Trouble getting SPI functioning on PIC24 - c

Question

I am trying to configure a PIC24FJ512GA610 to use SPI to interface with an ADC module. As such, I am configuring it in two-wire mode (only SCK and SDI, CS controlled by ports). Below is the relevant code I am using to configure the SPI. A header file contains most definitions:
// SPI3 is used to read from 22-bit ADC tied to RTDs
// SPI3CON1L
#define SPI3EN 0<<15 // SPI Enable Pin
// unimplemented
#define SPI3SIDL 0<<13 // Stop when CPU is in idle mode
#define SPI3DSSDO 1<<12 // Disable SDO (no data sent to ADC module)
#define SPI3MODE1 0<<11 // Data mode: (0,0) transmits 8-bits
#define SPI3MODE0 0<<10 //
#define SPI3SMP 1<<9 // Sampling phase: samples at the end of output data time
#define SPI3CKE 0<<8 // Transmits on transition from idle to active
#define SPI3SSEN 0<<7 // only used in slave mode
#define SPI3CKP 0<<6 // clock idle state is low, active state is high
#define SPI3MSTEN 1<<5 // Master mode enabled
#define SPI3DISSDI 0<<4 // Disable SDI (no data received the board))
#define SPI3DISSCK 0<<3
#define SPI3MCLKEN 1<<2
#define SPI3SPIFE 0<<1
#define SPI3ENHBUF 0<<0
#define SPI3CON1L_MASK (SPI3EN | SPI3SIDL | SPI3DSSDO | SPI3MODE1 | SPI3MODE0 | SPI3SMP |\
SPI3CKE | SPI3SSEN | SPI3CKP | SPI3MSTEN | SPI3DISSDI | SPI3DISSCK | SPI3MCLKEN |\
SPI3SPIFE | SPI3ENHBUF )
// SPI3CON1H
#define AUDEN3 0<<15 // Audio codec enable
#define SPI3SGNEXT 0<<14 // Sign Extend Enable
#define IGNROV3 0<<13 // Ignore Receive Overflow
#define IGNTUR3 0<<12 // Ignore Transmit Underrun
#define AUDMONO3 0<<11 // Audio data format
#define URDTEN3 0<<10 // Transmit underrun data enable
#define AUDMOD31 0<<9 // Audio protocol mode select
#define AUDMOD30 0<<8 //
#define FRMEN3 1<<7 // Framed SPI support
#define FRMSYNC3 0<<6 // Frame sync pulse direction control
#define FRMPOL3 0<<5 // Frame sync/slave select polarity bit
#define MSSEN3 0<<4 // Master mode slave select enable
#define FRMSYPW3 0<<3 // Frame sync pulse width bit
#define FRMCNT32 0<<2 // Frame sync pulse counter bits
#define FRMCNT31 0<<1 //
#define FRMCNT30 0<<0 //
#define SPI3CON1H_MASK (AUDEN3 | SPI3SGNEXT | IGNROV3 | IGNTUR3 | AUDMONO3 | URDTEN3 | AUDMOD31 |\
AUDMOD30 | FRMEN3 | FRMSYNC3 | FRMPOL3 | MSSEN3 | FRMSYPW3 | FRMCNT32 |\
FRMCNT31 | FRMCNT30 )
The function that configures the SPI is found elsewhere:
void SPI3Init(void)
// SPI 3 is the thermocouple interface chip or
// the external adc interface for the cryostat
// **************************************
{
#if PROCESSOR == __PIC24FJ512GA610__
// Used for SPI communication to read RTDs through ADC
IEC3bits.SPI3RXIE = 0; // disable all SPI3 interrupts
IEC5bits.SPI3TXIE = 0;
IEC5bits.SPI3IE = 0;
SPI3BUFL = 0; // clear buffer
SPI3BUFH = 0;
OSCCONbits.IOLOCK = 0;
RPINR28bits.SDI3R = 30; // assign SPI3 SDI to pin 52 (RP30)
// RPINR28bits.SCK3R = 15; // assign SPI3 SCK to pin 53 (RP15)
RPOR7bits.RP15R = 24; // Set SCK pin to SPI3 SCK OUT (function 24)
RPOR8bits.RP16R = 23; // Map SDO to N/C pin 51 (function 23)
OSCCONbits.IOLOCK = 1;
SPI3CON1Lbits.SPIEN = 0; // disable the port
SPI3IMSKH = 0; // disable all interrupts
SPI3CON1L = 0; // disable SPI
SPI3CON1H = 0; // turn off AUDEN & FRMEN
SPI3BRGL = 832; // 3=16mhz / (2* (3+1)) = 2mhz, 7=16/2(7+1) = 1mhz, 832 -> 9600 )
SPI3CON1L = SPI3CON1L_MASK; // Write config masks
SPI3CON1H = SPI3CON1H_MASK;
SPI3CON2L = 7; // 8 bit data
SPI3STATLbits.SPIROV = 0; // clear any overflow status
SPI3CON1Lbits.SPIEN = 1; // enable the port
SPI3BUFL = 0;
ADC_CS_HIGH;
ADC_CS_LOW;
ADC_CS_HIGH;
#endif
return;
}
Actually reading the ADC calls this function three times (one for each 8 bits of resolution)
unsigned char Read_SPI3()
{
unsigned char ioByte = 0;
if(SPI3STATLbits.SPIROV)
SPI3STATLbits.SPIROV = 0; // clear overflow
SPI3BUFL = ioByte; // CLK out data on falling edge
while(!SPI3STATLbits.SPIRBF)
;
ioByte = SPI3BUFL; // CLK in data on rising edge
return ioByte;
}
I have been going through my code for the last day or two and, after combing the internet, cannot figure out what I am doing wrong. Putting a scope on the SCK line indicates that the serial clock is not running. I have confirmed that the config bits are being successfully written. Can anybody tell me what might be wrong?

Answer 1

A few thoughts:
Your setting for DISSDI is enabling SDI (comment says disable)
TRIS register settings are not given, but the pin for SDI must be 1, and the pin for SCLK must be set to 0
by setting MCKLEN, your clock source is REFO - assume it is configured correctly? (e.g. ROEN=1)

Answer 2

With this:
RPOR7bits.RP15R = 24;
you are mapping your SCLK pin to RF8. But you had to switch this pin to an output with.
TRISFbits.TRISF8 = 0;
And you had to switch this pin to digital with
ANSELFbits.ANSELF8 = 0;
Same thing for the SDO pin.
And as #Dan already mentiones, you need a unlock/lock sequence for the PPS. Have aloo at page 163 in the datasheet

Answer 3

The statement you use to unlock the pin selector:
OSCCONbits.IOLOCK = 0;
Is not what Microchip requires.
Because all of the writable bits in the OSCCON register are "protected" a built in compiler extension command must be used to change them.
The command to unlock the PPS looks like this:
__builtin_write_OSCCONL(OSCCON & _OSCCON_IOLOCK_MASK);
The command to lock the PPS looks like this:
__builtin_write_OSCCONL(OSCCON | _OSCCON_IOLOCK_MASK);
Depending on how you have set the configuration words the PPS may be configured so it cannot be unlocked, and once locked it takes a power-on-reset for the PPS to become unlocked.