding the protocol of
transmission between the DSP and the AIC can be found in the
TLC32040 AIC data sheet (SLAS014).
Initializing the TLC32040 AIC on the TMS320C5x DSK 9
The TLC32040 AIC integrates a lowpass, switched-capacitor,
output-reconstruction filter. In order for the switched-capacitor
lowpass or bandpass to meet their transfer function specifications,
the frequency of the clock inputs of the switched-capacitor filters
must be 288 kHz. If the frequencies of the clock inputs are not 288
kHz, the filter transfer function frequencies are scaled by the ratios
of the clock frequencies to 288 kHz, as it appears in the two
formulas above.
The sampling frequency is given by the following formula:
f MCLK
fs =
2 × TA × TB
Once f MCLK is fixed, F s depends on the TA × TB value and Fc LP
depends only on the TA value Therefore, TA and TB are entirely
defined by F s and Fc LP with the following constraints:
1) TA is an integer in the range 4 to 31 (5 unsigned bits).
2) TB is an integer in the range 2 to 63 (6 unsigned bits).
3) F c as near as possible, but less than F s /2, to satisfy the
Shannon criteria.
The procedure to determine TA and TB parameters is the following:
Step 1: Calculate TA × TB related to the desired sampling
frequency. Note that if the sampling frequency is not a constraint for
your application, then it has to be chosen at least 2xF c according to
the Shannon criteria and so that TA × TB is an integer.
Step 2: Considering that TA × TB is an integer, within the list of all
possible integer values of TA and TB, only some of them satisfy the
two first conditions.
Step 3: The user must then choose between these results with the
one which satisfies the third condition, on calculating Fc LP .
Example
For an application requiring a sampling frequency F s = 9.6 kHz and
in order to satisfy the Shannon criteria, the bandwidth must be
limited to 4.8 kHz. Hence, it is recommended to filter the input signal
by a lowpass filter with a cutoff frequency of
Fc = Fs/2 =4.8 kHz.
10 SPRA275
According to the explanations above, the results are:
f MCLK 3.6 62.5kHz
f cLP = × =
2 × TA 288 TA
f MCLK
TA × TB = = 625 = 54
2 × fs
And the constraints are: 31 ≥ TA ≥ 4
63 ≥ TB ≥ 2
The easiest way to study the different possibilities to obtain the
desired sampling frequency and the appropriate cutoff frequency is
to fill up a table as shown in Figure 1. The fixed parameter is f s =
9.6 kHz, which involves a product TA × TB = 54. First, fill the first
column by giving to TB all the possible combinations of the prime
factors.
Figure 1. Possible values of TA and TB according to desired fc and fs
TB TA TA x TB fC (kHz)
5 125 > 31
25 25 625 2.5
125 > 63
Only one combination is acceptable:
TA=25 and TB=25 giving a cutoff frequency of 2.5 kHz.
To select one of these set of values, you have to consider the
desired cutoff frequency of your antialiasing filter and, then, choose
the most appropriate. For the 9.6 kHz sampling frequency, the ideal
antialiasing filter rejects all frequencies above 4.8 kHz. Hence, you
will have to adopt the combination: TA=25 and TB=25 and to satisfy
with FCLP= 2.5 kHz.
According to the Shannon criteria, the optimized operating
conditions are tained when fC ≤ fs/2 . Therefore, in most of the cases,
we recommend to have fc as near as possible, but less than fs/2 . In
case of filtering a signal under aliasing conditions, we let the user
choose the most appropriate values for TA and TB registers which
depend on the nondisturbed passband authorized for the
application.
Initializing the TLC32040 AIC on the TMS320C5x DSK 11
Besides the three steps of establishing the communications between
a DSP and the TLC32040 AIC, interrupt service routines must be
defined to set up the action to follow after a data word has been
received by the AIC or transmitted to it. The main program calls the
different routines corresponding to the initialization process, enables
or disables the interrupt registers whenever required, then waits for
data to be handled by the receive interrupt service routine.
This program, called AICDSK50.ASM, can be downloaded from the
TI DSP BBS or ftp site mirror. It can be assembled and linked by the
tools supplied with a DSK. The following procedure tells you how to
assemble this program to have it run on your board:
DSK5A AICDSK50.ASM (generates the AICDSK50.DSK file)
DSK5L AICDSK50
Operating Recommendations
Analog Input: Maximum Absolute Input Voltage: 3 V
Frequency range: 200 Hz–2500 Hz for F s =8 kHz
Under these recommendations, you should obtain the following
results:
Software Hardware
Fs (in kHz) TA TB Fc (in kHz) Band Pass (fc) Aliasing
4 25 50 2.5 200 – 1500 Hz Yes
8 25 25 2.5 200 – 2500 Hz No
10 20 25 3.125 260 – 3000 Hz No
10 10 50 6.25 530 – 3600 Hz Yes
The results obtained when Fs = 8 or 10 kHz (first case) correspond
to the case where Fs > 2 Fc (see Figure 2).
12 SPRA275
Figure 2. Shannon criteria satisfied
The Shannon criteria is satisfied. The bandpass obtained matches
with the values of the cutoff frequencies calculated by the formulas
above. The first and last result correspond to the case
where Fs < 2 Fc.
The results obtained for these values of TA and TB confirm the
presence of aliasing in the output signal between 1.5 kHz and 2.5
kHz for the first case and between 3.6 and 6.25 kHz for the last
result. The output signal is not stable on the screen of the
oscilloscope, when synchronized by the input signal.
Interpretation
Theoretically, let us consider a lowpass filter instead of a bandpass
filter to simplify the reasoning (see Figure 3). Then, all the
frequencies between 0 and Fs/2 are sampled according to the
Shannon rule, but the frequencies in [Fs/2, Fc] are under aliasing.
Because of the resulting superposition in the spectrum, the resulting
nondisturbed bandwidth is actually [0, F s –F c ] (as represented in
the shaded portion in Figure 3).
Initializing the TLC32040 AIC on the TMS320C5x DSK 13
Figure 3. Shannon criteria unsatisfied: aliasing
Experimentally, for the first case, we observe that [200–1500 Hz] is
not at all modified by aliasing. In the range [1.5, 2.5 kHz], the signal
is subject to aliasing. After 2.5 kHz, the amplitude of the signal falls
down.
14 SPRA275
