COP87L88FH Datasheet, PDF (17/45 Pages) National Semiconductor (TI) – 8-Bit CMOS OTP Microcontrollers with 16k Memory,

Multi-Input Wakeup (Continued)
both enabled and pending. Consequently, the user has the
responsibility of clearing the pending flags before attempting
to enter the HALT mode.
WKEN, WKPND and WKEDG are all read/write registers,
and are cleared at reset.
PORT L INTERRUPTS
Port L provides the user with an additional eight fully select-
able, edge sensitive interrupts which are all vectored into the
same service subroutine.
The interrupt from Port L shares logic with the wake up cir-
cuitry. The register WKEN allows interrupts from Port L to be
individually enabled or disabled. The register WKEDG speci-
fies the trigger condition to be either a positive or a negative
edge. Finally, the register WKPND latches in the pending
trigger conditions.
The GIE (Global Interrupt Enable) bit enables the interrupt
function.
A control flag, LPEN, functions as a global interrupt enable
for Port L interrupts. Setting the LPEN flag will enable inter-
rupts and vice versa. A separate global pending flag is not
needed since the register WKPND is adequate.
Since Port L is also used for waking the device out of the
HALT or IDLE modes, the user can elect to exit the HALT or
IDLE modes either with or without the interrupt enabled. If he
elects to disable the interrupt, then the device will restart ex-
ecution from the instruction immediately following the in-
struction that placed the microcontroller in the HALT or IDLE
modes. In the other case, the device will first execute the in-
terrupt service routine and then revert to normal operation.
(See HALT mode for clock option wakeup information.)
Note: There is always the possibility of an interrupt occurring during an in-
struction which is attempting to reset the GIE bit or any other interrupt
enable bit. If this occurs when a single cycle instruction is being used
to reset the interrupt enable bit, the interrupt enable bit will be reset but
an interrupt may still occur. This is because interrupt processing is
started at the same time as the interrupt bit is being reset. To avoid this
scenario, the user should always use a two, three, or four cycle instruc-
tion to reset interrupt enable bits.
USART
The device contains a full-duplex software programmable
USART. The USART (Figure 12) consists of a transmit shift
register, a receiver shift register and seven addressable reg-
isters, as follows: a transmit buffer register (TBUF), a re-
ceiver buffer register (RBUF), a USART control and status
register (ENU), a USART receive control and status register
(ENUR), a USART interrupt and clock source register
(ENUI), a prescaler select register (PSR) and baud (BAUD)
register. The ENU register contains flags for transmit and re-
ceive functions; this register also determines the length of
the data frame (7, 8 or 9 bits), the value of the ninth bit in
transmission, and parity selection bits. The ENUR register
flags framming, data overrun and parity errors while the US-
ART is receiving.
Other functions of the ENUR register include saving the
ninth bit received in the data frame, enabling or disabling the
USART’s attention mode of operation and providing addi-
tional receiver/transmitter status information via RCVG and
XMTG bits. The determination of an internal or external clock
source is done by the ENUI register, as well as selecting the
number of stop bits and enabling or disabling transmit and
receive interrupts. A control flag in this register can also se-
lect the USART mode of operation: asynchronous or
synchronous.
FIGURE 12. USART Block Diagram
17
DS101135-16
www.national.com