at42qt1481 ATMEL Corporation, at42qt1481 Datasheet - Page 20

• Manufacturer Part Number: at42qt1481
• Description: 48-key Qmatrix Ic
• Manufacturer: ATMEL Corporation

3. Serial Communications
3.1
3.2
20
Introduction
DRDY Pin
AT42QT1481
The QT1481 uses either SPI or UART communications modes; it cannot use both at the same
time. The QT1481 responds on whichever interface it receives a command. The QT1481 also
includes a Debug output interface, which can be used to monitor many operating variables
during product development.
The host device always initiates communications sequences; the QT1481 is incapable of
chattering data back to the host. This is intentional for FMEA and EN60730 purposes so that the
host always has total control over the communications with the QT1481.
A command from the host always ends in a response of some kind from the QT1481. Some
transmission types from the host or the QT1481 employ a CRC check byte to provide for robust
communications.
A DRDY line that handshakes transmissions is provided. This is needed by the host from the
QT1481 to ensure that transmissions are not sent when the QT1481 is busy or has not yet
processed a prior command. In UART mode this line is bidirectional, and the QT1481 can use it
to suspend transmissions back to the host if the host is busy.
If the host does not observe the correct DRDY timing, random communication errors may result.
Initiating or Resetting Communications: After a reset, or should communications be lost due
to noise or out-of-sequence reception, the host should repeatedly wait for a period not less than
the QT1481 communications time-out (110 ms ±5 ms), and send a 0x0F (return last command)
command until the complement of 0x0F, which is 0xF0, is received. Then, the host can resume
normal run mode communications from a clean start.
Poll rate: The typical poll rate in normal run operation should be no faster than once per 10 ms.
Even 50 ms is more than fast enough to extract status data using the 0x06 command (report first
key (see
command (dump setups (see
0x07 can and should pace at the maximum possible rate.
Run Poll Sequence: In normal run mode the host should limit traffic with a minimalist control
structure (see
something requires a deeper state inspection. If there is more than one key in detect, the host
should use 0x07 to find which additional keys are in detect. If there is an error, the host should
ascertain the error type based on command 0x0B and take appropriate action.
DRDY is an open-drain output (in SPI mode) or bidirectional pin (in UART mode) with an internal
20 k – 50 k pull-up resistor.
Most communications failures are the result of failure to properly observe the DRDY timing.
• In SPI mode the QT1481 is a slave, so that even return data following a command is
• In UART mode, the QT1481 still only responds to the host after a command, but the
controlled by the host.
responses are not controlled by the host.
Section 4.7 on page
Section 4.19 on page
Section 4.10 on page
28)) in most situations. Streaming commands like the 0x0D
33). The host should just send a 0x06 command until
31)) or multibyte response commands like
9621B–AT42–06/11