10 I/O PORTS (P)
S1C17704 TECHNICAL MANUAL
EPSON
10-25
10.9 Precautions
Operating clock
• Supply PCLK from the clock generator before the I/O ports can be accessed.
Furthermore, a prescaler output clock is required for operating the P0 port chattering filter. Turn the prescaler
on when using the chattering filter.
Pull-up resistor
• When changing the port pin from low level to high level with the built-in pull-up resistor, a delay in the
waveform rise time will occur depending on the time constant of the pull-up resistor and the load capacitance
of the pin. Therefore it is necessary to set an appropriate wait time for reading an I/O port. This wait time
should be the amount of time or more calculated by the following expression.
Wait time = R
IN
×
(C
IN
+ load capacitance on the board)
×
1.6 [seconds]
R
IN
: Pull-up resistance Max. value
C
IN
: Pin capacitance Max. value
• For unused ports, enable the pull-up resistors.
P0 and P1 port interrupt
• To avoid occurrence of unnecessary interrupts, be sure to reset the interrupt flags P0IF[7:0] (0x5207) or
P1IF[7:0] (0x5217) corresponding to the ports used to generate an interrupt before the interrupt is enabled
using the P0_IMSK register (0x5205) or P1_IMSK register (0x5215).
• Set the P0 and P1 interrupt trigger mode in the ITC to level trigger.
After an interrupt occurs, reset the interrupt flag in the P0IF[7:0] (0x5207) or P1IF[7:0] (0x5217) of the P
port module in the interrupt handler routine (this also resets the interrupt flag in the ITC).
P0 port chattering filter
• Input interrupts cannot be accepted in SLEEP mode if the CPU enters SLEEP mode when the chattering filter
is active. The chattering filter should be disabled (off) before executing the
slp
instruction.
• Be sure to disable the P0 port interrupt before changing the P0_CHAT register (0x5208). Unnecessary
interrupt may occur if the register is changed when the P0 port interrupt has been enabled.
• The check time to eliminate chattering means the maximum pulse width that can be eliminated. The valid
interrupt input needs a pulse width of the set check time (minimum) to twice that of the check time (maximum).
• The internal signal may oscillate if the rise/fall time of the input signal is too long because the input
signal level transition to the threshold level duration of time is too long. This causes the input interrupt to
malfunction, therefore setup the input signal so that the rise/fall time is 25 ns or less.
P0 port key-entry reset
• When using the P0 port key-entry reset function, make sure that the designated input ports will not be
simultaneously set to low level while the application program is running.
• The P0 port key-entry reset function cannot be used for power-on reset as it must be enabled with software.
• The P0 port key-entry reset function cannot be used in SLEEP mode.
Содержание S1C17704
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