STR71x ADC conversion speed-up

AN1798

12/16

4

STR71x ADC conversion speed-up

This section gives an example of how to reduce the conversion time of the A/D Converter of

the STR71x microcontroller, when only one input channel needs to be converted. This is

achieved using Round Robin mode instead of Single Channel mode to convert an analog

input signal which must be connected to all four analog input pins of the converter. By

configuring the conversion time to 1ms per channel, the conversion Speed-Up technique

allow conversion time to be reduced down to 250 µs.

4.1

Hardware implementation

In Single Channel mode, a valid sample for the selected channel is produced only every

2048 clock cycles, with the same output frequency as Round Robin mode which samples

the four channels (512 clock cycles for each channel). Consequently, the conversion time for

one analog voltage can be reduced by using Round Robin mode instead of Single Channel

mode.

The four ADC inputs have to be connected to the same analog voltage input in order to use

Round Robin conversion mode. As shown in Figure 9, only one input can be used when this

technique is applied.

Figure 9.

Speed-Up hardware implementation

4.2

Firmware description

To use the procedure described above, the ADC has to be driven in polling mode because in

Round Robin mode, an interrupt is generated only when the four input channels have been

converted and all the Data Available flags in the Control Status Register (CSR) are set. In

polling mode, it is possible to get the conversion result of the channel corresponding to the

DAn flag that has been set without waiting for the end of conversion on all the channels.

First, all the STR71x analog pins must be configured as high impedance analog inputs.

Next, the STR71x ADC has to be selected in Round Robin conversion mode. Then, the

prescaler has to be configured. Next, the ADC has to be enabled by setting the ADC_En bit

in the PCU_BOOTCR register. To speed-up the ADC conversion you have to check all the

AIN2

AIN3

AIN1

AIN0

STR71x

Analog input

(only one input possible)

Note: you must connect the 4 channels

together.

0.1µF

3.3V