ADP5586ACBZ-00-R7 datasheet(12/44 Pages) AD

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ADP5586

Data Sheet

Rev. 0 | Page 12 of 44

The EVENT_INT interrupt (Register 0x01, Bit 0) can be triggered

by both press and release key events. As shown in Figure 12, if

Key 3 is pressed, EVENT_INT is asserted, EC[4:0] is updated,

and the FIFO is updated. During the time that the key remains

pressed, it is possible for the FIFO to be read, the event counter

decremented to 0, and EVENT_INT cleared. When the key is

finally released, EVENT_INT is asserted, the event counter is

incremented, and the FIFO is updated with the release event

information.

KEY 3

KEY 3 PRESS

KEY 32 RELEASE

KEY SCAN

EVENT_INT

EVENT_INT CLEARED

EC[4:0]

FIFO

READ

FIFO

Figure 12. Asserting the EVENT_INT Interrupt Keypad Extension

As shown in Figure 10, the keypad can be extended if each row

is connected directly to ground by a switch. If the switch placed

between R0 and ground is pressed, the entire row is grounded.

When the key scanner completes scanning, it normally detects

Key 1 to Key 5 as being pressed; however, this unique condition

is decoded by the ADP5586, and Key Event 31 is assigned to it.

Up to five more key event assignments are possible, allowing the

keypad size to extend up to 30. However, if one of the extended

keys is pressed, none of the keys on that row is detectable. The

activation of a ground key causes all other keys sharing that row

to be undetectable.

Precharge Time

During a scan sequence, a row scans through the columns

sequentially. Each row/column combination is tested at a rate

that is defined by the KEY_POLL_TIME bits (Register 0x39,

Bits[1:0]). Within each of these scan times, each column is

scanned for a time defined by the PRECHARGE_TIME bit

(Register 0x39, Bit 3). As shown in Figure 13, the resistance

capacitance (RC) time constant, which is defined by the series

resistance (from pull-up/pull-down, for example) and parallel

capacitance that is seen on the individual columns, affects the

sampling of a key press event.

R1 SCAN

ACTIVE

KEY 8 (R1, C2) SAMPLED

PRECHARGE TIME

KEY 9 (R1, C3) SAMPLED

VC2

VC3

Figure 13. Precharge Time

The ADP5586 samples the state of the row/column pairs near

the end of the precharge time. By extending this time, higher RC

time constants can be accommodated. For applications that use

physical buttons, the RC time constant is usually not an issue, but

if external relay switches or multiple external muxes are attached to

columns, the RC constant may increase. Using a smaller pull-up

resistor on the rows (Register 0x3C, Bit 7) reduces the RC time

constant.

Ghosting

Ghosting is an occurrence where, given certain key press

combinations on a keypad matrix, a false positive reading of

an additional key is detected. Ghosting is created when three

or more keys are pressed simultaneously on multiple rows or

columns (see Figure 14). Key combinations that form a right

angle on the keypad matrix may cause ghosting.

COL0

ROW0

ROW1

ROW2

ROW3

PRESS

GHOST

PRESS

COL1

COL2

Figure 14. Ghosting Example: Column 0/Row 3 is a Ghost Key Due to a Short

Among Row 0, Column 0, Column 2, and Row 3 During Key Press

The solution to ghosting is to select a keypad matrix layout that

takes into account three key combinations that are most likely

to be pressed together. Multiple keys that are pressed across one

row or across one column do not cause ghosting. Staggering

keys so that they do not share a column also avoids ghosting.

The most common practice is to place keys in the same row or

column that are likely to be pressed at the same time. Some

examples of keys that are likely to be pressed at the same time

are as follows:

•

The navigation keys in combination with the Select key

•

The navigation keys in combination with the space bar

•

The reset combination keys, such as CTRL + ALT + DEL

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