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STR71x ADC Driving an Analog Keyboard

1 HARDWARE CONFIGURATION

To reach the goal of this application note we need to connect an analog keyboard to one of the

analog inputs of the STR71x ADC.

When a key is pressed, the voltage at the ADC input is given by the activated voltage divider.

This analog voltage is converted by the ADC and the digital value is used to determine which

switch is closed.

Figure 1. STR71x ADC and analog keyboard configuration

1.1 STR71X ADC

The STR71x Analog to Digital converter is a 12-bit Sigma-Delta converter with an input range

between 0 and 2.5V. It has up to 4 multiplexed analog input channels.

It offers two conversion modes:

■

Round-robin: this repeats the conversion process for each of the four channels

continuously in turn.

■

Single channel: this converts only the selected analog input.

Enabling or disabling the converter is done by setting or clearing the ADC_EN bit in the

PCU_BOOTCR register.

The STR71x ADC provides four 16-bit data registers to store the conversion result for each

channel. The availability of the conversion result in the data register is shown by the four

DA[3:0] flags in the Control/ Status Register (ADC_CSR).

An interrupt is generated depending on the conversion mode, to confirm the end of conver-

sion. In Single channel mode, an interrupt is generated if the interrupt bit and Data Available

flag in the ADC_CSR are set, corresponding to the selected channel. In Round robin mode, an

STR71x

V3.3

ADC

AINx

AVss

AVdd

Vss

key0

key1

key14

key15

Vdd

Vss

Keyboard

2.5v

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