Jameco Electronics 2000, 3000 manual Serial Port Software Suggestions, Push AF IOI LD A,SCSR

12.9 Serial Port Software Suggestions

The receiver and transmitter share the same interrupt vector, but it is possible to make the receive and transmit interrupt service routines (ISRs) separate by dispatching the interrupt to either of two different routines. This is desirable to make the ISR less complex and to reduce the interrupt off time. No interrupts will be lost since distinct interrupt flip-flops exist for receive and transmit. The dispatcher can test the receiver data register full bit to dispatch. If this bit is on, the interrupt is dispatched for receive, otherwise for transmit. The receiver receives first consideration because it must be serviced attentively or data could be lost.

The dispatcher might look as follows.

interrupt:
PUSH AF	; 10
IOI LD A,(SCSR)	; 7	get status	register serial port C
JP m,receive	; 7	go service	the receive interrupt
	;	else service transmit interrupt

The individual interrupts would assume that register AF has been saved and the status reg- ister has been loaded into Register A.

The interrupt service routines can, as a matter of good practice and obtaining optimum performance, remove the cause of the interrupt and re-enable the interrupts as soon as pos- sible. This keeps the interrupt latency down and allows the fastest transmission speed on all serial ports.

All the serial ports will normally generate priority level 1 interrupts. In exceptional circum- stances, one or more serial ports can be configured to use a higher priority interrupt. There is an exception to be aware of when a serial port has to operate at an extremely high speed. At 115,200 bps, the highest speed of a PC serial port, the interrupts must be serviced in 10 baud times, or 86 µs, in order not to lose the received characters. If all six serial ports were operating at this receive speed, it would be necessary to service the interrupt in less than 21.5 µs to assure no lost characters. In addition, the time taken by other interrupts of equal or higher priority would have to be considered. A receiver service routine might appear as follows below. The byte at bufptr is used to address the buffer where data bits are stored. It is necessary to save and increment this byte because characters could be han- dled out of order if two receiver interrupts take place in quick succession.

receive:
PUSH HL	; 10 save HL
PUSH DE	; 10 save DE
LD HL,struct	; 6
LD A,(HL)	; 5 get in-pointer
LD E,A	; 2 save in pointer in E
INC HL	; 2 point to out-pointer
CMP A,(HL)	; 5 see if in-pointer=out-pointer (buffer full)
JR Z,roverrun	; 5 go fix up receiver over run
INC A	; 2	incement the in pointer
AND A,mask	; 4	mask such as 11110000 if 16 buffer locs
DEC HL	; 2
192		Rabbit 3000 Microprocessor