How to recover from I2C bus collision BCLIF?

Question

I posted this a couple of days ago on the Microchip Forum (here) but the only response has been crickets. The I2C code below works most of the time but occasionally on power-up there is a bus collision (BCLIF) and the I2C module is unable to recover after the BCLIF. The I2C lines are pulled up by 3.3K ohms. Using REALICE and breakpoints I can see that i2c_write() resets BCLIF and returns FALSE when BCLIF is set. I have used a scope to verify that the I2C bus has flat-lined. Re-initializing the PIC18F25K20 I2C module (see init_i2c() below) when i2c_write() returns FALSE does not help. The PIC18F25K20 I2C is connected to a single slave device (MCP4018 I2C Digital POT). I have used this same code on previous PIC18 projects without issue so I replaced the MCP4018 suspecting a bad part but see no difference. Is there a way to reset the PIC18F25K20 I2C module when it is locked up?

void init_i2c(I2C_BAUD_RATE baud_rate, float freq_mhz) 
{ 
    UINT32 freq_cycle; 
    /* Reset i2c */ 
    SSPCON1 = 0; 
    SSPCON2 = 0; 
    PIR2bits.BCLIF = 0; 
    /* Set baud rate */ 
    /* SSPADD = ((Fosc/4) / Fscl) - 1 */ 
    freq_cycle = (UINT32) ((freq_mhz * 1e6) / 4.0); 
    if (baud_rate == I2C_1_MHZ) 
    { 
        SSPADD = (UINT8) ((freq_cycle / 1000000L) - 1); 
        SSPSTATbits.SMP = 1;        /* disable slew rate for 1MHz operation */ 
    } 
    else if (baud_rate == I2C_400_KHZ) 
    { 
        SSPADD = (UINT8) ((freq_cycle / 400000L) - 1); 
        SSPSTATbits.SMP = 0;        /* enable slew rate for 400kHz operation */ 
    } 
    else /* default to 100 kHz case */ 
    { 
        SSPADD = (UINT8) ((freq_cycle / 100000L) - 1); 
        SSPSTATbits.SMP = 1;        /* disable slew rate for 1MHz operation */ 
    } 
    /* Set to Master Mode */ 
    SSPCON1bits.SSPM3 = 1; 
    SSPCON1bits.SSPM2 = 0; 
    SSPCON1bits.SSPM1 = 0; 
    SSPCON1bits.SSPM0 = 0; 
    /* Enable i2c */ 
    SSPCON1bits.SSPEN = 1; 
} 
BOOL i2c_write(UINT8 addr, const void *reg, UINT16 reg_size, const void *data, UINT16 data_size) 
{ 
    UINT16 i; 
    const UINT8  *data_ptr, *reg_ptr; 

    /* convert void ptr to UINT8 ptr */ 
    reg_ptr  = (const UINT8 *) reg; 
    data_ptr = (const UINT8 *) data; 
    /* check to make sure i2c bus is idle */ 
    while ( ( SSPCON2 & 0x1F ) | ( SSPSTATbits.R_W ) ) 
        ; 
    /* initiate Start condition and wait until it's done */ 
    SSPCON2bits.SEN = 1; 
    while (SSPCON2bits.SEN) 
        ; 
    /* check for bus collision */ 
    if (PIR2bits.BCLIF) 
    { 
        PIR2bits.BCLIF = 0; 
        return(FALSE); 
    } 
    /* format address with write bit (clear last bit to indicate write) */ 
    addr <<= 1; 
    addr &= 0xFE; 
    /* send out address */ 
    if (!write_byte(addr)) 
        return(FALSE); 
    /* send out register/cmd bytes */ 
    for (i = 0; i < reg_size; i++) 
    { 
        if (!write_byte(reg_ptr)) 
            return(FALSE); 
    } 
    /* send out data bytes */ 
    for (i = 0; i < data_size; i++) 
    { 
        if (!write_byte(data_ptr)) 
            return(FALSE); 
    } 
    /* initiate Stop condition and wait until it's done */ 
    SSPCON2bits.PEN = 1; 
    while(SSPCON2bits.PEN) 
        ; 
    /* check for bus collision */ 
    if (PIR2bits.BCLIF) 
    { 
        PIR2bits.BCLIF = 0; 
        return(FALSE); 
    } 
    return(TRUE); 
} 
BOOL write_byte(UINT8 byte) 
{ 
    /* send out byte */ 
    SSPBUF = byte; 
    if (SSPCON1bits.WCOL)       /* check for collision */ 
    { 
        return(FALSE); 
    } 
    else 
    { 
        while(SSPSTATbits.BF)   /* wait for byte to be shifted out */ 
            ; 
    } 
    /* check to make sure i2c bus is idle before continuing */ 
    while ( ( SSPCON2 & 0x1F ) | ( SSPSTATbits.R_W ) ) 
        ; 
    /* check to make sure received ACK */ 
    if (SSPCON2bits.ACKSTAT) 
        return(FALSE); 
    return(TRUE); 
} 

Answer 1

This Errata needs to be added to PIC18F25K20 Errata.

PIC18F2455/2550/4455/4550 Rev. A3 Silicon Errata

17 Module: MSSP

It has been observed that following a Power-on Reset, I2C mode may not initialize properly by just configuring the SCL and SDA pins as either inputs or outputs. This has only been seen in a few unique system environments.

A test of a statistically significant sample of preproduction systems, across the voltage and current range of the application's power supply, should indicate if a system is susceptible to this issue.

Work around

Before configuring the module for I2C operation:

1. Configure the SCL and SDA pins as outputs by clearing their corresponding TRIS bits.
2. Force SCL and SDA low by clearing the corresponding LAT bits.
3. While keeping the LAT bits clear, configure SCL and SDA as inputs by setting their TRIS bits.

Once this is done, use the SSPCON1 and SSPCON2 registers to configure the proper I2C mode as before.