PSoC
®
Thermal Management Expansion Board Kit Guide, Doc. No. 001-89649 Rev. **
18
Designers can enter duty cycle data in the graphical user interface (GUI) of the fan controller component in PSoC Creator;
this component automatically configures and optimizes the firmware and hardware inside PSoC to control fans with these
parameters.
It is important to note that
fans don’t always behave in the same way at low duty cycles. Some fans stop rotating as the duty
cycle approaches 0 percent, whereas others rotate at a nominal specified minimum rpm. In both cases, the duty-cycle-to-
speed relationship can be nonlinear or unspecified. When entering duty-cycle-to-speed data in the fan controller component
customizer interface, select two data points from the linear region where the behavior of the fan is well defined.
Four-wire DC fans include Hall-effect sensors that sense the rotating magnetic fields generated by the rotor as it spins. The
output of the Hall-effect sensor is a pulse train that has a period inversely proportional to the rotational speed of the fan. The
number of pulses that are produced per revolution depends on how many poles are used in the electromechanical
construction of the fan.
For the most common four-pole brushless DC fan, the tachometer output from the Hall-effect sensor will generate two high
and low pulses per fan revolution. If the fan stops rotating due to mechanical failure or other fault, the tachometer output
signal will remain static at either a logic-low level or a logic-high level.
The fan controller component measures the period of the tachometer pulse train for all fans in the system using a custom
hardware implementation. The firmware APIs provided convert the measured tachometer periods into revolutions per
minute to enable development of fan control algorithms that are firmware based. The same hardware block can generate
alerts when it detects that a fan has stopped rotating
—a condition referred to as a stall event.
For details on implementing four-wire fan control using various PSoC devices, read the following application notes.
AN78692
– PSoC 1 – Intelligent Fan Controller
AN89346
– PSoC 4 – Intelligent Fan Controller
AN66627
– PSoC 3 and PSoC 5LP – Intelligent Fan Controller
These application notes provide example projects that demonstrate different usage modes of four-wire fan control. In
addition, they include a thermal management example project that uses the temperature sensors on the CY8CKIT-036 EBK
to control the speed of the fans associated with a thermal zone.
3.3 PWM Output Digital Temperature Sensor
There are two PWM output TMP05 temperature sensors on the CY8CKIT-036 EBK. TMP05 is a monolithic temperature
sensor that generates a PWM serial digital output. The duty cycle of the PWM output varies in direct proportion to the
ambient temperature of the devices. The high period (T
H
) of the PWM remains static over all temperatures, whereas the low
period (T
L
) varies. It offers a high temperature accuracy of ±1° C (from 0° C to 70° C), with excellent transducer linearity.
The ratio of T
H
/T
L
provides a method for determining the temperature according to the formula:
Temperature (°C) = 421
– (751 × T
H
/T
L
)
The TMP05 sensor has a two-pin interface. The CONV/IN input, when pulsed by the PSoC chip, initiates a new temperature
measurement. The output provides a PWM signal that can be decoded using the formula above to determine the ambient
temperature. TMP05 sensors support a daisy-chain mode of operation in which the OUT signal of the first sensor can be
directly connected to the CONV/IN input of the subsequent sensor. The OUT of the second sensor carries the PWM signals
from both sensors. Many sensors can be daisy-chained in this fashion, with the final OUT signal carrying the PWM
temperature encoding from all sensors in the daisy chain.
For details on interfacing with a TMP05 temperature sensor using various PSoC devices, read the following application
notes:
AN78737
– PSoC 1 – Temperature Sensing Solution Using a TMP05/TMP06 Digital Temperature Sensor
AN65977
– PSoC 3 and PSoC 5LP: Creating an Interface to a TMP05/TMP06 Digital Temperature Sensor
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