Effective Accuracy - Analog Devices SHARC ADSP-214 Series Hardware Reference Manual

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configuration registers, duty cycle registers and the

result, it is possible to alter both the characteristics (switching frequency

and dead time) as well as the output duty cycles at the mid-point of each

PWM cycle. Consequently, it is possible to produce PWM switching pat-

terns that are no longer symmetrical about the mid-point of the period

(asymmetrical PWM patterns).

In double update mode, it may be necessary to know whether operation at

any point in time is in either the first half or the second half of the PWM

cycle. This information is provided by the

period (between the rising edge of the original PWM interrupt latch pulse

and the rising edge of the new PWM interrupt pulse introduced in double

update mode). The

operation in the second half of each PWM period. This status bit allows

programs to make a determination of the particular half-cycle during

implementation of the PWM interrupt service routine, if required.

The advantage of the double update mode is that the PWM process can

produce lower harmonic voltages and faster control bandwidths are possi-

ble. However, for a given PWM switching frequency, the interrupts occur

at twice the rate as in double update mode. Since new duty cycle values

must be computed in each PWM interrupt service routine, there is a larger

computational burden on the processor in the double update mode.

Alternatively, the same PWM update rate may be maintained at half the

switching frequency to give lower switching losses.

Effective Accuracy

The PWM has 16-bit resolution but accuracy is dependent on the PWM

period. In single update mode, the same values of

to define the on times in both half cycles of the PWM period. As a result,

the effective accuracy of the PWM generation process is 2 x

for a 200 MHz clock). Incrementing one of the duty cycle registers by one

changes the resultant on time of the associated PWM signals by 2 x

ADSP-214xx SHARC Processor Hardware Reference

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bit of the the

Pulse Width Modulation

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