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Application brief BR1544/D

Started by Roelof Oelofsen February 12, 2002
Hi all,

In the motorola app. brief to design a class D amplifier, the author
states that to work out the PWM value register, the following formula
applies:

PWM Value = (Input Data Sample) x (PWM Modulus)

However, in the 56f80x user's manual, it is stated then whenever the
PWM Value is greater than or equal to the PWM Modulus, the output of the
PWM will be 0 permanently (which makes sense).
When using above formula, the PWM value will always be more than the
PWM modulus except when the input data samples = 0. (The input data
samples is from a audio CODEC which have a 16 bit resolution). Obviously
I am reading the formula incorrectly. Can someone please shed more light
on this subject for me.

Thank you in advance...

Roelof Oelofsen
Telkom Development Laboratory
Telkom SA




Using fractional math, if the input data sample is a FRACT16 number, the
formula holds true. I believe you will want to deal with the sign of the
sample separately, as to which output to enable.

-----Original Message-----
From: Roelof Oelofsen [mailto:]
Sent: Tuesday, February 12, 2002 2:21 AM
To:
Subject: [motoroladsp] Application brief BR1544/D Hi all,

In the motorola app. brief to design a class D amplifier, the author
states that to work out the PWM value register, the following formula
applies:

PWM Value = (Input Data Sample) x (PWM Modulus)

However, in the 56f80x user's manual, it is stated then whenever the
PWM Value is greater than or equal to the PWM Modulus, the output of the
PWM will be 0 permanently (which makes sense).
When using above formula, the PWM value will always be more than the
PWM modulus except when the input data samples = 0. (The input data
samples is from a audio CODEC which have a 16 bit resolution). Obviously
I am reading the formula incorrectly. Can someone please shed more light
on this subject for me.

Thank you in advance...

Roelof Oelofsen
Telkom Development Laboratory
Telkom SA

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Thank you, Jerry (It is Jerry I assume?) It's much clearer now.
There is another problem though. If I have a PWM output stage switching
at 1Mhz, the maximum resolution will be 40 if you use an 56807 running
at 40MIPS. Am I correct?

Thanks again

Roelof Oelofsen
Telkom Development Laboratories
Telkom SA

>>> "Johnson, Jerry" <> 02/13/02 04:31PM >>>
Using fractional math, if the input data sample is a FRACT16 number,
the
formula holds true. I believe you will want to deal with the sign of
the
sample separately, as to which output to enable.

-----Original Message-----
From: Roelof Oelofsen [mailto:]
Sent: Tuesday, February 12, 2002 2:21 AM
To:
Subject: [motoroladsp] Application brief BR1544/D Hi all,

In the motorola app. brief to design a class D amplifier, the author
states that to work out the PWM value register, the following formula
applies:

PWM Value = (Input Data Sample) x (PWM Modulus)

However, in the 56f80x user's manual, it is stated then whenever the
PWM Value is greater than or equal to the PWM Modulus, the output of
the
PWM will be 0 permanently (which makes sense).
When using above formula, the PWM value will always be more than the
PWM modulus except when the input data samples = 0. (The input data
samples is from a audio CODEC which have a 16 bit resolution).
Obviously
I am reading the formula incorrectly. Can someone please shed more
light
on this subject for me.

Thank you in advance...

Roelof Oelofsen
Telkom Development Laboratory
Telkom SA

_____________________________________
Note: If you do a simple "reply" with your email client, only the
author of
this message will receive your answer. You need to do a "reply all" if
you
want your answer to be distributed to the entire group.

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About this discussion group:

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