![](http://datasheet.mmic.net.cn/30000/MC68307CFG16_datasheet_2368698/MC68307CFG16_163.png)
Serial Module
MOTOROLA
MC68307 USER’S MANUAL
8-3
8.1.2 Baud Rate Generator Logic
The Baud rate Generator logic consists of an internal UART clock input which is not avail-
able as an external pin, but is a derivative of the IMBP system clock, a baud-rate generator
(BRG) and a programmable 16-bit timer. The clock serves as the basic timing reference for
the baud-rate generator and other internal circuits.
The 16-bit timer is used to produce a 16X clock for any baud rate by counting down the sys-
tem clock, it acts as a programmable divider. This feature is especially useful for non-stan-
dard system clock frequencies and baud rates.
8.1.3 Baud Rate Generator/Timer
The 16-bit timer is used as a baud rate generator, and provides a synchronous clock mode
of operation when used as a divide-by-1 clock and an asynchronous clock mode when used
as a divide-by-16 clock. This allows flexible baud rates for various system clock rates of the
MC68307, the divisor value being directly programmable.
8.1.4 Interrupt Control Logic
An internal interrupt request signal (IRQ) is provided to notify the MC68307 interrupt control-
ler of an interrupt condition. The output is the logical NOR of all (up to eight) unmasked inter-
rupt status bits in the interrupt status register (UISR).
The interrupt level of the serial module is programmed in the MC68307 interrupt controller
external to the serial module. When an interrupt at this level is acknowledged, the serial
module provides an automatic vector if UART is highest priority at this level.
8.1.5 Comparison of Serial Module to MC68681
The MC68307 is code compatible with the MC68681, except that only channel A is imple-
mented, and the timer/counter is used as a baud rate clock, dividing the MC68307 clock fre-
quency. The input and output port lines (IPx and OPx) are not implemented except for CTS
and RTS functions.
8.2 SERIAL MODULE SIGNAL DEFINITIONS
The following paragraphs contain a brief description of the serial module signals.
Figure 8-2 shows both the external and internal signal groups.
NOTE
The terms
assertion and negation are used throughout this sec-
tion to avoid confusion when dealing with a mixture of active-low
and active-high signals. The term
assert or assertion indicates
that a signal is active or true, independent of the level represent-
ed by a high or low voltage. The term
negate or negation indi-
cates that a signal is inactive or false.