MPR121

Sensors

Freescale Semiconductor

19

The normal setup of the system is to set this register to 0x0B or 0b00001011. This means that the FFI is 00, but if the FFI in

the AFE Configuration Register is different, it must be changed to match. For a description of this register, please refer to AN3890.

The RETRY is disabled because in production systems, this will not be required. The BVA is set to 10 which allows the baseline

to be updated. 10 is used instead of 11 because this guarantees that the baseline will be lower than the data. This is preferable

as it protects against false touches. If somehow the baseline started higher than the data, a touch would be triggered and the

detection system would have to be reset to work correctly. Last, both the automatic configuration and automatic reconfiguration

are enable. Reconfiguration will trigger any time the baseline drifts outside the range set by the USL and the LSL.

There is also a set of flags which show when the automatic configuration has failed. For normal sized touch electrodes, this

cannot occur without the USL, LSL and TSL being incorrectly set. The most likely configuration error is to set the USL (upper

limit) at a lower value than the LSL (lower limit). Thus, as the algorithm searches for settings that work, it would always result in

a fail throwing the OOR (Out Of Range) status flag.

The ARFF and ACFF also tell the user which type of configuration cycle caused the error. If it was triggered during an initial

calibration, the ACFF will trigger. If the fail occurs during a reconfiguration, the ARFF will trigger.

ELE0-7 OUT OF RANGE STATUS REGISTER

Figure 16. ELE0-7 Out Of Range Status Register

Table 17. AUTO-CONFIG Control Register Field Descriptions

Field

Description

7:6

AFES

First Filter Iterations – The first filter iterations field selects the number of samples taken

as input to the first level of filtering. This value must match the FFI set in the AFE

Configuration register for proper AUTO-CONFIG functionality.

00 Encoding 0 – Sets samples taken to 6

01 Encoding 1 – Sets samples taken to 10

10 Encoding 2 – Sets samples taken to 18

11 Encoding 3 – Sets samples taken to 34

5:4

RETRY

Retry – The Retry value determines under what circumstances the auto-configuration

system will retry.

00 – Retry disabled

01 – Retry enabled

10 – Retry enabled

11 – Retry enabled

3:2

BVA

Baseline Value Adjust – The baseline value adjust determines the initial value of the

baseline registers after auto-configuration completes.

00 – Baseline is not changed

01 – Baseline is cleared

10 – Baseline is set to the AUTO-CONFIG baseline with the lower 3 bits cleared

11 – Baseline is set to the AUTO-CONFIG baseline

1

ARE

Automatic Reconfiguration Enable – The automatic reconfiguration enable, enables or

disables automatic reconfiguration.

0 – ARE is disabled

1 – ARE is enabled

0

ACE

Automatic Configuration Enable – The automatic configuration enable, enables or

disables automatic configuration.

0 – ACE is disabled

1 – ACE is enabled

7

6

543210

R

E7S

E6S

E5S

E4S

E3S

E2S

E1S

E0S

W

Reset:

0

0

000000

= Unimplemented