i2c — I2C Device Management
Inter-Integrated Circuit (I2C) is a simple two-wire serial communication protocol. The two lines are the serial data line (SDA) and the serial clock line (SCL). A controller, or master, node on the bus can send commands to and read data from target, or slave, nodes on the bus. Texas Instruments provides A Basic Guide to I2C.
As outlined in the DL-12xx, DL-25xx and DL-7xx Design Guidelines there are several I2C interfaces available to the DL7450. At present the i2c module gives application developers an interface to the I2C1 bus via the SDK. The SDK samples include simple examples of drivers for I2C devices.
Tips For Writing an I2C Driver
The application developer is responsible for the placement of the I2C device on the bus, and understanding how its address is assigned. The base address of the device is a 7-bit number determined by the vendor. Often vendors allow the application developer to specify the lowest 2 or 3 bits by providing 2 or 3 pins which can be brought high or low using resistors. For example the MCP9808 Temperature Sensor uses a base address of 0x18, or 0b0011000. Any of the 3 least significant bits can be set using the address pins, giving a possible range of addresses from 0x18 to 0x1F. Please read the vendor data sheet carefully.
Sometimes a vendor datasheet will provide two 8-bit device addresses, one for read operations and one for write operations. To use the DL7450 i2c module correctly, you must obtain the correct 7-bit address in such cases. The 8-bit number arises from including the read-write bit of the register in the device address. To extract the correct 7-bit address, use the most-significant 7 bits and apply a right shift (half). For example if the datasheet might give a write address of 0x92 (0b10010010) and a read address of 0x93 (0b10010011). To use the DL7450 SDK use a device address of 0x49 (0b1001001).
Finally, there are some addresses that are reserved.
000 0xxx (0x00 to 0x07) reserved.
0x08 to 0x77 valid address range.
111 1xxx (0x78 to 0x7f) reserved
The read and write methods in the DL7450 i2c module will raise an error if the device address is outside the valid range of 7-bit addresses, or if there is not device present at the given address. In all cases, it is the responsibility of the application developer to provide the correct device address. An incorrect address will result in an I2cError being raised.
from i2c import I2C, I2cError
BASE_ADDRESS = 0x18
class MCP9808
def __init__(self):
self.i2c = I2C()
try:
self.i2c.read(BASE_ADDRESS, 0, 1)
except I2cError:
# invalid base address.
pass