VSC7440 datasheet(378/444 Pages) MICROSEMI

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VCore-III System and CPU Interfaces

VMDS-10492 VSC7440 Datasheet Revision 4.2

361

The PWM output pin can be configured to act as a normal output or as an open-collector output, where

the output value of the pin is kept low, but the output enable is toggled. The open-collector output mode

is enabled by setting FAN_CFG.PWM_OPEN_COL_ENA.

Note: By using open-collector mode, it is possible to externally pull-up to a higher voltage than the maximum

GPIO I/O supply. The GPIO pins are 3.3V-tolerable.

The speed of the fan is measured using a 16-bit counter that counts the rising edges on the TACHO

input. A fan usually gives one to four pulses per revolution, depending on the fan type. The counter value

is available in the FAN_CNT register. Depending on the value of FAN_CFG.FAN_STAT_CFG, the

FAN_CNT register is updated in two different ways:

•

If FAN_CFG.FAN_STAT_CFG is set, the FAN_CNT register behaves as a 16-bit wrapping counter

that shows the total number of ticks on the TACHO input.

•

If FAN_CFG.FAN_STAT_CFG is cleared, the FAN_CNT register is updated one time per second

with the number of TACHO ticks received during the last second.

Optionally, the TACHO input is gated by the polarity-corrected PWM output by setting

FAN_CFG.GATE_ENA, so that only TACHO pulses received while the polarity corrected PWM output is

high are counted. Glitches on the TACHO input can occur right after the PWM output goes high. As a

result, the gate signal is delayed by 10 µs when PWM goes high. There is no delay when PWM goes low,

and the length of the delay is not configurable. Software must read the counter value in FAN_CNT and

calculate the RPM of the fan.

An example of how to calculate the RPM of the fan is if the fan controller is configured to 100 Hz and a

20% duty cycle, each PWM pulse is high in 2 ms and low in 8 ms. If gating is enabled, the gating of the

TACHO input is open in 1.99 ms and closed in 8.01 ms. If the fan is turning with 100 RPMs and gives two

TACHO pulses per revolution, it will ideally give 200 pulses per minute. TACHO pulses are only counted

in 19.99% of the time, so it will give 200 × 0.1999 = 39.98 pulses per minute. If the additional 10 µs

gating time is ignored, the counter value is multiplied by 2.5 to get the RPM value, because there is a

20% duty cycle with two TACHO pulses per revolution. By multiplying with 2.5, the RPM value is

calculated to 99.95, which is 0.05% off the correct value (due to the 10 µs gating time).

4.8.11

Temperature Sensor

This section provides information about the on-die temperature sensor. When enabled the temperature

sensor logic will continually monitor the temperature of the die and make this available for software.

The following table lists the registers associated with the temperature monitor.

The temperature sensor is enabled by setting TEMP_SENSOR_CTRL.SAMPLE_ENA. The temperature

sensor then samples the temperature every 500 µs and shows the current temperature in

TEMP_SENSOR_STAT. The formula for converting TEMP field value to centigrade temperature is:

Temp (°C) = 177.4 – 0.8777 × TEMP_SENSOR_STATE.TEMP

It takes approximately 500 µs after setting SAMPLE_ENA until the first temperature sample is ready. The

TEMP_SENSOR_STAT.TEMP_VALID field is set when the temperature value is available.

4.8.12

Memory Integrity Monitor

Soft errors happen in all integrated circuits as a result of natural alpha decay, cosmic radiation, or

electrical disturbances in the environment in which the device operates. The chance of soft errors

happening in a memory (RAM) is higher than for flip-flop based logic, because the memory structures are

physically small and changes require less outside force than in flip flops. The device has built-in

protection from soft errors by using error correcting code (ECC) on critical memories. In addition, the

device allows monitoring and reporting of soft error events.

Table 268 •

Temperature Sensor Registers

Description

HSIO::TEMP_SENSOR_CTRL

Enabling of sensor

HSIO::TEMP_SENSOR_STAT

Temperature value

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