LM8322 datasheet(15/42 Pages) NSC

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During a scan cycle, only one KP-Yx output pin will be driven

low at any time, while the others are driven high or undriven.

At the time scale used in Figure 8, the low phase of a KP-Yx

output during a scan cycle is not visible. The KP-Xx input pins

are pulled high by weak pullups.

There are capacitive loads on the KP-Xx inputs and KP-Yx

outputs due to protection circuits, wiring, etc. The LM8322 in-

serts delays to allow complete charging or discharging of

these loads before sampling the input levels on the KP-Xx

inputs. The maximum parasitic load capacitance on the KP-

Xx inputs is 5 nF.

After detecting a key-press or key-release event, the de-

bounce time specified by the SET_DEBOUNCE command

(0x8F) sets the minimum time for confirming the event before

the IRQ output is asserted.

If more than two keys are pressed simultaneously, the pattern

of key closures may be ambiguous, in which case the the in-

terrupt code indicates an error and the IRQ output is asserted

(if enabled).

The SF keys connect KP-Xx inputs directly to ground. There

can be up to eight SF-keys. If any of these keys are pressed,

other keys that use the same KP-Xx pin are ignored.

12.2.1 Timing Parameters

Two timing parameters affect scanning of the keypad:

•

Debounce Time — minimum delay between detecting a

keypad event and confirming the event before asserting

IRQ. The default debounce time is 3 scan cycles (about

12 milliseconds), but the host can set values in the range

1–255 cycles (4–1020 milliseconds).

•

Active Time — period without detecting a state change in

the keypad that triggers entry into Halt mode, during which

keypad scanning is suspended. The default active time is

500 milliseconds, but the host can set it values in the range

4–1020 milliseconds. The active time must be greater than

the debounce time.

12.2.2 Multiple Key Pressings

If more than two keys are pressed at the same time, the

LM8322 stores all key pressed and released events in the

FIFO buffer in the sequence in which they were decoded.

For multiple key pressings the following circumstances have

to be respected:

•

A multiple key-press event is given if two or more key-

press events are reported but no corresponding key-

release event.

•

With the activity time set between the minimum and

maximum time (4 msec to 1 second) it is not safe to detect

two simultaneous key pressings in one input row (see

Figure 9 on the left hand side.)

•

If all key pressings (two or more) are located in different

input rows (see Figure 9 on the right hand side) then the

key pressed events will be correctly found in the FIFO

buffer without any restriction.

30013609

FIGURE 9. Simultaneous Keys Pressed

•

In order to securely detect and store the key codes of

simultaneous key pressings in the same input row the

following precautions must be taken from the host side:

“As soon as the host device has detected a key pressed event

the host must send the SET_ACTIVE Command with the pa-

rameter set to “00”. This will prevent the LM8322 from enter-

ing HALT mode. If all keyboard events are resolved (no

remaining key pressed status in the LM8322 anymore) then

the host must send the SET_ACTIVE Command again with

the parameter setting the desired duration for the active time.

This will enable the LM8322 to enter low power HALT mode

once the activity time has passed without detecting any

events.

•

Once one or more key (pressed and/or released) events

have been read from the host with the help of the READ

FIFO command there are two conditions cleaning the

FIFO buffer contents:

— A second execution of the READ FIFO Command or,

— A new key event detected from the LM8322.

12.3 EXAMPLE KEYPAD CONFIGURATION

Figure 10 shows an 8 × 4 keypad matrix. This configuration

occupies all scanning inputs (KP-X0 through KP-X7) and four

scanning outputs (KP-Y0 through KP-Y3). The remaining

scanning outputs KP-Y4 through KP-Y11 are available for use

as GPIO pins.

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