基于ATSAMG55J19的MCP79411 RTC I2C/TWI通信问题求助

Let's break down the issues in your current code and walk through the correct I2C transaction flow for the MCP79411—ASF's TWI library requires explicit control over start/stop/repeated start signals, which your code is missing, leading to failed reads.

Key Issues in Your Current Code #

• Reinitializing TWI on every operation: Your i2c_start() function resets the TWI peripheral every time it's called, which breaks bus state consistency. TWI initialization should happen once at system startup, not per transaction.
• Missing critical bus control signals: ASF's TWI library doesn't auto-send start/stop/repeated start signals. You need to explicitly call functions like twi_master_send_start(), twi_master_send_repeated_start(), and twi_master_send_stop() to manage the bus.
• Incorrect read transaction flow: To read from an RTC register, you first write the register address, then send a repeated start before switching to read mode. Your code skips this critical step.
• Improper byte read handling: When reading multiple bytes, send an ACK for all bytes except the last one (send a NACK to signal the end of the read).

Step-by-Step Corrected Implementation #

1. Global TWI Initialization (Run Once) #

First, initialize the TWI peripheral once at system startup (e.g., in main() before calling rtc_Init()):

#include <asf.h>

// MCP79411 I2C Address (7-bit base address is 0x6F)
#define RTCC_ADDR 0x6F
#define RTCC_WRITE_ADDR (RTCC_ADDR << 1) | 0x00
#define RTCC_READ_ADDR  (RTCC_ADDR << 1) | 0x01

// MCP79411 Register Addresses
#define RTCC_REG_SEC   0x00
#define RTCC_REG_MIN   0x01
#define RTCC_REG_HR    0x02
#define RTCC_REG_CTRL  0x07

static void twi_init(void) {
    flexcom_enable(FLEXCOM4);
    flexcom_set_opmode(FLEXCOM4, FLEXCOM_TWI);
    
    twi_options_t twi_options;
    twi_options.master_clk = sysclk_get_cpu_hz();
    twi_options.speed = 100000; // 100kHz standard I2C mode
    twi_options.smbus = 0;
    
    if (twi_master_init(TWI4, &twi_options) != TWI_SUCCESS) {
        // Handle initialization error (e.g., trap in loop)
        while(1);
    }
}

2. Corrected RTC Initialization #

The MCP79411 requires setting the ST bit (bit 7 of the seconds register) to start its oscillator. Here's the proper transaction flow:

// Helper functions (ensure these are defined in your code)
uint8_t bcd2bin(uint8_t bcd) { return ((bcd >> 4) * 10) + (bcd & 0x0F); }
uint8_t bin2bcd(uint8_t bin) { return ((bin / 10) << 4) + (bin % 10); }

void rtc_Init(void) {
    uint8_t seconds;
    
    // Step 1: Read current seconds register value
    twi_master_send_start(TWI4, RTCC_WRITE_ADDR, TWI_WRITE);
    twi_master_write_byte(TWI4, RTCC_REG_SEC, TWI_MASTER_ACTIVE);
    twi_master_send_repeated_start(TWI4, RTCC_READ_ADDR, TWI_READ);
    
    // Read seconds (send NACK since it's the only byte, then stop)
    twi_master_read_byte(TWI4, &seconds, TWI_MASTER_STOP);
    
    // Step 2: Set ST bit to start oscillator, preserve current seconds value
    seconds = bcd2bin(seconds & 0x7F); // Mask out existing ST bit
    uint8_t sec_bcd = bin2bcd(seconds) | 0x80; // Set ST bit
    
    twi_master_send_start(TWI4, RTCC_WRITE_ADDR, TWI_WRITE);
    twi_master_write_byte(TWI4, RTCC_REG_SEC, TWI_MASTER_ACTIVE);
    twi_master_write_byte(TWI4, sec_bcd, TWI_MASTER_STOP);
    
    // Step 3: Disable square wave output (control register 0x07, bit 7 = 1 disables)
    twi_master_send_start(TWI4, RTCC_WRITE_ADDR, TWI_WRITE);
    twi_master_write_byte(TWI4, RTCC_REG_CTRL, TWI_MASTER_ACTIVE);
    twi_master_write_byte(TWI4, 0x80, TWI_MASTER_STOP);
}

3. Corrected Time Read Function #

To read hours, minutes, and seconds, follow the standard I2C read flow for register-based devices:

void rtc_get_time(uint8_t *hr, uint8_t *min, uint8_t *sec) {
    // Step 1: Send start, write device address, write starting register (0x00)
    twi_master_send_start(TWI4, RTCC_WRITE_ADDR, TWI_WRITE);
    twi_master_write_byte(TWI4, RTCC_REG_SEC, TWI_MASTER_ACTIVE);
    
    // Step 2: Repeated start to switch to read mode
    twi_master_send_repeated_start(TWI4, RTCC_READ_ADDR, TWI_READ);
    
    // Step 3: Read bytes - send ACK for first two, NACK + stop for last
    twi_master_read_byte(TWI4, sec, TWI_MASTER_ACTIVE); // Seconds (ACK)
    twi_master_read_byte(TWI4, min, TWI_MASTER_ACTIVE); // Minutes (ACK)
    twi_master_read_byte(TWI4, hr, TWI_MASTER_STOP);    // Hours (NACK + Stop)
    
    // Convert BCD to binary and mask out non-time bits
    *sec = bcd2bin(*sec & 0x7F);
    *min = bcd2bin(*min & 0x7F);
    *hr = bcd2bin(*hr & 0x3F); // Mask out 12/24 mode and AM/PM bits for 24h format
}

Important Notes #

• Error Handling: For robustness, add checks for return values of all TWI functions (e.g., twi_master_send_start() returns TWI_SUCCESS or an error code) to catch issues like no ACK from the RTC module.
• Hardware Checks: Ensure 4.7kΩ pull-up resistors are connected to SDA and SCL lines (standard for 100kHz I2C).
• Debugging: If issues persist, use an oscilloscope or logic analyzer to verify:
  • Presence of valid start/stop signals
  • Correct device and register addresses being sent
  • RTC sending ACKs after each transmitted byte

内容的提问来源于stack exchange，提问作者Edoardo Lobbiani