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emPC-CX+
(
System Reference
Manual)
System Control
©
Janz Tec AG
Rev. 1.2
3.2
FAN Controller
The fan controller sets the fan rotation speed based on the temperature.
It controls the fan by generating a pulse width modulated output signal (PWM). This signal is either
directly used by the fan (4-wire fans) or used to control the DC operating voltage of the fan. A PWM
setpoint of 100% results in maximum fan operation speed.
The fan controller can utilize two basic temperature sources:
An internal temperature sensor inside the controller chip. Although offset calibration is
possible, this sensor has limited accuracy. Furthermore the fan controller IC might be placed
in some distance to the interesting temperature spots.
Two temperature values supplied to the fan controller over it's I2C interface. The system
software might have access to temperature sensors of interest and can supply them to the fan
controller. Suppose a CPU fan: If the cores are idle, the fan speed can be low even at high
ambient temperatures. You can supply two such remote temperature values to the fan
controller. The highest priority temperature source is used. If not updated within a 2 second
timeout, the next priority temperature source will be used. The last resort is the internal
sensor.
The fan controller utilizes a simple linear temperature to PWM setpoint calculation as shown in figure
7. The T_LOW and T_HIGH parameters can be configured independently for all three temperature
sources and are stored in the controllers internal EEPROM (refer to section 3.2.5).
Rotation speed of the FAN is measured and can be monitored via the controllers I2C interface.
An open drain fault output is available, which sinks current when the rotation speed falls below a
configured minimum (not utilized on emPC-CX+). The PWM setpoint is forced to 100% during a fault
condition.
3.2.1
I2C Communication
The I2C interface is used to transfer data between I2C master and fan controller. Note that the fan
controller always acts as slave in I2C communication.
This section describes the fan controller integrated on the emPC-CX+ baseboard.
In the emPC-CX+ design you have no direct access to the I2C bus where the fan
controller is located. Instead you have to do indirect access via the PMON firmware.
Refer to the FANCON_RDWR register for more information. The I2C protocols described
are left for reference.
Tx_LOW
Tx_HIGH
MIN_PWM
100%
PWM
setpoint
Temperature
figure 7: fan controller setpoints
Summary of Contents for emPC-CX+
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