SAMD21_Master_I2C

Using one SAMD20 and one SAMD21 Xplained pro demo boards, (one as Master, one as Slave) and communicating over I2C. Example from AT11628

Documents:

AT11628 App Note

DS40001882D Datasheet

Create New Project

Starting with the Demo Board, ideally port to Userboard

1. New GCC C ASF Board Project
2. Choose SAMD21J18 -> Xplained Pro ATSAMD21J18A and verify LED Toggle with SW0 is working.

Overview

• Master will transmit a data buffer of a few bytes to the slave.
• Slave will re-transmit the same data buffer to the master.
• Using SERCOM2 (PA08 = SDA, PA09 = SCL). These I2C lines are available on all 3 EXT headers on the Xplained Pro board, can use any.

Hardware

• SAMD21 Xplained pro has on-board 4.7k pullup resistors on I2C lines (R305, R306) (Left of PB06 on EXT1) which have been replaced with 2k pullup resistors to improve signal timing.

For the SAMD21J18A: worst case (Fast Mode) tr = 100ns, Cb = 400pF,

Configuration

Clocks

Master and slave application uses OSC8M as the clock source for Generator 0 (Undivided = 8MHz)

I2C Pin and Register Init (I2C_Master_Init)

Take care - there is an error in 5.1.6 regarding the pin_set_peripheral_function. See source code

• Switch pin functionality to peripheral function D (SERCOM-ALT: i2c) per datasheet Follow I2C Configuration Steps in Datasheet (pg 553)

• Configure CTRL Reg A: i2C STANDARD_MODE_FAST_MODE (100-400khz), SDA Hold (300-600ns); Runstandby clock enabled in sleep; I2C Config as Master

• Configure CTRL Reg B: SMart Mode Enabled

• Configure Sync busy (wait until not busy)

• Configure BAUD rate: We are configuring the SCL clock.
  ) So if fSCL = 1MHz, fGCLK = 48MHz and trise = 100ns we can calculate that 2BAUD = about 33

If BAUD and BAUDLOW are being configured separately (eg for high speed or fast+ mode): Convention seems to be that t_{low} is approx 2x t_{high} (per data sheets) Therefore, BAUD = 11, BAUDLOW = 22.

Note: It is recommended to set the slave's baud rate higher than the master baud rate and give a sufficient margin to meet the I2C timing.

Interrupt Enable on Master-on-bus and Slave-on-bus. pg610/611 of datasheet Enable SERCOM interrupts

Transactions

Setup

1. Create a global iteration count variable (uint8_t i)
2. Create global boolean values rx_done, tx_done (init to false)

CTRLB Register - ACK

Setup the ACK behavio. The ACKACT field is used to define the I2C master’s acknowledge behavior after a data byte is received from the I2C slave. The acknowledge action will be executed when a command is written to CTRLB.CMD bits or after a transfer with Smart Mode is enabled. After a data byte is received. Wait for sync

Interrupt Handler (breakout from master_transaction)

This will override the weak definition of the interrupt handler from the device specific header files

Once the Slave Address has been written to the register, we have initiated a transfer and triggered the interrupt. The SERCOM2_Handler controls the data transfer process of buffer length (counted by i ) until all data has been transferred. Then it sets tx_done to true, and returns to the master_transaction function, exiting the while loop.

Repeat similarly for RX: Continue sending bytes until complete. Check for last byte and send ACK/STOP when appropriate. Return to transaction function.

Additional

There's a calculate Baud function to calculate baud rate integer. Use that for lower speeds, use the high/low for higher speeds (unsupported by SAMD20) Any globals or #defines used in the functions need to be added, as well as any prototypes. Otherwise MASTER seems to be completed. Compiles and loads successfully; works with SAMD20 I2C Slave