MC33884 Просмотр технического описания (PDF) - Motorola => Freescale

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MC33884

Switch Monitor Interface

Motorola => Freescale 

MC33884

PIN FUNCTIONAL DESCRIPTION (continued)

SB1, SB2 Pins

The SB pins are switch–to–battery sensing inputs only.

Transient battery/supply voltages greater than 40 volts must

be clamped by external device. Surface mount 0805 MOVs

and transient voltage suppressors (TVS) are available in

SOT–23 packages. The sensed input is compared with an

internal 4.0 volts reference. Voltages greater than 4.0 volts are

interpreted as a CLOSED switch. Sensed voltages less than

4.0 V are interpreted as an OPEN switch. Programming can

set wetting currents or Tri–State the input. Programming

methods are provided in the following section.

SG1 – SG6 Pins

The SG pins are switch–to–ground inputs only. The input is

compared with the internal 4.0 volt reference. Voltages greater

than 4.0 volts are interpreted as an OPEN switch. Voltages

less than 4.0 V are interpreted as a CLOSED switch.

Programming can set the wetting currents or Tri–State the

input. Programming methods are provided in the following

section.

FUNCTIONAL DESCRIPTION

POWER UP

On initial power up, all MC33884 registers will be cleared

and the device will enter the Sleep mode. To exit Sleep mode

a valid command word is required to be received from the

microprocessor.

Polling mode allows the user to reduce quiescent

current by disabling sink and source currents during

swtich ”OFF” periods.

Sleep Command

Sleep mode can be entered by a SPI Sleep command or

asserting the RSTB pin. In Sleep mode all inputs are Tri–State

and all internal active pull up and pull down currents are

disabled. Sleep mode reduces the current drain to a quiescent

current level of 10 µA and disables the IC. Sleep mode

provides lowest quiescent current for the IC. Exit from sleep

mode requires a valid SPI RUN, TRI–STATE, or METALLIC

command.

RUN Command

Run command places the IC in one of three operating

modes; Normal, Polling, and Polling + INT Timer. The

command also programs the SP1 to SP4 sense inputs

(switch–to–battery [1] or switch–to–ground [0]). See Table 1.

Normal mode is the normal operating mode of the

MC33884. In Normal Mode the status of the input

switches are latched on falling edge of CSB and data is

sent back to MCU via SPI. All programmed

combinations of source and sink currents, used for

sensing purposes, are always active in this mode. In

normal mode an interrupt is generated and sent to the

microprocessor whenever an external switch changes

its OPEN or CLOSED state. Prior to a switch closing,

the MC33884 sources 0.75 mA of sustain current.

When the voltage at the input crosses the comparator

threshold, 14 mA of current is allowed to flow. The 14

mA wetting current shuts off after a 20 ms timer

expires.

Polling mode reads a switch status periodically and

interrupts the microprocessor only when an external

switch is sensed as being CLOSED. If the MC33884

senses all external switches to be OPEN, the Polling

mode of operation continues. If a switch is sensed

CLOSED, an interrupt is sent to the microprocessor

and the MC33884 transfers it’s operational mode to the

Normal mode. The Polling mode provides a reduction

in quiescent current by turning OFF all source and sink

currents during sensed switch OFF periods. The

Polling + INT Timer mode of operation is similar to the

Polling mode above, with the addition of an interrupt

being sent to the microprocessor if a switch is sensed

CLOSED or upon the internal interrupt timer ”timing

out”. An interrupt is always ultimately sent to the

microprocessor in this mode. The microprocessor can

be programmed to read (or ignore) the MC33884’s

reported switch status upon receiving the interrupt. If a

switch is sensed CLOSED, operation reverts

automatically to the Normal mode. If all switches are

sensed OPEN, and the wake–up timer (INT Timer)

times out, the MC33884 continues to operate in the

Polling + INT Timer mode. The wake–up timer duration

may be set much longer than the Polling time.

TRI–STATE Command

TRI–STATE command places all switch inputs into

Tri–State. All comparators on inputs are disabled in this mode.

The device will return [0] for the switch status.

SPI PROGRAMMING

The MC33884 uses the SPI in full duplex synchronous slave

mode for communication with the microprocessor. The

MC33884 is programmed via a 16 bit word command from the

MCU. The word is sent to the device with the MSB first. The

command word sent to the MC33884 sets the mode of

operation in the device. Data received back from the

MC33884 is the status of the sensed input switch on falling

edge of CSB. Sixteen clock periods are required for each

transmission to be valid. After the 16 clocks, CSB is returned

to the inactive state (logic [1]), command words are no longer

accepted into SI, and the SO pin is Tri–Stated. The response

to a SPI command returns status based on previous command

word. This previous command could be a hardware reset as

well as any of the other commands discussed in this section.

MOTOROLA TSG IC DEVICE DATA

11

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