130 Rabbit 2000 Microprocessor
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 four serial
ports were operat-
ing 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 nec-
essary to save and increment this byte because characters could be handled 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 getin-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
LD (HL),A ; 6 update the in pointer
IOI LD A,(SCDR) ; 11 get data register port C, clears interrupt request
IPRES ; 4 restore the interrupt priority
; 68 clocks to here
; to level before interrupt took place
; more interrupts could now take place,
; but receiver data is in registers
; now handle the rest of the receiver interrupt routine
LD HL,BUFBASE ; 6
LD D,0 ; 6
ADD HL,DE ; 2 location to store data
LD (HL),A ; 6 put away the data byte
POP DE ;7
POP HL ; 7
POP AF ; 7
RET ; 8 from interrupt
; 117 clocks to here
This routine gets the interrupts turned on in about 68 clocks or 3.5 µs at a clock speed of
20 MHz. Although two characters may be handled out of order, this will be invisible to a
higher level routine checking the status of the input buffer because all the interrupts will
be completed before the higher level routine can perform a check on the buffer status.
A typical way to organize the buffers is to have an in-pointer and an out-pointer that incre-
ment through the addresses in the data buffer in a circular manner. The interrupt routine
manipulates the in-pointer and the higher level routine manipulates the out-pointer. If the in-
pointer equals the out-pointer, the buffer is considered full. If the out-pointer plus 1 equals
the in-pointer, the buffer is empty. All increments are done in a circular fashion, most easily
accomplished by making the buffer a power of two in length, then anding a mask after the
increment. The actual memory address is the pointer plus a buffer base address.
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