FUNCTIONAL DEVICE OPERATION
I2C BUS SPECIFICATION
During manual mode, all internal Registers are in Default
Configuration, refer Table 6, under this configuration the PIN
and NIN pins are enabled to scale the current capability per
string and may be disable by setting 2.2 V in the
corresponding terminal.
PWM signal applied to PWM pin will be in charge of
controlling LED dimming and enable the device every time
the PWM is active. For this configuration EN pin should be
LOW.
POWER DOWN MODE
Also in this mode, the device can be enabled as follows:
If the input voltage falls below the UVLO threshold, the
device enters automatically into power down mode. When in
power down, the supply current is reduced below 2.0 μA
when there is no I2C activity, and it rises up when I2C
interface is enabled.
+ EN pin + PWM signal (Two Signals): In this
configuration, the PWM signal applied to PWM pin will be in
charge of controlling the LED dimming and a second signal
will enable or disable the chip through the EN pin. Figure 20
+ PWM Signal tied to SDA pin (Just ONE signal): In this
configuration the PWM pin should be tied to SDA pin. The
I2C BUS SPECIFICATION
MC34844 is a unidirectional device that can only be written by an external control unit. Since the device is a 7 bit address
device (1110110), the control unit needs to follow a specific data transfer format which is shown below.
Figure 7. A complete data transfer
For a complete data transfer, please use this format in the
following order:
The receiver (MC34844) must pull down the SDA line
during this acknowledge pulse to indicate that the data
was correctly written.
1. START condition
• Bits in the first byte: The first seven bits of the first bite
make up the slave address. The eighth bit is the LSB (least
significant bit), which determines the direction of the
message (Write = 0)
2. MC34844 device address and Write instruction (R/W =
0)
3. First data pack, it corresponds to the MC34844
Register that needs to be written. (refer to Table 5)
For the MC34844 device, when an address is sent, each
of the devices in a system compares the first seven bits
after the START condition with its address. If they match,
the device considers itself addressed by the control unit as
a slave-receiver.
4. Second data pack, it corresponds to the value that
should be written to that register. (refer to Table 5)
5. STOP condition
I2C variables description:
• STOP: this condition occurs when SDA changes from
LOW to HIGH while SCK is HIGH
• START: this condition occurs when SDA changes from
HIGH to LOW while SCK is HIGH.
• ACKNOWLEDGE: The acknowledge clock pulse is
generated by the Master (Control Unit).
• The transmitter releases the SDA line (HIGH) during the
acknowledge clock pulse.
For more information about “I2C BUS SPECIFICATION”
please refer to the following link:
http://www.nxp.com/acrobat_download/literature/
9398/39340011.pdf
34844
Analog Integrated Circuit Device Data
Freescale Semiconductor
19
FREESCALE [ Freescale ]
