60 Software Compatibility

Two of the conditions present in the INS8250-B are required

in many of these personal computers (see Items 1 and 2 in

Section 20) These two detect multiple pending interrupts

from the INS8250-B and test the baud rate These two con-

ditions were eliminated in the revision part and all parts

thereafter Thus the more recent UARTs require that one of

the following recommendations or a similar change is made

to the target system Changing the software or hardware

allows the more recent UARTs to replace the INS8250-B If

the target system services the UART via polling rather than

interrupts then all of the more recent parts will be plug-in

replacements for the INS8250-B

Note

The NS16550AF has two pins with new functions (see the data sheet

for specifics)

61 USING THE INS8250A NS16450 INS82C50A

NS16C450 AND NS16550AF WITH EDGED-TRIGGERED

ICUs

Using these UARTs with an edge-triggered ICU as in some

of the popular microcomputers requires a signal edge on the

INTR pin for each pending UART interrupt Otherwise when

multiple interrupts are pending the interrupt line will be con-

stantly high active and the edge-triggered ICU will not re-

quest additional service for the UART

62 CREATING AN INTERRUPT EDGE VIA SOFTWARE

This is done by disabling and then re-enabling UART inter-

rupts via the Interrupt Enable Register (IER) before a specif-

ic UART interrupt handling routine (line status errors re-

ceived data available transmitter holding register empty or

modem status) is exited To disable interrupts write H’00 to

the IER To re-enable interrupts write a byte containing ones

to the IER bit positions whose interrupts are supposed to be

enabled

63 CREATING AN INTERRUPT EDGE IN HARDWARE

This is done externally to the UART One approach is to

connect the INTR pin of the UART to the input of an AND

gate The other input of this AND gate is connected to a

signal that will always go low active when the UART is ac-

cessed (see

Figure 3 ) The output of the AND gate is used

as the interrupt to the ICU

Note

This simple hardware recommendation will result in one invalid inter-

rupt being generated so the software routine should be able to han-

dle this The example shown below was tested using a modified asyn-

chronous card in a few 8088-based microcomputer systems

70 Acknowledgements

The editor expresses his gratitude to all of the applications

design and field applications engineers whose laboratory

and field research have discovered most of the technical

information used in this document

TLC9320 – 4

FIGURE 3 Creating an INTR Edge in Hardware

3