ATA6286C_14 Datasheet, PDF (101/183 Pages) ATMEL Corporation – Embedded AVR Microcontroller with LF Receiver and UHF Transmitter

Figure 3-56. Sensor Interface Block
Motion Sensor
Interface
Unit
(MSIU)
CLK_SEN
S0 (C_EX0)
S1 (C_EX1)
S2 (C_EX2)
Capacitor Sensor
Interface
f_C
Unit
(CSIU)
Voltage Sensor
Interface
Unit
MUX
(VSIU)
f_V
MSENOS
MSENO
MSIE
SENO
SENINT
Control SMS
SEN (1:0)
For further information on the resolution of the three different units MSIU, CSIU and VSIU see Section 5.2 “Operating
Characteristics of Atmel ATA6289” on page 169ff.
3.15.1 Motion Sensor Interface Unit (MSIU)
This section describes how a measurement with a motion sensor at pin S2 is executed and how a motion wake-up can be
generated.
The SMEN bit in the SCR register must be set (one) to enable the MSIU. The MSIU needs the timer0timer0 clock (CLK_SEN)
for starting the motion measurements (starting the state machine) periodically, i.e., timer0 has to be activated and
TOBS[2..0] bits in the T0CR register must be set so that CLKSEN is directed to the motion sensor interface (see also
Section 3.13.3 “Timer0 with Watchdog/Interval Timer” on page 62ff). Before changing the T0CR register the MSIU needs to
be disabled. After the T0CR register is set the motion sensor interface can be enabled again. In that way a safe restart of the
state machine of MSIU is guaranteed.
The capacitance of the external motion sensor C_M (at pin S2) together with the internal capacitance C_Ref forms a capacitive
voltage divider (see Figure 3-57 on page 102). If switch Sw1 is closed while controlled by the state machine, a voltage VSEN
is generated which is a function of C_M.
V_SEN
=
--------C----^R---^e--^f--------
C_Ref + C_M
×
V_CC
V_CC is the supply voltage of the ATA6289.
If the application starts to rotate, the capacitance of the motion/acceleration sensor increases, while at the same time the
sensor voltage V_SEN decreases. A low-offset-auto-zero comparator stage compares the sensor input voltage on pin S2 with
the internally calibrated reference voltage (V_Ref). If the sensor voltage on pin S2 is lower than the internal reference voltage
V_Ref,, the MSENOS signal goes to high and the MSENO bit in the SSFR register is set. By polling the MSENO bit the
ATA6289 can be woken up if this bit is set.
ATA6286C [DATASHEET] 101
9308C–RFID–09/14