Figure 9. SPI Transfer Format with CPHA = 1
SCK CYCLE #
1
2
3
4
5
6
7
8
(FOR REFERENCE)
SCK (CPOL=0)
SCK (CPOL=1)
MOSI
(FROM MASTER)
MSB
MSB
6
5
5
4
3
3
2
1
1
LSB
MISO
(FROM SLAVE)
6
4
2
LSB
*
SS (TO SLAVE)
*Not defined but normally LSB of previously transmitted character
mented. User software should not write 1s to these bit posi-
tions, since they may be used in future AT89 products.
Interrupts
The AT89S53 has a total of six interrupt vectors: two exter-
nal interrupts (INT0 and INT1), three timer interrupts (Tim-
ers 0, 1, and 2), and the serial port interrupt. These inter-
rupts are all shown in Figure 10.
Timer 2 interrupt is generated by the logical OR of bits TF2
and EXF2 in register T2CON. Neither of these flags is
cleared by hardware when the service routine is vectored
to. In fact, the service routine may have to determine
whether it was TF2 or EXF2 that generated the interrupt,
and that bit will have to be cleared in software.
Each of these interrupt sources can be individually enabled
or disabled by setting or clearing a bit in Special Function
Register IE. IE also contains a global disable bit, EA, which
disables all interrupts at once.
The Timer 0 and Timer 1 flags, TF0 and TF1, are set at
S5P2 of the cycle in which the timers overflow. The values
are then polled by the circuitry in the next cycle. However,
the Timer 2 flag, TF2, is set at S2P2 and is polled in the
same cycle in which the timer overflows.
Note that Table 10 shows that bit position IE.6 is unimple-
mented. In the AT89C51, bit position IE.5 is also unimple-
Table 10. Interrupt Enable (IE) Register
Figure 10. Interrupt Sources
(MSB)
EA
(LSB)
EX0
—
ET2
ES
ET1
EX1
ET0
Enable Bit = 1 enables the interrupt.
Enable Bit = 0 disables the interrupt.
Symbol
Position
Function
EA
IE.7
Disables all interrupts. If EA = 0, no interrupt
is acknowledged. If EA = 1, each interrupt
source is individually enabled or disabled by
setting or clearing its enable bit.
—
IE.6
IE.5
IE.4
IE.3
IE.2
IE.1
IE.0
Reserved.
ET2
ES
Timer 2 interrupt enable bit.
SPI and UART interrupt enable bit.
Timer 1 interrupt enable bit.
External interrupt 1 enable bit.
Timer 0 interrupt enable bit.
External interrupt 0 enable bit.
ET1
EX1
ET0
EX0
User software should never write 1s to unimplemented bits, because
they may be used in future AT89 products.
AT89S53
4-232
ATMEL [ ATMEL ]
