© Quantum Research Group Ltd.
Bypass capacitors and series resistors can be used to
prevent these effects as shown in Figures 4-4 and 4-5.
3.22 ESD / Noise Considerations
In general the QT60xx5 will be well protected from static
discharge during use by the overlying panel. However, even
with a dielectric panel transients currents can still flow into
scan lines via induction or in extreme cases, dielectric
breakdown. Porous or cracked materials may allow a spark to
tunnel through the panel. In all cases, testing is required to
reveal any potential problems. The devices have diode
protected pins which can absorb and protect the device from
most induced discharges, up to 5mA.
4 Serial Interface
QT60xx5 devices use an SPI serial interface to a host MCU.
This port uses a protocol described in Section 5.
4.1 Serial Port specifications
QT60xx5's use an SPI synchronous serial interface with the
following specifications at 6MHz oscillator frequency:
The X lines are not usually at risk during operation, since they
are low-resistance output drives. The YCn lines are not
directly connected to the matrix and so are not at risk.
However the PLD and the QS3251 are connected to the Y
lines and may require additional ESD protection.
Max clock rate, Fck
Data length
Host command space, Tcm
Response delay to host, Tdr1
Drdy delay from response, Tdr2
Multi-byte return spacing, Tdr3
1.5MHz
8 bits
m 50µs
Table 4-1, also, Sec. 7
1µs to 1ms
15µs to 2ms
Diode clamps can be used on the X and Y matrix lines. The
diodes should be high speed / high current types such as
BAV99 dual diodes, connected from Vdd to Vss with the
diode junction connected to the matrix pin.
The host can clock the SPI at any rate up to and including the
maximum clock rate Fck. The maximum clock rate of the part
in Master mode is determined by Setup ^Q.
Capacitors placed on the X and Y matrix lines can also help
to a limited degree by absorbing ESD transients and lowering
induced voltages. Values up to 100pF can be used without
causing circuit problems.
The part can operate in either master-slave mode or
slave-only mode, and is thus compatible with virtually all
SPI-capable microcontrollers.
The circuit can be further protected by inserting series
resistors into the X and/or Y lines to limit peak transient
current. Values up to 500 ohms can be used in most cases,
but if the dwell time is short this resistance can cause a
reduction in signal gain. RC networks as shown in Figures
4-4 and 4-5 can provide enhanced protection against ESD
while also limiting the effects of external fields.
The SPI interface should not be used over long distances due
to problems with signal ringing and introduced noise etc.
unless suitably buffered or filtered with RC networks as
shown in Figures 4-4 and 4-5. Slower data rates with longer
RC timeconstants will provide enhanced resistance to noise
and ringing problems.
Conversion to asynchronous UART format can be
accomplished by using a microcontroller with conversion
firmware. Using such a conversion device the part can
communicate with Quantum's QmBtn PC software. Consult
Quantum for details.
External field interference can occur in some cases; these
problems are highly dependent on the interfering frequency
and the manner of coupling into the circuit. PCB layout
(Section 3.21) and external wiring should be carefully
designed to reduce the probability of these effects occurring.
Of particular note is the length of the connection from the
circuit to the key panel. This connection will act as an
antenna that will resonate at various radio frequencies to
cause interference, and thus should be very short. If RFI
pickup is a problem, the connections should be damped
using ferrite beads or low-value (22 - 100 ohm) series
resistors in all lines including any ground and power lines
running in parallel to the panel.
4.2 Protocol Overview
The SPI protocol is based entirely on polled data
transmission, that is, the part will not send data to the host of
its own volition but will do so only in response to specific
commands from the host.
Run-time data responses, such as key detection or error
information, requires simple single-byte functions to evoke a
response from the part.
SPI data noise: In some applications the host MCU can be
some distance from the sensor, with the interface coupled via
ribbon cable. The SPI link is particularly vulnerable to noise
injection on these lines; corrupted or false commands can be
induced from transients on the power supply or ground wiring.
Setup mode interactions mostly use 2-byte functions from the
host to cause the part to alter its behavior; these functions
also cause writes to the internal eeprom.
The concept of 'scope' is used to allow functions to operate
on individual keys or groupings of keys. The scope of
subsequent functions can be altered by short initial scope
instructions.
Figure 4-1 SPI Connections
Slave-Only
Master-Slave
See Section 5 for protocol details.
Host MCU
QT60xx5
Host MCU
QT60xx5
4.3 SPI Slave-Only Mode
DRDY
SS
P_IN
P_OUT
SCK
DRDY
SS
Refer to Figures 4-1 and 4-2. Select Slave-only by
floating Pin 37 (MS) or tying high via a m10K resistor. Pin
37 also functions as an oscilloscope sync output (Section
3.20) and should never be tied directly to a supply rail. In
Slave mode the host must always be in Master mode, as
it controls all SPI activity including clocking of the
SS
SCK
SCK
MISO
MOSI
MS
SCK
MISO
MOSI
MS
MISO
MOSI
SS
MISO
MOSI
Vdd
10K
lQ
16
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