COP884CG Datasheet, PDF (19/42 Pages) National Semiconductor (TI) – 8-Bit Microcontroller with UART and Three Multi-Function Timers

UART Operation (Continued)
FIGURE 12 Framing Formats
TL DD 9765 – 19
UART INTERRUPTS
The UART is capable of generating interrupts Interrupts are
generated on Receive Buffer Full and Transmit Buffer Emp-
ty Both interrupts have individual interrupt vectors Two
bytes of program memory space are reserved for each inter-
rupt vector The two vectors are located at addresses 0xEC
to 0xEF Hex in the program memory space The interrupts
can be individually enabled or disabled using Enable Trans-
mit Interrupt (ETI) and Enable Receive Interrupt (ERI) bits in
the ENUI register
The interrupt from the Transmitter is set pending and re-
mains pending as long as both the TBMT and ETI bits are
set To remove this interrupt software must either clear the
ETI bit or write to the TBUF register (thus clearing the TBMT
bit)
The interrupt from the receiver is set pending and remains
pending as long as both the RBFL and ERI bits are set To
remove this interrupt software must either clear the ERI bit
or read from the RBUF register (thus clearing the RBFL bit)
Baud Clock Generation
The clock inputs to the transmitter and receiver sections of
the UART can be individually selected to come either from
an external source at the CKX pin (port L pin L1) or from a
source selected in the PSR and BAUD registers Internally
the basic baud clock is created from the oscillator frequency
through a two-stage divider chain consisting of a 1 – 16 (in-
crements of 0 5) prescaler and an 11-bit binary counter
(Figure 13) The divide factors are specified through two
read write registers shown in Figure 14 Note that the 11-bit
Baud Rate Divisor spills over into the Prescaler Select Reg-
ister (PSR) PSR is cleared upon reset
As shown in Table I a Prescaler Factor of 0 corresponds to
NO CLOCK NO CLOCK condition is the UART power down
mode where the UART clock is turned off for power saving
purpose The user must also turn the UART clock off when
a different baud rate is chosen
The correspondences between the 5-bit Prescaler Select
and Prescaler factors are shown in Table I Therer are many
ways to calculate the two divisor factors but one particularly
effective method would be to achieve a 1 8432 MHz fre-
quency coming out of the first stage The 1 8432 MHz pre-
scaler output is then used to drive the software programma-
ble baud rate counter to create a x16 clock for the following
baud rates 110 134 5 150 300 600 1200 1800 2400
3600 4800 7200 9600 19200 and 38400 (Table II) Other
baud rates may be created by using appropriate divisors
The x16 clock is then divided by 16 to provide the rate for
the serial shift registers of the transmitter and receiver
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