2.8 Fast Detect Mode

In many applications, it is desirable to sense touch at high

speed. Examples include scrolling ‘slider’ strips or ‘Off’

buttons. It is possible to place the device into a ‘Fast Detect’

mode that usually requires under 15 ms to respond. This is

accomplished internally by setting the Detect Integrator to

only two counts, i.e. only two successive detections are

required to detect touch.

In LP mode, ‘Fast’ detection will not speed up the initial

delay (which could be up to 360ms typical depending on the

option setting). However, once a key is detected the device

is forced back into normal speed mode . It will remain in this

faster mode until another LP pulse is received.

When used in a ‘slider’ application, it is normally desirable to

run the keys without AKS.

In both normal and ‘Fast’ modes, the time required to

process a key release is the same: it takes six sequential

confirmations of non-detection to turn a key off.

Fast Detect mode can be enabled as shown in Tables 1.2

and 1.6.

2.9 Simplified Mode

A simplified operating mode which does not require the

majority of option resistors is available. This mode is set by

connecting a resistor labeled SMR between pins SNS6K and

SNS7. (see Figure 1.2).

In this mode there is only one option available - AKS enable

or disable. When AKS is disabled, Fast Detect mode is

enabled; when AKS is enabled, Fast Detect mode is off.

AKS in this mode is global only (i.e. operates across all

functioning keys).

The other option features are fixed as follows:

DETECT Pin: Push-pull, active high

SYNC/LP Function: LP mode, ~200ms response time

Max On-Duration: 60 seconds

See also Tables 1.6 and 1.7.

2.10 Unused Keys

Unused keys should be disabled by removing the

corresponding Cs, Rs, and Rsns components and

connecting SNS pins as shown in the ‘Unused’ column of

Table 1.1. Unused keys are ignored and do not factor into

the AKS function (Section 2.6).

2.11 Serial 1W Interface

The 1W serial interface is an RS-232 based auto baud rate

serial asynchronous interface that requires only one wire

between the host MCU and the QT1101. The serial data are

extremely short and simple to interpret.

Auto baud rate detection takes place by having the host

device send a specific character to the QT 1101, which

allows the QT1101 to set its baud rate to match that of the

host.

One feature of this method is that the baud rate can be any

rate between 8,000 and 38,400 bits per second. Neither the

QT1101 nor the host device has to be accurate in their

transmission rates, i.e. crystal control is not required.

Depending on the timing of a 1W host transmission, the

QT1101 device may need to abort an acquisition burst, and

rerun it after the transmission is complete and a reply has

been sent. As a consequence, each host request can

potentially result in a small, unnoticeable increase in

detection delay.

1W Connection: The 1W pin should be pulled high with a

resistor. When not in use it floats high, hence this causes no

increase in supply current.

During transmission from the host, the host may drive the

1W line with either an open-drain or a push-pull driver.

However, if the host uses push-pull driving, it must release

the 1W line as soon as it is done with its stop bit so that

there is no drive conflict when the QT1101 sends its reply.

If open-drain transmission is used by the host, the value of

the pull-up resistor should be optimized for the desired baud

rate: faster rates require a lower value of resistor to prevent

rise-time problems. A typical value for 19,200 baud might be

100K

✡. An oscilloscope should be used to confirm that the

resistor is not causing excessive timing skew that might

cause bit errors.

The QT1101 uses push-pull drive to transmit

data out on the 1W line back to the host. When

the stop bit level is established, 1W is floated;

for this reason, a pull-up resistor should always

be used on the 1W pin to prevent the signal from

drifting to an undefined state. A 100K

✡ pull-up

resistor on 1W is recommended, unless the host

uses open-drain drive to the QT1101, in which

case a lower value may be required (see prior

paragraph).

2.11.1 Basic 1W Operation

The basic sequence of 1W serial operation is

shown in Figure 2.1. The 1W line is bi-directional

and must be pulled high with a resistor to

prevent a floating, undefined state (see previous

section).

7

QT1101 R4.06/0806

Figure 2.1 Basic 1W Sequence

*See Figure 2.3

Figure 2.2 1W UART Host Pattern

1 ~ 3 bit periods

1W

/CHANGE

key state

change

request

from host

(1 byte)

driven reply

from QT1101

(2 bytes)*

floating

floating

floating

floating

S 0 1234

7 S

Serial bits

56

1W

(from host)