Rainbow Electronics AT89C5122 - Manual (Page 159)

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AT8xC5122/23
4202E–SCR–06/06
Interrupt System
Introduction The AT8xC5122/23 implements an interrupt controller with 15 inputs but only 9 are used
for :
– two external interrupts (INT0 and INT1)
– two timer interrupts (timers 0, 1),
– the UART interface
– the SPI interface
– the keyboard interface
– the USB interface
– the Smart Card Interface.
Interrupt System
Description Each of the interrupt sources can be individually enabled or disabled by setting or clear-
ing a bit in the Interrupt Enable registers (Table 98 on page 162 and Table 99 on page
163). These registers also contain a global disable bit, which must be cleared to disable
all interrupts at once.
Each interrupt source can also be individually programmed to one out of four priority lev-
els by setting or clearing a bit in the Interrupt Priority Low registers (Table 101 on page
164 and Table 103 on page 166) and in the Interrupt Priority High register (Table 102 on
page 165 and Table 105 on page 168) shows the bit values and priority levels associ-
ated with each combination.
A low-priority interrupt can be interrupted by a high priority interrupt, but not by another
low-priority interrupt. A high-priority interrupt can’t be interrupted by any other interrupt
source.
If two interrupt requests of different priority levels are received simultaneously, the
request of higher priority level is serviced. If interrupt requests of the same priority level
are received simultaneously, an internal polling sequence determines which request is
serviced first. Thus within each priority level there is a second priority structure deter-
mined by the polling sequence.
Table 97. Priority Level Bit Values
IPH.x IPL.x Interrupt Level Priority
0 0 0 (Lowest)
0 1 1
1 0 2
1 1 3 (Highest)