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AT42QT1111-MU Datasheet(PDF) 10 Page - ATMEL Corporation |
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AT42QT1111-MU Datasheet(HTML) 10 Page - ATMEL Corporation |
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10 / 50 page  10 9571A–AT42–02/10 AT42QT1111-MU/AT42QT1111-AU 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 QT1111 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’s data from MISO at the trailing edge. The QT1111 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 QT1111 SPI interface can operate at any SCK frequency up to 750 KHz. In multibyte communications, the master must pause for a minimum delay of 300 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 QT1111 so that the QT1111 knows how to respond to, or how to interpret, the following bytes. If there is a delay of >100 ms between bytes while the QT1111 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 QT1111 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. |
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