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External interrupts – Rainbow Electronics AT89C5132 User Manual

Page 39

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39

AT8xC5132

4173A–8051–08/02

External Interrupts

INT1:0# Inputs

External interrupts INT0 and INT1 (INTn, n = 0 or 1) pins may each be programmed to
be level-triggered or edge-triggered, dependent upon Bits IT0 and IT1 (ITn, n = 0 or 1) in
TCON register as shown in Figure 22. If ITn = 0, INTn is triggered by a low level at the
pin. If ITn = 1, INTn is negative-edge triggered. External interrupts are enabled with Bits
EX0 and EX1 (EXn, n = 0 or 1) in IEN0. Events on INTn set the interrupt request flag IEn
in TCON register. If the interrupt is edge-triggered, the request flag is cleared by hard-
ware when vectoring to the interrupt service routine. If the interrupt is level-triggered, the
interrupt service routine must clear the request flag and the interrupt must be deas-
serted before the end of the interrupt service routine.

INT0 and INT1 inputs provide both the capability to exit from Power-down mode on low
level signals as detailed in Section “Exiting Power-down Mode”, page 48.

Figure 22. INT1:0# Input Circuitry

KIN3:0 Inputs

External interrupts KIN0 to KIN3 provide the capability to connect a matrix keyboard. For
detailed information on these inputs, refer to Section “Keyboard Interface”, page 134.

Input Sampling

External interrupt pins (INT1:0 and KIN3:0) are sampled once per peripheral cycle (6
peripheral clock periods) (see Figure 23). A level-triggered interrupt pin held low or high
for more than 6 peripheral clock periods (12 oscillator in standard mode or 6 oscillator
clock periods in X2 mode) guarantees detection. Edge-triggered external interrupts
must hold the request pin low for at least 6 peripheral clock periods.

Figure 23. Minimum Pulse Timings

0

1

INT0/1#

IT0/1

TCON.0/2

EX0/1

IEN0.0/2

INT0/1#

Interrupt

Request

IE0/1

TCON.1/3

Edge-Triggered Interrupt

Level-Triggered Interrupt

1 cycle

1 cycle

>

1 peripheral cycle

1 cycle

>

1 peripheral cycle