# AMIS30543 | Data-Sheet understanding Status register

I was reading the data Sheet of AMIS-30543 stepper motor driver and I was not able to understand how to combine the result of the Micro step position status register MSP[8:0]. I know that its a 9 bit value . Help me understand how to get the value.

In the data sheet it is given like

in SR3 -> MSP[8:2], and in SR4 -> MSP[6:0]. How I am I suppose to get the 9 bit out of these.

In the library I have seen that , `(sr3<<2) | (sr4 & 3) ` . I don't understand why they are doing and `&` here.

Please explain . also If there was no library how am I suppose to know how to combine these registers from the dataSheet .

Link to DataSheet

## 1 Answer

Most of the data is duplicated between the registers. Both registers contain MSP[6:2].

If MSP contains `111001001` the registers (ignoring the MSB "PAR" bit) would look like:

``````SR3: 1110010
SR4: 1001001
``````

If we line those up it's then like:

``````SR3: 1110010
SR4    1001001
``````

You can now see the overlap.

So taking SR3 and shifting it left two places (`sr3 << 2`) gives you:

``````SR3:   1110010
<<2: 111001000
``````

Taking the lowest two bits of SR4 with `& 3` gives you:

``````SR4:   1001001
& 3:   0000001
``````

Then ORing the two results together:

``````SR3<<2: 111001000
SR4 &3: 000000001
-----------------
MSP   : 111001001
``````
• How can I understand this from data sheet like from sr3 and sr4 Oct 15, 2020 at 21:10
• The 8:2 and 6:0 refer to the range of bits stored in that register. It's a little unusual to have overlap like that. Oct 15, 2020 at 21:17
• So is it okay to do like :: ‘(sr3 << 2 | sr4)’ Oct 15, 2020 at 21:19
• No, because of the PAR bit that's in the MSB that will corrupt SR3. Oct 15, 2020 at 21:20
• Practice and experience. It's the first time I have seen overlapping bits like that, but the code snippet you showed and the 8:2 and 6:0 in the datasheet allowed me to infer it. It's really not clear at all in the datasheet, so it's not surprising you were struggling with it. Oct 15, 2020 at 21:23