T9000
Preliminary Data Sheet
November 2000
ISDN Network Termination Node (NTN) Device
12 PWM Module (continued)
12.4 PWM Auto Mode Example (continued)
As mentioned earlier, when PWV is greater than 28, some ROM values will be skipped. Another way of stating this
is that, when Ka is less than 28, some ROM values will be skipped. This is true by definition, since Ka represents
the number of samples output during one period of a sine wave, and there are 28 ROM samples. If one criterion of
the generated sinewave is that the amplitude be as accurate as possible (i.e., no ROM values are skipped), then
the choice of entries in Table 103 is limited to those with values of Ka that are greater than or equal to 256. Note
that only three of the entries meet this requirement in this example.
This example is provided to illustrate some of the considerations when selecting values for range and granularity. It
may be useful to construct a spreadsheet that reproduces Table 103 (which itself was generated using a spread-
sheet) for the frequency value of interest. Other engineering trade-offs, such as low-pass filter complexity vs. PWM
output accuracy, are beyond the scope of this document but may be important to consider depending on the spe-
cific system requirements.
2.5%—97.5% SINE MODULATION SAMPLES
35
30
25
20
15
10
5
0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
SAMPLE (PERIOD) #
5-6524F
Figure 23. Widths of PWM Pulses Generated with a 2.5%—97.5% Modulation Width
Lucent Technologies Inc.
99
AGERE [ AGERE SYSTEMS ]
