T9000
Preliminary Data Sheet
November 2000
ISDN Network Termination Node (NTN) Device
12.2 PWM Auto Operation (Sine) Mode
12 PWM Module (continued)
The auto mode uses a sine modulator controller
(PWSM) to substantially reduce the overhead require-
ment of the microcontroller. In this mode, the width of
the pulses automatically follows the amplitude of a sine
wave of frequency Fs. A 256-byte ROM is used to store
discrete values of amplitude for one period of a sine
wave, where each successive ROM location, n,
represents the sine amplitude at a normalized time
of t = n/256.
For low-frequency tones (Hz range), the algorithm that
defines the width of the pulse is easily accomplished
with microcontroller routines (manual mode). However,
implementing higher frequency tones (kHz range)
requires a large degree of microcontroller intervention.
To address this issue, the PWM generators were
designed to operate in two different modes: manual/
timer mode and auto mode.
Figure 22 shows a simplified architecture of the PWM
block (the shaded areas indicate the extra logic
12.1 PWM Manual/Timer Operation Mode
required for implementing the sine wave functionality).
The 8-bit ROM address is derived from the upper 8 bits
of the 16-bit accumulator output. The accumulator sim-
ply adds the 16-bit value formed by the PWxVH and
PWxVL registers (PWV) to its output every cycle,
where the cycle time is determined by the pulse period,
PP. Consider then, how PWV and PP affect the output.
In manual mode, the user may implement any desired
algorithm to define the width of the pulses. Two impor-
tant parameters that are controlled via the PWxCF reg-
ister are pulse-width granularity and pulse-width range.
Pulse-width granularity defines the minimum duration
(or tick) of a pulse width. Pulse-width range denotes
the number of possible ticks in a pulse period or, in
other words, the number of different width values with
which the pulse can be modulated. The tick size and
pulse period may be expressed as:
When PWV is <28, each ROM value will be output for a
least one cycle, and possibly even more (depending on
how far below 28 the PWV value is). Conversely, when
PWV is >28, some ROM values will be skipped. Thus,
as the value of PWV drops below 28, it has the effect
of increasing the quantization error in the amplitude of
the sinewave output. PWV, then, can be thought of as
controlling the ROM step size (where the step size can
be <1).
Tick = Granularity x 65 ns
(1)
PP = Range x Tick = Range x Granularity x 65 ns (2)
Concerning the above relationships, note the following:
■ A small granularity allows for a finer resolution of the
resulting output signal in time, and therefore requires
less filtering.
When PP is large, the rate at which each newly formed
ROM address is output is slower than when PP is
small. Therefore, if all other factors are equal, a larger
PP will result in a lower frequency sinewave output. PP,
then, can be thought of as controlling the ROM step
rate.
■ A large range allows for a finer resolution, in ampli-
tude, of the resulting output signal.
■ Power consumption is roughly inversely proportional
to the granularity value, so the larger the granularity,
the less power the circuit will consume.
In auto mode, range and granularity take on a some-
what different meaning than in manual mode. In auto
mode, Equations (1) and (2) still hold with respect to
range and granularity, but tick does not play a direct
role in this case. Rather, it is the combination of range
and granularity that determines the frequency and
amplitude resolution of the output waveform as
explained above.
■ As granularity and range are increased, the equiva-
lent oversampling rate is decreased (i.e., the pulse
period, PP, increases as shown in equation 2 above).
At the start of a pulse period, the controller loads the
value contained in register PWxVH and generates a
pulse with a width PWxVH multiplied by the tick value
(where only the appropriate MSBs of PWxVH are used
according to the tick value, see register PWxVH). The
value in PWxVL determines the rate at which the
PWIR[PWxI] interrupt register bit will be asserted. The
module asserts the PWIR (PWxI) interrupt register bit
every PWxVL + 1 pulse period intervals. The interrupt
is generated only if the PWxCF (PWxIE) bit is set. The
interrupt is asserted even if GPIO pin is not assigned to
the PWMx generator. The interrupt register is reset
upon a register read operation.
94
Lucent Technologies Inc.
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