at42qt2100 ATMEL Corporation, at42qt2100 Datasheet - Page 12

• Manufacturer Part Number: at42qt2100
• Description: Qtouch Touch Sensor Ic
• Manufacturer: ATMEL Corporation

4.2
4.2.1
4.2.2
12
Communications
AT42QT2100
Introduction
Change Pin
The QT2100 communicates as a slave device over a full-duplex 4-wire (MISO, MOSI, SCK,
SS) SPI interface. In addition there is a DRDY pin which indicates when the QT2100 is
carrying out acquisition bursts and a CHANGE pin which is asserted when a change occurs in
the status of the touch sensors (see
Table 4-1.
See
The host must always transfer three bytes in succession within the allotted time (10 ms
maximum). If all bytes are not received in this interval it is treated by the QT2100 as an error.
In this case the exchange is reset and the next read will contain the first data byte of a new
exchange.
Messages from the host to the QT2100 carry configuration information; return data from the
QT2100 carries key state information. For details of the message contents see
page
Figure 6-1
the clocking and controls the timing of the transfers from the QT2100. Transfers are always
clocked as a set of three bytes, Byte 1, 2 and 3.
DRDY stays high for 500 µs. It falls again after Byte 3 has shifted to indicate completion.
DRDY goes high after each burst.
After the host asserts SS low, it should wait >22 µs in low power mode before starting SCK; in
Free run mode, a delay of 2 µs is sufficient. The QT2100 reads the MOSI pin with each rising
edge of SCK, and shifts data out on the MISO pin on falling edges. The host should do the
same to ensure proper operation.
Between the end of the Byte 1 shift and the start of the Byte 2 shift (and between Byte 2 and
Byte 3), the host may raise SS again, but this is not required. SS should be held high when not
communicating; if SS is low this is taken as an indication of impending communications.
In this case, extra current is drawn, as the QT2100 does not enter its lowest power sleep
mode.
All timings not mentioned above should be as in
The QT2100 has a CHANGE output pin which allows for key state change notification. Use of
the CHANGE signal relieves the host of the burden of regularly polling the QT2100 to get key
states. CHANGE goes high when an event occurs that causes a change to the contents of the
“Normal Data” bytes; that is, when a new key is pressed, or released, or a movement is
detected on the slider/wheel.
Pin
DRDY
CHANGE
Section 6.5 on page 31
20.
and
Additional Pins
Figure 6-2 on page 32
for details of the SPI Configuration and Timing Parameters.
High
Burst is complete
New touch data
Table
show the basic timing for SPI operation. The host does
4-1).
Figure 6-2 on page
Burst is active
Low
Latest data has already been
read by host
32.
9554CX–AT42–11/11
Section 5 on