[1dc9759] | 1 | ; ---------------------------------------------------------------
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| 2 | ; Copyright 2010, Adrien Destugues <pulkomandy@pulkomandy.ath.cx>
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| 3 | ; Distributed under the terms of the MIT Licence
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| 4 |
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| 5 | ; Firmware for µSerial expansion board
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| 6 |
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| 7 | ; Vectors
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| 8 | ; reset
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| 9 | ; int0
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| 10 | RJMP cpc_write
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| 11 | ; int1
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| 12 | RJMP cpc_read
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| 13 | ; ...
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| 14 |
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| 15 | ; Interrupt vectors for external INT pins (read and write).
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| 16 | ; we have to react very quick.
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| 17 | ; A read operation for the CPC lasts 3 clock cycles at 4MHz, that's 15
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| 18 | ; AVR cycles. But the interrupt latency is as follow :
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| 19 | ; Lowlevel detection ; 2 cycles
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| 20 | ; End of running instruction ; up to 2 cycles
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| 21 | ; Save PC ; 4 cycles
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| 22 | ;Vector RJMP ; 2 cycles
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| 23 | ; TOTAL => 10 cycles
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| 24 |
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| 25 | ; --- READ INTERRUPT ---
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[cb32b0b] | 26 | cpc_read
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[1dc9759] | 27 | ; That means we only have 5 cycles left to output the value on the BUS!
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| 28 | ; We have no time to do anything, so we assume that X is already pointing at
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| 29 | ; the right place and we just OUT it to the data port. We have no time for
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| 30 | ; PUSHing and loading it, anyway.
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| 31 |
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| 32 | ; Note you can read from either port and get the same result. Two reasons to
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| 33 | ; that : you can already access all the registers and part of the SRAM,
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| 34 | ; and there's no time to do something more clever.
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| 35 |
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| 36 | ; There is no time to push/pop regs, so we just use X as is. R27 is part of X.
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| 37 |
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| 38 | ; We assume X (R26:R27) points to the current reg
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| 39 | ; So we can load it and react fast enough to the interrupt
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[cb32b0b] | 40 | LDI R0,ALL_OUT ; 1 ; peut être économisé si on sacrifie un reg
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| 41 | OUT DATADIR,R0 ; 1
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| 42 | LD R27,(X) ; 2 cycles ; peut être économisé si un reg. contient
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[1dc9759] | 43 | ; déjà la valeur à envoyer
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| 44 | ; (mais qui l'update ?)
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[cb32b0b] | 45 | OUT DATA, R27 ; 1 cycle
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[1dc9759] | 46 |
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| 47 | ; Here data is sent, the CPC read operation is handled.
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| 48 | ; We now wait for the end of the read cycle.
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| 49 | ; This is not the end of the time-constrained nightmare, however :
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| 50 | ; In the worst case, the CPC can do another OUT or IN right after,
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| 51 | ; so we don't have an infinite number of cycles to handle the interrupt.
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| 52 | ; it is much more relaxed, as we have 12 CPC cycles = 60 AVR cycles free.
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| 53 |
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| 54 | ; Restore R27
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[cb32b0b] | 55 | LD R27,curregbak
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| 56 | LD R27,(X)
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[1dc9759] | 57 |
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| 58 | ; release the bus
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[cb32b0b] | 59 | LDI R0,ALL_IN
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| 60 | OUT DATADIR, R0
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[1dc9759] | 61 |
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| 62 | ; Restore R27 to selected reg. (we erased it to do the OUT)
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| 63 | RETI
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| 64 |
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| 65 | ; --- WRITE INTERRUPT ---
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[cb32b0b] | 66 | cpc_write
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[1dc9759] | 67 | ; The timing is a bit less constraining here.
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[cb32b0b] | 68 | PUSH R0
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| 69 | IN R0,DATA
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[1dc9759] | 70 | ; we also need to know A0 state...
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[cb32b0b] | 71 | SBIS CTRL,A0
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[1dc9759] | 72 | ; This was actually a reg select operation!
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| 73 | ; Jump to the proper code
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| 74 | RJMP regSel
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| 75 | ; We have read the CPC data. End of the heavy-constraint area
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| 76 |
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| 77 | ; Register write
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| 78 | ST X,R0 ; Normal register write
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| 79 | RJMP intEnd
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| 80 |
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[cb32b0b] | 81 | regSel
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| 82 | LD R27,curregbak
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| 83 | ST (X),R0
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[1dc9759] | 84 | MOV R27,R0
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| 85 |
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| 86 | POP R0
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| 87 | RETI
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| 88 |
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| 89 |
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| 90 | ; --- RESET VECTOR ---
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| 91 | ; Here we perform the hardware initialization.
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| 92 | ; At a bare minimum :
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| 93 | ; * Set up the INT0 and INT1 so the CPC can do the rest of the setup itself
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