L9654 datasheet(46/59 Pages) STMICROELECTRONICS

46 / 59 page

Functional description

L9654

46/59

DocID14218 Rev 3

4.14.1

Current sensor

Each output channel senses the current drawn by the remote satellite sensor; the circuit

modulates the load current into logic voltage levels for post processing by a Manchester

decoder. Each channel has an internal comparator with programmable currents trip points

selectable through the appropriate setting in the CCR Register for each of the 2 satellite

channels. The current sense comparator provides a hysteresis, which can be enabled

through appropriate setting in the CCR Register. Each comparator output has a de-glitch

filter as a function of the protocol speed as defined in the AC characteristic table. Each

current is limited to 150mA maximum and includes a fault timer.

If the output for a given channel is in current limited for a period of time exceeding the output

fault timer, the IC reports an over current fault via SPI and latches off the affected channel,

the channel remains in latch off mode until the user sends an SPI command to re-activate

the channel. When the output is shorted to battery, an internal comparator senses the output

voltage level then turns off an internal series transistor to provide blocking diode for the

current going through the output channel, the output resumes normal operation once the

fault condition is removed. The comparator has 20 to 50mV input offset to prevent turning off

the output under an open circuit condition. In case of loss of VCC all outputs remain off.

4.14.2

Manchester decoding

The L9654 decodes satellite messages based on Manchester decoding, each of the two

satellite channels has a Manchester decoder that can be enabled or disabled through the

CCR register. An example of Manchester decoding is given below; logic 0 is defined as a

signal transition from 0 to 1 at 50% Duty cycle, logic 1 is defined as a signal transition from 1

to 0 at 50% Duty cycle.

The IC starts decoding the satellite message after it receives two SYNC bits defined as logic

00, the SYNC bits are used to determine the bit rate of the incoming message and are used

as the time base in decoding the following bits; different bit rates ranges are programmable

via SPI, in case the measured bit rate obtained using the 2 SYNC bits doesn’t fall within the

range selected by the SPI as defined in Table 35, the device declares a bit time error,

reverts to the minimum bit time of the selected range, and waits for idle.

Figure 19. Manchester decoding

L9654 Datasheet(PDF) 46 Page - STMicroelectronics