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 INT1:0 Input Circuitry. 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 inter-
rupt request flag IEn in TCON register. If the interrupt is edge-triggered, the request flag
is cleared by hardware 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 de-asserted 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 21.
Figure 29. INT1:0 Input Circuitry
INT0/1
Interrupt
Request
0
1
INT0/1
IE0/1
TCON.1/3
EX0/1
IEN0.0/2
IT0/1
TCON.0/2
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 239.
Input Sampling
External interrupt pins (INT1:0 and KIN3:0) are sampled once per peripheral cycle (6
peripheral clock periods) (see Minimum Pulse Timings). 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 30. Minimum Pulse Timings
Level-Triggered Interrupt
> 1 Peripheral Cycle
1 cycle
Edge-Triggered Interrupt
> 1 Peripheral Cycle
1 cycle
1 cycle
58
AT85C51SND3Bx
7632A–MP3–03/06
ATMEL [ ATMEL ]
