TMS320F28075: __byte() Intrinsic: Signed 8 Bit Numbers

Niloc2004

Part Number: TMS320F28075

Hi,

I need to store and retrieve signed 8 bit numbers into an EEPROM using the C28x Processor.

I know that the __byte() intrinsic allows us to pack and unpack data into 8 bit chunks. However, retrieving the negative 8 bit values results in corrupted data.

I'm assuming this is because the __byte() intrinsic doesn't handle sign extension correctly. I.e. the MSB of the resultant is set to 0. Is the best workaround for this to just subtract 256 from the value if it is greater than 127? Or is there some casting incantation that would address this behavior?

Thanks,

-Colin

over 2 years ago

0 Sira Rao80 over 2 years ago

TI__Mastermind 26875 points

Colin, assigning this to the right expert to address.

Thanks,

Sira

+1 George Mock over 2 years ago in reply to Sira Rao80

TI__Guru**** 251200 points

The __byte intrinsic never does sign extension. That is how the underlying instruction MOVB works. For the details about MOVB, search the C28x instruction set manual for the phrase Load Byte Value. The __byte intrinsic has worked that way for many years. Thus, no changes can be made. However, the documentation of __byte should mention this detail, so I filed the entry EXT_EP-11153 to have the documentation updated. You are welcome to follow it with that link.

Thanks and regards,

-George

C2000™︎ microcontrollers

C2000 microcontrollers forum

TMS320F28075: __byte() Intrinsic: Signed 8 Bit Numbers