Keypress Detection and Debounce

At power-up, the MAX11041/MAX11042 begin to moni-

tor the SENSE input for keypresses. When the

MAX11041/MAX11042 detect a keypress at SENSE,

they attempt to debounce the SENSE input. After suc-

cessful debouncing of the input, the corresponding

keypress result is inserted into the FIFO. In addition,

INT goes low to signal a keypress to the µP.

Keypress FIFO and Time Duration

After detecting and debouncing a key, the decoded

key is stored in one byte of the 8-word FIFO. A 7-bit

internal timer starts counting the duration of the key-

press (one count = 32ms) and the result is stored after

each increment in another byte of the 8-word FIFO. The

8th bit in the time duration byte is an overflow bit that

is set when the count reaches 128. After the count

reaches 128, the 7-bit timer rolls over to 0 and contin-

ues to count while the 8th bit becomes set and stays

set until the associated FIFO entry is cleared. For key-

press durations longer than 8.16s, see the

Extended

Keypresses section.

When the device detects another change in resistance

at SENSE (either by key release or another keypress),

the count resets and the FIFO begin recording the next

keypress/duration. This allows the 8-word FIFO to store

time duration and key-type information for up to four

keypresses and releases. When the FIFO is full and a

key is pressed, the oldest keypress information in the

FIFO is written over. Writing to the power-down bit (bit

6) in the control register or bringing SHDN low clears

the FIFO to its power-on-reset (POR) state.

Wired Remote Controllers

8

_______________________________________________________________________________________

➂

➁

➀

➃

1. DEBOUNCED KEYPRESS STORED IN FIFO AND INT GOES LOW, DURATION

TIMER STARTS.

2. PROCESSOR READS FIFO AND INT GOES HIGH. KEY TYPE AND CURRENT

KEYPRESS DURATION TIME SENT. FIFO IS NOT CLEARED.

3. KEYPRESS RELEASES AND INT GOES LOW. KEY TYPE AND FINAL KEYPRESS

DURATION TIME STORED IN FIFO.

4. PROCESSOR READS THE FIFO AND INT GOES HIGH. KEYPRESS INFORMATION

STORED IN FIFO FROM STEP 3 IS CLEARED.

KEY TYPE

TIME

TIME

Figure 3. Reading the FIFO While the Key is Still Pressed

➂

➁

➀

TIME

TIME

VINT

1. DEBOUNCED KEYPRESS STORED IN FIFO AND INT GOES LOW.

DURATION TIMER STARTS.

2. KEYPRESS RELEASES. KEY TYPE AND KEYPRESS TIME

DURATION INFORMATION STORED IN FIFO.

3. PROCESSOR READS FIFO COMPLETELY AND INT GOES HIGH.

PREVIOUS KEYPRESS INFORMATION CLEARED.

KEY TYPE

Figure 4. Reading the FIFO After the Key is Released

BIT NAMES

CHIP ID

I7

I6

I5

I4

I3

I2

I1

I0

MAX11041

0

0000

000

MAX11042

0

0000

001

Table 3. Chip ID Data Format