UM0851 datasheet(184/245 Pages) STMICROELECTRONICS

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Human interface device (HID) drivers

UM0851

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Doc ID 16604 Rev 2

Figure 49.

ADC block diagram

When you scan a picture with a scanner what the scanner is doing is an analog-to-digital

conversion: it is taking the analog information provided by the picture (light) and converting

into digital. When you record your voice or use a VoIP solution on your computer, you are

using an analog-to-digital converter to convert your voice, which is analog, into digital

information.

Digital information is not only restricted to computers. When you talk on the phone, for

example, your voice is converted into digital (at the central office switch, if you use an analog

line, or at you home, if you use a digital line like ISDN or DSL), since your voice is analog

and the communication between the phone switches is done digitally.

SPEAr includes an Analog-to-Digital Converter (ADC), which is connected to the APB bus. It

is a successive approximation ADC and its main features are:

●

10 bit resolution

●

8 analog input (AIN) channels, ranging from 0 to 2.5 V

●

For each input, the number of samples to be collected for average calculation can be

one (no average) or up to 128 as 2's power (2, 4, 8...).

●

Normal or enhanced mode: In normal mode, a single conversion is perfomed on one

channel. In enhanced mode, the ADC converts the selected channels continuously

inserting a selectable amount of time between two conversions.

●

Positive and negative reference voltages can be supplied by dedicated pins to select

different conversion range (Default 0 - 2.5V)

7.3.2

Software overview

The ADC can be accessed from two levels in Linux:

●

From the user level by operating on nodes created for ADC channels

●

From other kernel modules

The ADC converts the analog voltage present on the analog input pin (AIN). The A/D

conversion is done as configured by the ADC driver. The converted voltage then is returned

as a digital value to the ADC driver. The ADC software system architecture is shown in the

following figure.

ADC

Configuration registers

Conversion data

APB bus

8 analog inputs

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