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USART Interrupt
– The USART Interrupt is generated by RI
(Receive Interrupt) OR TI (Transmit Interrupt).
– When the USART Interrupt is generated, the
corresponding request flag must be cleared by
the software. The interrupt service routine will
have to check the various USART registers to
determine the source and clear the
corresponding flag.
– Both USART’s are identical, except for the
additional interrupt controls in the Bit 4 of the
additional interrupt control registers (A7H,
B7H).
Interrupt Priority Structure
Each interrupt source can be assigned one of two
priority levels. Interrupt priority levels are defined
by the interrupt priority special function register IP
and IPA.
0 = low priority
1 = high priority
A low priority interrupt may be interrupted by a
high priority interrupt level interrupt. A high priority
interrupt routine cannot be interrupted by any oth-
er interrupt source. If two interrupts of different pri-
ority occur simultaneously, the high priority level
request is serviced. If requests of the same priority
are received simultaneously, an internal polling
sequence determines which request is serviced.
Thus, within each priority level, there is a second
priority structure determined by the polling se-
quence.
Interrupts Enable Structure
Each interrupt source can be individually enabled
or disabled by setting or clearing a bit in the inter-
rupt enable special function register IE and IEA. All
interrupt source can also be globally disabled by
the clearing Bit EA in IE (see Table 19). Please
see Tables 20, 21, 22, and 23 for individual bit de-
scriptions.
Table 18. Priority Levels
Table 19. SFR Register
Source Priority with Level
Int0 0 (highest)
2nd USART 1
Timer 0 2
I²C 3
Int1 4
reserved 5
Timer 1 6
USB 7
1st USART 8
Timer 2+EXF2 9 (lowest)
SFR
Addr
Reg
Name
Bit Register Name
Reset
Value
Comments
76543210
A7 IEA — — — ES2 — —
EI
2
C
EUSB 00
Interrupt
Enable (2nd)
A8 IE EA — ET2 ES ET1 EX1 ET0 EX0 00
Interrupt
Enable
B7 IPA — — — PS2 — —
PI
2
C
PUSB 00
Interrupt
Priority (2nd)
B8 IP — — PT2 PS PT1 PX1 PT0 PX0 00
Interrupt
Priority