[AVR] 7 Bit Transfer on SPI

[arash rezaee]

Hi every one

I have a chip which is HMC830 and I need to send 7 bit is SPI mode. I am using ATxmega128 and I know I cannot use SPI from ATXmega because it sends 8 bit in each iteration. So how can I write my own software to have 7 bit?

Regards,
Arash

 

[alexan_e]

So, if you send an additional dummy bit (hardware SPI) will this be a problem?

The alternative is to use generic I/O pins, one for clock and one for data output (plus the enable output).
If you want read the SPI data from HMC830 then you'll need a data input too.

Refer to bit banging https://en.wikipedia.org/wiki/Bit_banging

 

[FvM]

I have a chip which is HMC830 and I need to send 7 bit is SPI mode.

How did you arrive at this conclusion? SPI is transporting data in 8-bit entities by specification, and as far as I'm aware of there's not a single chip with SPI interface on the market that can't work with bytewise transfer.

Did you read the VCO programming specifications at all? https://www.hittite.com/content/documents/operating_guide/synth_operating_guide_rf_vcos.pdf

 

[arash rezaee]

How did you arrive at this conclusion? SPI is transporting data in 8-bit entities by specification, and as far as I'm aware of there's not a single chip with SPI interface on the market that can't work with bytewise transfer.

Did you read the VCO programming specifications at all? https://www.hittite.com/content/documents/operating_guide/synth_operating_guide_rf_vcos.pdf

I have read it more than you think and I also have evaluation board of HMC830 and I god SDI with CLK and I saw the first data to HMC is 7 bit with 7 CLK and then 3 Bytes of data and at the end just one CLK after a while. On the reference you also mention please go to page A-35 figure 24. you will find it.

 

[FvM]

The data is transferred in multiples of 8-Bit, as required by the SPI specification. The special point is that the 24 bit data is shifted one bit so that the byte boundaries are crossed. That's not particularly user-friendly but can be e.g. handled by copying the data data into a C union and send it byte-wise. Surely easier than programming a non-standard bit-banging 7-bit SPI interface.