4.2.3 Event Packet Structure

Every qualified event detected by the capacitive sensing input engine generates a single packet that can be retrieved by the host pro-

cessor through the event buffer I2C interface. The packet is an atomic data unit that fully describes the generated event.

Note: The bytes in the packet are transmitted MSB first.

Each packet has a standard structure that can be parsed by the host.

Table 4.1. Standard Packet Structure

Byte #

Designator

0

I2C Slave Address + read bit

1

Packet counter and event type

2

Event description (byte 1)

3

Event description (byte 2)

The packet counter is a 4-bit number stored in the upper bits of byte 1. Each new event will be assigned a counter value that is +1 from

the last qualified event. After event 15, the counter wraps back to 0 for the next event. The counter captures the temporal nature of

touch events so that a host can reconstruct a sequence of events over time. Also, the host can use the counter value to determine if a

packet has been lost due to a buffer overflow.

The event type is a 4-bit value describes the originator of the event. For instance, the source could be a capacitive sensing button. The

event type is stored in the lower 4 bits of byte 1.

The event description bytes define characteristics of the event that have been qualified. Event descriptions are defined relative to the

event source. An event source that is a capacitive sensing input will have a defined set of valid event description values. Those same

values will mean something different for a different type of event source, such as a slider. Event description values are defined relative

to the event type field of byte 1.

Touch

Event

I2C Slave Address

+ read bit

CSxx index

reserved

byte 0

byte 1

byte 2

byte 3

Touch

Release

Event

I2C Slave Address

+ read bit

CSxx index

reserved

byte 0

byte 1

byte 2

byte 3

Slider

Event

I2C Slave Address

+ read bit

slider position

(MSB)

slider position

(LSB)

byte 0

byte 1

byte 2

byte 3

packet

counter

0010

event

type

0000

packet

counter

0011

event

type

0001

packet

counter

0100

event

type

0010

Figure 4.5. I2C Event Buffer Packet Structure

CPT112S Data Sheet

Functional Description

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