Atmega164 spi byte string

[Pferd]

Hello Forum,

I am again in need of some programming advice... I have six shift registers type 74HC595 in series on the spi interface of an Atmega164. I wrote a send function which takes 6 bytes of data and shifts them out, after all 6 bytes are through the MCU latches the registers. This all works perfectly. The problem here is that I have 6-bit and 4-bit circuits connected to the shift registers parallel outputs, but for the sake of programming ease I want every value to be stored in a normal uint8_t variable in software. So I somehow need to cut down those variables to the information I need and then combine them again to the 8-bit b<tes I am then giving the SPI send function. Is there a clever way of bitmasking this or is there a trick in existence?
I´ll attach a sketch to hopefully give some additional help of what my circuit needs to achieve.
Thanks a lot in advance!:)

[Ian.M]

No, as ATmegas don't have a barrel shifter.   Also that 4 bit chunk in the middle doesn't do you any favors if you want a neat solution like shift and mask in a loop.

If the hardware design isn't set in stone, your best option would be to rewire so each 6 bit chunk uses the same bits of each '595, preferably LSB aligned, then you only need to split and mask in the  two halves of the 4 bit chunk into the high bits of two of the bytes and mask in the five individual bits.

[madires]

You could use an array of 6 bytes for the 595s and overlay a uint16_t and a uint32_t pointer based on the boundaries. If any of the bitfield bytes for driver/signal lines change then copy and shift the changed bitfield byte in a temporary variable, clear the corresponding bits in the overlay (AND ~bitmask) and copy the new bitfield by a simple OR. After that resend the array. Instead of changing single bitfield bytes in the overlay you could also build the complete 2 or 4 byte pack and copy it directly to the overlay.