START-STOP Software Interrupt

The START-STOP interrupt is a method for the slave

device to initiate a signal over the 2-wire interface with-

out the need for a third (interrupt) wire. A START-STOP

interrupt is a start condition followed by a stop condi-

tion; in other words, SMBDATA goes low and then high

with SMBCLK high (Figure 5 shows the START-STOP

interrupt and a subsequent Alert Response transmis-

sion used to clear the interrupt). The START-STOP

function can be disabled (masked) by setting the data

register mask SS (bit 6) high.

In order to avoid bus collisions, the START-STOP inter-

rupt will not occur when the bus is busy. If the device

begins a start condition simultaneously with another

transmitter on the bus, it recognizes the falling SMB-

CLK as a collision and re-transmits the interrupt when

the bus becomes available. Upon thermal shutdown or

a transition on an I/O line, the device issues only one

START-STOP interrupt, and won’t repeat it unless there

has been a collision. However, thermal-shutdown faults,

not being edge triggered, may result in a continuous

stream of START-STOP bits.

Input/Output Pins

Each input/output (I/O) is protected by an internal

20mA (typical) current-limit circuit. The I/O current limit

depends on the supply voltage and the voltage applied

to the I/O pins (see

Typical Operating Characteristics).

as well as the voltage on the I/O. Typical pull-down on-

respectively. I/O source and sink capability can affect

the rise and fall times of external power MOSFETs com-

monly used in power-switching applications. Other fac-

FET, and the pull-up resistor value used in the circuit.

Typical MOSFET gate capacitance ranges from 150pF

to 2000pF. Increasing the RC time constant slows down

the MOSFET’s response, but provides for a smoother

transition.

Power-On Reset

The power-on reset circuit keeps the external MOSFETs

off during a power-up sequence. When the supply volt-

age falls below the power-on reset threshold voltage,

the MAX1662/MAX1663’s outputs reset to a high-

impedance state, and the MAX1661’s outputs reset to a

increases, the ALERT pin goes low and then high,

which indicates the device is powered on. The time

between the low and high state on ALERT is the power-

states can’t be enforced, and the I/O pins of all ver-

sions exhibit increasingly weak pull-down current capa-

bility, eventually becoming high impedance.

Thermal Shutdown

These devices have internal thermal-shutdown circuitry

that turns off all output stages (I/O pins) when the junc-

tion temperature exceeds +140°C typical. Thermal

shutdown only occurs during an overload condition on

the I/O pins. The device cycles between thermal shut-

down and the overcurrent condition until the overload

condition is removed. This could cause a sustained

START-STOP interrupt and, in the extreme case, tie up

the master controller. However, the device asserts

ALERT low, indicating this fault status.

Serial-to-Parallel/Parallel-to-Serial Converters and

Load-Switch Controllers with SMBus Interface

10

______________________________________________________________________________________

START

STOP

DATA LINE HELD

LOW BY SLAVE

SMBCLK

SMBDATA

START-STOP INTERRUPT

ALERT RESPONSE ADDRESS

(0001100)

ACTUAL SLAVE ADDRESS

(0100000 IN THIS EXAMPLE)

DUMMY BIT (1)

ALERT

SLAVE

ACKNOWLEDGE

(ACK)

Figure 5. START-STOP Software Interrupt Timing Diagram and Alert Response