Rainbow Electronics MAX11042 - Manual (Page 8)

MAX11041/MAX11042
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
V INT
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 0 0 0 0 0 0 0
MAX11042 0 0 0 0 0 0 0 1
Table 3. Chip ID Data Format