PXP PS Scaling

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PXP PS Scaling

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steveschefterti
Contributor III

Can someone clarify the setting of the PS Scale Factor Register (PXP_PS_SCALE) in the Pixel Pipeline?

The description in the iMX6 RMs is a tad confusing/incomplete.

The manual says that each X/Y scale is "two bit integer and 12 bit fractional representation" (total 14 bits) but shows the scales occupying 15 bits each (plus a single reserved bit to make up one 16 bit half-word).  Am I correct to assume they are really 14 bits each (bits 29-16, 13-0) plus two reserved bits?

Next, the manual says "The reciprocal of the scale factor should be loaded into this register", but it is not clear how to make a "reciprocal" out of the above bit fields.

The following up scaling examples are provided:
00.0100_0000_0000    scale up by 4
00.0010_0000_0000    scale up by 2
00.1010_0000_0000    scale up by 8/5

Especially with the last one I don't see where they are getting the register settings.

The 2x and 4x examples might make sense to me, except that I wonder why the most significant nibble is being set.  And how, in that case, would you represent a scaling factor of 0x20 for example?

Thanks,

    Steve

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Bio_TICFSL
NXP TechSupport
NXP TechSupport

Hello Steve,

I find no reference to a PXP register PS_SCALE in any MX6 documentation.

 

Is this issue really about the MX6? If not, what device is being used?

Regards

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steveschefterti
Contributor III

The PXP, including the PXP_PS_SCALE register, is part of the iMX6UL,

iMX6ULL, iMX6Solo/DualLite. It's not found in the iMX6D/Q which have an

IPU instead.

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Bio_TICFSL
NXP TechSupport
NXP TechSupport

The manual says that each X/Y scale is "two bit integer and 12 bit fractional representation" (total 14 bits) but shows the scales occupying 15 bits each (plus a single reserved bit to make up one 16 bit half-word).  Am I correct to assume they are really 14 bits each (bits 29-16, 13-0) plus two reserved bits?

>>> Bits 30 and 14 are not usable per the note in the RM that states that "The maximum value programmed should be 2 since scaling down by a factor greater than 2 is not supported with the bilinear filter". 

 

Next, the manual says "The reciprocal of the scale factor should be loaded into this register", but it is not clear how to make a "reciprocal" out of the above bit fields.

>>> The value to be programmed is 1/desired_scale_factor. The radix point is between bits 28-27 and 12-11 for each register. The two bits to the left of the radix point are the "integer" (valid values are 0 through 2) and the bits to the right are the fractional part. The examples given in the RM match the algorithm here for converting a binary fraction to decimal. 

 

00.0100_0000_0000    scale up by 4      => scale = reciprocal of ( 1/2^2 ) = 1/ (1/4) = 4
00.0010_0000_0000    scale up by 2      => bit field here is incorrect, should be 00.1000_0000_0000, which is  1/2, reciprocal is 2
00.1010_0000_0000    scale up by 8/5   => scale = reciprocal of ( 1/2 + 1/8 ) = 1 / (0.5 + 0.125) = 8

Regards

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steveschefterti
Contributor III

Good to have a further indication that it is just the figure showing the register bits is incorrect in the RM.

Thanks a lot for the link to the algorithm.  That's just what I was looking for.

    Steve

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