SPI - The basics

[boroko]

Thanks Steve!

I appreciate that you took the time to give me the reference. Very helpful.

Bo

[boroko]

Interesting, that explanation doesn't show up in either the MCP4821 or MCP4921 datasheets.

Thanks again for the heads up

Bo

[boroko]

After studying this for a few nights, the fog is beginning to clear a bit, but not completely.
On the above mentioned inclinometer (sca61t, pages 11 &12), you send a command and then concurrently it sends back a reply with the data. I think I can see how to send the command, and extend the transmission to 19 clocks so that it can fit in the response by leaving CS low with the [var\19] in SEROUT. What I don't see how to grab those reply bits using SEROUT and SERIN. Each one would execute in order and not catch the response as both share the clock.

I VAR WORD
D VAR WORD 
CS_I      VAR           PORTC.2  ' chip select Inclinomteter
CS_D      VAR           PORTC.3  ' chip select D/A
SCK       VAR           PORTC.5 ' clock
SDI       VAR           PORTC.4 ' data OUT
SDO       VAR           PORTC.7 ' data IN
IncIn     var           word 
HIGH CS_I   
HIGH CS_D
LOW SCK
I = %0001000000000000     'send RDAX & fill to get 11 bits of data 
D = %0001111111111111        '2x, active, max output for test

'*** Inclinometer Commands ***********
'MEAS         00000000           'Measure (Exit self test)
'RWTR         00001000           'R/W Temp     
'RDSR         00001010           'Read Status register
'RLOAD        00001011           'relaod NV to o/p
'STX          00001110           'activate self test
'RDAX         00010000           'read acceleration

Main:
     GOSUB IncRdg
     GOSUB DACout
goto main
'**** Subs *********************************
;---- Sub to read the inclinometer ------------------
IncRdg:
  LOW CS_i                       ' enable writes to Inclinometer
  SHIFTOUT SDI,SCK,0,[I\19]      'sends command to READ
' SHIFTIN SDO,SDK,0,[IncIn\11]  ' I don't see this working
  HIGH CS_I 
  PAUSE 5
  RETURN

''---- Sub to output to the D/A converter ----- 
'DACout:
'  LOW CS_D                       ' enable writes to D/A converter
'  SHIFTOUT SDI,SCK,0,[D\16]
'  HIGH CS_D 
'  PAUSE 5
'  return

Will test further, but I suspect that I might have to resort to the hardware SPI.

Any thoughts?
Bo

[HenrikOlsson]

Hi,
I just took a quick look at this but as far as I can see the commands are 8 bits and the response from the device is 11 bits starting at the 9th clock pulse. So, if reading the acceleration you first SHIFTOUT the 8 bit RDAX command, then you SHIFTIN 11 bits for a total of 19bits.

[boroko]

After banging around for a while, I was able to get this to give me good data from the inclinometer:

CS_I      VAR           PORTD.0  ' chip select Inclinomteter
CS_D      VAR           PORTC.3  ' chip select D/A
SCK       VAR           PORTD.3 ' clock
SDI       VAR           PORTD.2 ' data IN , MISO
SDO       VAR           PORTD.1 ' data OUT, MOSI
IncIn     var           word
HIGH CS_I   
HIGH CS_D
LOW SCK
HIGH CS_I
'I = %00001000      'send TeMP command
I = %00010000     'send RDAX 

Main:
     GOSUB iNCrDG
     hserout ["IncRdg ",bin IncIn," ",DEC IncIn," ",10,13]
     PAUSE 100
goto main
'**** Subs *********************************
;---- Sub to read the inclinometer ------------------
IncRdg:
  LOW CS_I                       ' enable writes to Inclinometer
  SHIFTOUT SDO,SCK,1,[I]      'sends command to READ
  SHIFTIN SDI,SCK,0,[IncIn\11]  
  HIGH CS_I 
  RETURN

My LOGIC pod had a bit of a rough time trying to make sense of the unusual length of data and that threw me for a bit, but the HSEROUT showed that all was good. .

Hope that help someone.

Bo