source: avrstuff/CPC stuff/cpc_serial_2313/code/main.asm@ 1dc9759

; ---------------------------------------------------------------
; Copyright 2010, Adrien Destugues <pulkomandy@pulkomandy.ath.cx>
; Distributed under the terms of the MIT Licence

; Firmware for µSerial expansion board

; Vectors
; reset
; int0
        RJMP cpc_write
; int1
        RJMP cpc_read
; ...

; Interrupt vectors for external INT pins (read and write).
; we have to react very quick.
; A read operation for the CPC lasts 3 clock cycles at 4MHz, that's 15
; AVR cycles. But the interrupt latency is as follow :
; Lowlevel detection                    ; 2 cycles 
; End of running instruction    ; up to 2 cycles
; Save PC                                               ; 4 cycles
;Vector RJMP                                    ; 2 cycles
                                                                ; TOTAL => 10 cycles

; --- READ INTERRUPT ---
cpc_read
; That means we only have 5 cycles left to output the value on the BUS!
; We have no time to do anything, so we assume that X is already pointing at
; the right place and we just OUT it to the data port. We have no time for
; PUSHing and loading it, anyway.

; Note you can read from either port and get the same result. Two reasons to
; that : you can already access all the registers and part of the SRAM,
; and there's no time to do something more clever.

; There is no time to push/pop regs, so we just use X as is. R27 is part of X.

; We assume X (R26:R27) points to the current reg
; So we can load it and react fast enough to the interrupt
        LDI R0,ALL_OUT ; 1 ; peut être économisé si on sacrifie un reg
        OUT DATADIR,R0 ; 1
        LD R27,(X)                      ; 2 cycles ; peut être économisé si un reg. contient
                                                                        ; déjà la valeur à envoyer
                                                                        ; (mais qui l'update ?)
        OUT DATA, R27           ; 1 cycle

; Here data is sent, the CPC read operation is handled.
; We now wait for the end of the read cycle.
; This is not the end of the time-constrained nightmare, however :
; In the worst case, the CPC can do another OUT or IN right after,
; so we don't have an infinite number of cycles to handle the interrupt.
; it is much more relaxed, as we have 12 CPC cycles = 60 AVR cycles free.

; Restore R27
        LD R27,curregbak
        LD R27,(X)

; release the bus
        LDI R0,ALL_IN
        OUT DATADIR, R0

; Restore R27 to selected reg. (we erased it to do the OUT)
        RETI

; --- WRITE INTERRUPT ---
cpc_write
; The timing is a bit less constraining here.
        PUSH R0
        IN R0,DATA
; we also need to know A0 state...
        SBIS CTRL,A0
                ; This was actually a reg select operation!
                ; Jump to the proper code
        RJMP regSel
; We have read the CPC data. End of the heavy-constraint area

; Register write
        ST X,R0 ; Normal register write
        RJMP intEnd

regSel
        LD R27,curregbak
        ST (X),R0
        MOV R27,R0

        POP R0
        RETI


; --- RESET VECTOR ---
; Here we perform the hardware initialization.
; At a bare minimum :
; * Set up the INT0 and INT1 so the CPC can do the rest of the setup itself