AT42QT1110-MU View Datasheet(PDF) - Atmel Corporation
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AT42QT1110-MU Datasheet PDF : 50 Pages
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4. Detailed Operations
4.1 Communications
4.1.1
Introduction
All communication with the device is carried out over the Serial Peripheral Interface (SPI). This is a synchronous
serial data link that operates in full-duplex mode. The host communicates with the QT controller over the SPI using a
master-slave relationship, with the QT1110 acting in slave mode.
4.1.2
SPI Operation
The SPI uses four logic signals:
Serial Clock (SCK) – output from the host.
Master Output, Slave Input (MOSI) – output from the host, input to the QT controller. Used by the host to send
data to the QT controller.
Master Input, Slave Output (MISO) – input to the host, output from the QT controller. Used by the QT device to
send data to the host.
Slave Select (SS) – active low output from the host.
At each byte, the master pulls SS low and generates 8 clock pulses on SCK. With these 8 clock pulses, a byte of
data is transmitted from the master to the slave over MOSI, most significant bit (msb) first.
Simultaneously a byte of data is transmitted from the slave to the master over MISO, also most significant bit first.
The slave reads the status of MOSI at the leading edge of each clock pulse, and the master reads the slave data
from MISO at the trailing edge.
The QT1110 requires that the clock idles “high”, meaning that the data on MOSI and MISO pins are set at the falling
edges and sampled at the rising edges.
That is:
Clock polarity CPOL = 1
Clock phase CPHA = 1
The QT1110 SPI interface can operate at any SCK frequency up to 1.5 MHz.
In multibyte communications, the master must pause for a minimum delay of 150 µs between the completion of one
byte exchange and the beginning of the next.
Note that the number of bytes to be transmitted depends on the initial command sent by the host. This sets the mode
on the QT1110 so that the QT1110 knows how to respond to, or how to interpret, the following bytes. If there is a
delay of >100 ms between bytes while the QT1110 is waiting for data, or waiting to send data, then the incomplete
transmission is discarded and the device resets its SPI state machine. It will then interpret the next byte it receives as
a fresh command.
When the QT1110 SPI interface is receiving a new command, it returns the Idle status code (0x55) on MISO during
the first byte exchange to indicate to the master that it is in the correct state for receiving instructions.
4.1.3
CRC Bytes
If enabled, a CRC checking procedure is implemented on all communications between the SPI master and the
QT1110. In this case, each command or report request sent by the master must have a byte appended containing
the CRC checksum of the data sent. The QT1110 will not respond to commands until the CRC byte has been
received and verified.
Sample C code showing the algorithm for calculating the CRC of the data can be found in Appendix A..
AT42QT1110-MU / AT42QT1110-AU [DATASHEET] 10
9520J–AT42–05/2013
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