I2C Generic

The basic low-level words provide a rather raw access to the I2C bus and its devices. Since the bus has some standard actions, which are always the same, some combinations are useful factors.

i2c.begin ( addr – )

start a I2C communication with the device addr. This involves sending the start condition and the address with the write bit cleared.

In addition, the variable i2c.current gets the addr information to be available for user applications.

i2c.begin-read ( addr – )

start a I2C read communication with the device at addr. This means that the device address is used with the read bit set.

i2c.end ( – )

The communication ends with sending the I2C stop condition and the bus is released. The variable i2c.current is cleared.

With these commands an I2C transaction becomes

i2c.hw.id i2c.begin .... i2c.end

Inside the begin/end scope, the basic I2C routines for writing (i2c.tx) and reading can be used. They work with the device selected with i2c.begin.

Most I2C devices use rather small data packets they exchange with the host. It’s not uncommon to place the data on the data stack instead of providing a RAM buffer.

For these tasks the following words are provided. They to work within the begin/end scope described above.

i2c.c@ ( addr – c )

Start a bus cycle and read one byte from the device. Afterwards release the bus.

i2c.c! ( c addr – )

Start a bus cycle and write one byte to the device. Afterwards release the bus.

i2c.n! ( x_n .. x_1 n addr – )

Start a bus cycle and send n bytes to the device. Afterwards the STOP condition is sent and the bus is released.

i2c.n@ ( n addr – x_n .. x_1 )

Start a bus cycle and receive n bytes from the device. To acomplish that, a start is triggered with the read bit of the addr set. Afterwards the STOP condition is sent and the bus is released.

i2c.m!n@ ( n xm .. x1 m addr – x1 .. xn )

A combination of the two above. It creates the I2C transaction scope and sends m bytes to the device. Afterwards the data transfer direction is switched by sending a repeated start and n bytes are read from the device. Finally the STOP condition is sent and the bus is released.

Example - Port Expander

This example communicates with an I2C port expander PCF8574(a). The I2C address is usually between $30 and $3f.

Communication is not time critical, so the slow speed standard initialization is sufficient. To chack whether the device is present and works properly, an I2C bus scan is made first

(ATmega1280)> i2c.init.default
 ok
(ATmega1280)> i2c.detect
      0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F
 0:                       -- -- -- -- -- -- -- -- --
10:  -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20:  -- -- -- -- -- -- -- 27 -- -- -- -- -- -- -- --
30:  30 -- -- -- -- -- -- -- -- -- -- -- -- 3D -- --
40:  -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50:  50 51 -- -- -- -- -- -- -- -- -- -- -- -- -- --
60:  -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70:  -- -- -- -- -- -- -- --
 ok
(ATmega1280)>

A modification uses the value design pattern. With that, a new value is created that automatically fetches the data from the device when called and stores the new bit pattern with TO:

#require value.frt
#require quotations.frt

: i2c.cvalue ( n addr hwid -- )
  (value)
  dup , \ store the hwid
  [: dup @i ( hwid) i2c.c@ ;] ,
  [: dup @i ( hwid) i2c.c! ;] ,
  i2c.pe.c!  \ store inital data
;

Use it as follows

> $ff $3d i2c.cvalue keys ( sets all bits to HIGH)
ok
> $00 to keys ( set all bits to LOW )
ok
> keys $01 and ( if key 1 is pressed )

Big Data

Big data means that a device sends or receives more data than the data stack can hold. In this case, the i2c.begin and i2c.end in combination with the low level i2c.tx, i2c.rx etc should be used. One example is the I2C EEPROM block driver. It transfers 512 bytes in one transaction and uses a RAM buffer to actually hold the data.

See also

I2C EEPROM, I2C Bus Scanner, and I2C EEPROM Blocks. Values I2C Slave