Legacy-Free Keyboard/Embedded Controller with SPI and LPC Docking Interface
Chapter 19 Fan Tachometer Interface
19.1
Fan Tachometer Overview
The LPC47N350 implements a dual fan tachometer interface for systems with fans equipped with speed
monitoring outputs. The fan tachometer input pins are FAN_TACH1 and FAN_TACH2 (Table 2.2). These
pins are alternate functions of the GPIO15 and GPIO16 pins, respectively (See Table 2.4 and MICS[11
bit in Section 21.4, "Multiplexing_2 Register - MISC[16:9]"). The timebase for the fan tachometer
interface is the 32.768kHz RTC oscillator (Figure 19.1). A fan tachometer input gates the 32.768 kHz
RTC oscillator for one period of the input signal into an 8-bit counter. As shown in Figure 19.1, one fan
revolution, T consists of two fan tachometer pulses, T . The fan tachometer interface can generate
R,
P
an 8051 interrupt and wake event when the fan speed (RPM) drops below a predetermined value. The
fan tachometer interface is available when VCC2 is active and on the suspend supply, VCC1.
TR
Fan
Tachometer
Input
TP
TR =Revolution Time =60/RPM(sec)
TP = Pulse Time = T /2
(Two Pulses PerRRevolution)
Time-base
F =32kHz ÷Prescaler
Figure 19.1 Fan Tachometer Input and Time-Base
19.2
Theory of Operation
Each fan tachometer in the dual fan tachometer interface contains a Timebase Prescaler, a Fan Pulse
Counter, a Fan Pulse Counter Preload, a Fan Pulse Counter Read Latch, and a Fan Pulse Counter
Threshold Detector (Figure 19.2). Detailed descriptions of these components follow in the subsections,
below.
19.2.1 Timebase Prescaler
The timebase prescaler divides the fan tachometer timebase. The timebase prescaler can be used to
account for several fan types (i.e., fans where the RPM values do not depend on two fan tachometer
pulses per revolution) and a wide range of fan speeds. The prescaler for each fan tachometer is
programmable via the FAN Tachometer Timebase Prescaler Register (see Section 19.8, "FAN
Tachometer Timebase Prescaler Register"). The choices for the timebase prescaler are 1, 2, 4 and 8;
the default is 2.
SMSC LPC47N350
211
Revision 1.1 (01-14-03)
DATASHEET
SMSC [ SMSC CORPORATION ]
