AT42QT1245_14 Datasheet, PDF (18/59 Pages) ATMEL Corporation – 24-key QMatrix FMEA IEC/EN/UL60730 Touch Sensor

Serial communications pacing is controlled by DRDY. Use of DRDY is critical to successful communications with the
QT1245. The host is permitted to perform an SPI transfer only when DRDY has returned high. After each SPI byte
transfer DRDY goes low after a short delay and remains low until the QT1245 is ready for another transfer. A short
delay occurs before DRDY is driven low because the QT1245 may be otherwise busy and requires a finite time to
respond. DRDY may go low only for a few microseconds. During the period from the end of one transfer until DRDY
goes low and back high again, the host should not perform another transfer. Therefore, before each byte
transmission the host should first check that DRDY is high again.
Figure 4-1. SPI Slave-only Mode Timing
DRDY
(from QT)
high via pull-up resistor
S1
S6
S5
SS
(from Host)
S3
S9
CLK
(from Host)
MOSI
(Data from Host)
Data shifts into QT on rising edge
?
765 43 21 0
S2 ^{(header byte)}
S4
MISO 3-state
(Data from QT)
Data shifts out of QT on falling edge
?7 6 5 4 3 2 1 0
3-state
S7
S8
765 43 21 0
(header byte)
?7 6 5 4 3 2 1 0
765 43 21 0
(Null byte to get QT response)
?7 6 5 4 3 2 1 0
data response
S1: > 333 ns ⁽¹⁾
S5:  40 µs
S2:  20 ns
S6:  1 µs
S3:  25 ns
S7:  333 ns
S4:  20 ns
S8:  333 ns
S9:  667 ns
1.
Or > 3 µs if sleep is enabled
If the host wants to perform a byte transfer with the QT1245 it should behave as follows:
1. Wait at least 40 µs after the previous SPI transfer (time S5 in Figure 4-1: DRDY is guaranteed to go low before
this 40 µs expires).
2. Wait until DRDY is high (it may already be high again).
3. Perform the next SPI transfer with the QT1245.
The time it takes for DRDY to go high again after each transfer depends if the host is performing a setups write, or is
performing a read, as follows:
Setups write: 4.5 ms
Read:
1 ms
The DRDY times above are valid when the maximum operating frequency (FREQ0 = 1) is used. These times
increase as the operating frequency is reduced. With very low operating frequency add 5 ms to the above times.
Other DRDY specifications:
QT1245 [PRELIMINARY DATASHEET]
18
9632EX–AT42–12/13