MC33298 Motorola, MC33298 Datasheet - Page 11

• Manufacturer Part Number: MC33298
• Description: OCTAL SERIAL SWITCH (SPI Input/Output)
• Manufacturer: Motorola

CSB Pin
communicated with through the use of the CSB pin.
Whenever the pin is in a logic low state, data can be
transferred from the MCU to the MC33298 and vise versa.
Clocked–in data from the MCU is transferred from the
MC33298 shift register and latched into the power outputs on
the rising edge of the CSB signal. On the falling edge of the
CSB signal, drain status information is transferred from the
power outputs and loaded into the device’s shift register. The
CSB pin also controls the output driver of the serial output
pin. Whenever the CSB pin goes to a logic low state, the SO
pin output driver is enabled allowing information to be
transferred from the MC33298 to the MCU. To avoid any
spurious data, it is essential that the high–to–low transition of
the CSB signal occur only when SCLK is in a logic low state.
SCLK Pin
registers of the MC33298. The serial input pin (SI) accepts
data into the input shift register on the falling edge of the
SCLK signal while the serial output pin (SO) shifts data
information out of the shift register on the rising edge of the
SCLK signal. False clocking of the shift register must be
avoided to guarantee validity of data. It is essential that the
SCLK pin be in a logic low state whenever chip select bar pin
(CSB) makes any transition. For this reason, it is
recommended though not necessary, that the SCLK pin be
kept in a low logic state as long as the device is not accessed
(CSB in logic high state). When CSB is in a logic high state,
any signal at the SCLK and SI pin is ignored and SO is
tristated (high impedance). See the Data Transfer Timing
diagram of Figure 16.
SI Pin
information is read in on the falling edge of SCLK. A logic high
state present on this pin when the SCLK signal rises will
program a specific output “off,” and in turn, turns “off” the
specific output on the rising edge of the CSB signal.
Conversely, a logic low state present on the SI pin will
program the output “on,” and in turn, turns “on” the specific
output on the rising edge of the CSB signal. To program the
eight outputs of the MC33298 “on” or “off,” an eight bit serial
stream of data is required to be entered into the SI pin
starting with Output 7, followed by Output 6, Output 5, etc., to
Output 0. For each rise of the SCLK signal, with CSB held in
a logic low state, a databit instruction (“on” or “off”) is loaded
into the shift register per the databit SI state. The shift register
is full after eight bits of information have been entered. To
preserve data integrity, care should be taken to not transition
SI as SCLK transitions from a low to high logic state.
SO Pin
the shift register. The SO pin remains in a high impedance
state until the CSB pin goes to a logic low state. The SO data
reports the drain status, either high or low. The SO pin
changes state on the rising edge of SCLK and reads out on
the falling edge of SCLK. When an output is “off” and not
faulted, the corresponding SO databit is a high state. When
an output is “on,” and there is no fault, the corresponding
databit on the SO pin will be a low logic state. The SI/SO
shifting of data follows a first–in–first–out protocol with both
MOTOROLA ANALOG IC DEVICE DATA
T he s y s t em M CU s ele c ts th e MC 33 29 8 t o b e
The system clock pin (SCLK) clocks the internal shift
This pin is for the input of serial instruction data. SI
The serial output (SO) pin is the tri–stateable output from
PIN FUNCTION DESCRIPTION
MC33298
input and output words transferring the Most Significant Bit
(MSB) first. The SO pin is not affected by the status of the
Reset pin.
Reset Pin
SPI shift register and in doing so sets all output switches “off.”
With the device in a system with an MCU; upon initial system
power up, the MCU holds the Reset pin of the device in a
logic low state ensuring all outputs to be “off” until both the
V DD and V PWR pin voltages are adequate for predictable
operation. After the MC33298 is reset, the MCU is ready to
assert system control with all output switches initially “off.” If
the V PWR pin of the MC33298 experiences a low voltage,
following normal operation, the MCU should pull the Reset
pin low so as to shutdown the outputs and clear the input data
register. The Reset pin is active low and has an internal
pull–up incorporated to ensure operational predictability
should the external pull–up of the MCU open circuit. The
internal pull–up is only 20
interfacing to the MCU. The Reset pin of the MC33298
should be pulled to a logic low state for a duration of at least
250 ns to ensure reliable reset.
programmed through the use of an RC network comprised of
a shunt capacitor from the Reset pin to Ground and a resistor
to V DD (See Figure 15). Care should be exercised to ensure
proper discharge of the capacitor so as to not adversely
delay the reset nor damage the MCU should the MCU pull the
Reset line low and yet accomplish initialization for turn “on”
delay. It may be easier to incorporate delay into the software
program and use a parallel port pin of the MCU to control the
MC33298 Reset pin.
SFPD Pin
disable the over current latch–off. This feature allows control
of incandescent loads where in–rush currents exceed the
device’s analog current limits. Essentially the SFPD pin
determines whether the MC33298 output(s) will instantly shut
down upon sensing an output short or remain “on” in a
current limiting mode of operation until the output short is
removed or thermal shutdown is reached. If the SFPD pin is
tied to V DD = 5.0 V the MC33298 output(s) will remain “on” in
a current limited mode of operation upon encountering a load
short to supply. If the SFPD pin is grounded, a short circuit
will immediately shut down only the output affected. Other
outputs not having a fault condition will operate normally. The
short circuit operation is addressed in more detail later.
The MC33298 Reset pin is active low and used to clear the
A simple power “on” reset delay of the system can be
The Short Fault Protect Disable (SFPD) pin is used to
MCU
Reset
Figure 15. Power “On” Reset
V DD
R DLY
C DLY
A to afford safe and easy
MC33298
Reset
20 A
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