SPX-5 Reference Manual
9
Operation
Description
The SPX-5 incorporates the Freescale MC33879 device, which provides eight power MOSFET
switches that can be configured to drive small industrial loads such as solenoid valves, fans, and
lights. It is controlled through the VersaLogic SPX (SPI) interface, but has two channels that can
be configured via jumpers to accept direct control signals such as PWM, GPIO, or pushbutton
inputs. These inputs are 5V tolerant and can accept 3.3V or 5V CMOS outputs.
You must provide an external voltage source to supply the intended load(s). The switches are
independent and can use individual sources. The SPX-5 has a built-in 13.6VDC gate drive source
for driving the power MOSFETs. Each MOSFET switch incorporates automatic over-current and
over-temperature fault detection and will shutoff when the load current is within the range of
0.6A to 1.2A or when the device temperature is within the range of 155°C to 185°C. When the
over-current/temperature condition is relieved, the device will return to its previous state. A fault
register indicates when an error condition has occurred.
Note
: The switch can oscillate if subjected to continuous high loads and temperatures. The
MC33879 also incorporates internal voltage clamping of +45VDC and -20VDC for low
and high side configurations respectively.
See the
SPI
I
NTERFACE
The MC33879 is controlled with a 16-bit SPI frame as shown in the following tables.
Table 5: Command Register Bit Assignments
MSB
LSB
Bit 15
Bit 14
Bit 13
Bit 12
Bit 11
Bit 10
Bit 9
Bit 8
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
ON/
OFF
Open
Load
Detect
8
ON/
OFF
Open
Load
Detect
7
ON/
OFF
Open
Load
Detect
6
ON/
OFF
Open
Load
Detect
5
ON/
OFF
Open
Load
Detect
4
ON/
OFF
Open
Load
Detect
3
ON/
OFF
Open
Load
Detect
2
ON/
OFF
Open
Load
Detect
1
ON/
OFF
OUT 8
ON/
OFF
OUT 7
ON/
OFF
OUT 6
ON/
OFF
OUT 5
ON/
OFF
OUT 4
ON/
OFF
OUT 3
ON/
OFF
OUT 2
ON/
OFF
OUT 1
Table 6: Fault Register Bit Assignments
MSB
LSB
Bit 15
Bit 14
Bit 13
Bit 12
Bit 11
Bit 10
Bit 9
Bit 8
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
0
0
0
0
0
0
0
0
OUT 8
Status
OUT 7
Status
OUT 6
Status
OUT 5
Status
OUT 4
Status
OUT 3
Status
OUT 2
Status
OUT 1
Status
3
