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/* Name: usbdrvasm16.inc

 * Project: V-USB, virtual USB port for Atmel's(r) AVR(r) microcontrollers

 * Author: Christian Starkjohann

 * Creation Date: 2007-06-15

 * Tabsize: 4

 * Copyright: (c) 2007 by OBJECTIVE DEVELOPMENT Software GmbH

 * License: GNU GPL v2 (see License.txt), GNU GPL v3 or proprietary (CommercialLicense.txt)

 */

/* Do not link this file! Link usbdrvasm.S instead, which includes the

 * appropriate implementation!

 */

/*

General Description:

This file is the 16 MHz version of the asssembler part of the USB driver. It

requires a 16 MHz crystal (not a ceramic resonator and not a calibrated RC

oscillator).

See usbdrv.h for a description of the entire driver.

Since almost all of this code is timing critical, don't change unless you

really know what you are doing! Many parts require not only a maximum number

of CPU cycles, but even an exact number of cycles!

*/

;max stack usage: [ret(2), YL, SREG, YH, bitcnt, shift, x1, x2, x3, x4, cnt] = 12 bytes

;nominal frequency: 16 MHz -> 10.6666666 cycles per bit, 85.333333333 cycles per byte

; Numbers in brackets are clocks counted from center of last sync bit

; when instruction starts

USB_INTR_VECTOR:

;order of registers pushed: YL, SREG YH, [sofError], bitcnt, shift, x1, x2, x3, x4, cnt

    push    YL                  ;[-25] push only what is necessary to sync with edge ASAP

    in      YL, SREG            ;[-23]

    push    YL                  ;[-22]

    push    YH                  ;[-20]

;----------------------------------------------------------------------------

; Synchronize with sync pattern:

;----------------------------------------------------------------------------

;sync byte (D-) pattern LSb to MSb: 01010100 [1 = idle = J, 0 = K]

;sync up with J to K edge during sync pattern -- use fastest possible loops

;The first part waits at most 1 bit long since we must be in sync pattern.

;YL is guarenteed to be < 0x80 because I flag is clear. When we jump to

;waitForJ, ensure that this prerequisite is met.

waitForJ:

    inc     YL

    sbis    USBIN, USBMINUS

    brne    waitForJ        ; just make sure we have ANY timeout

waitForK:

;The following code results in a sampling window of < 1/4 bit which meets the spec.

    sbis    USBIN, USBMINUS     ;[-15]

    rjmp    foundK              ;[-14]

    sbis    USBIN, USBMINUS

    rjmp    foundK

    sbis    USBIN, USBMINUS

    rjmp    foundK

    sbis    USBIN, USBMINUS

    rjmp    foundK

    sbis    USBIN, USBMINUS

    rjmp    foundK

    sbis    USBIN, USBMINUS

    rjmp    foundK

#if USB_COUNT_SOF

    lds     YL, usbSofCount

    inc     YL

    sts     usbSofCount, YL

#endif  /* USB_COUNT_SOF */

#ifdef USB_SOF_HOOK

    USB_SOF_HOOK

#endif

    rjmp    sofError

foundK:                         ;[-12]

;{3, 5} after falling D- edge, average delay: 4 cycles [we want 5 for center sampling]

;we have 1 bit time for setup purposes, then sample again. Numbers in brackets

;are cycles from center of first sync (double K) bit after the instruction

    push    bitcnt              ;[-12]

;   [---]                       ;[-11]

    lds     YL, usbInputBufOffset;[-10]

;   [---]                       ;[-9]

    clr     YH                  ;[-8]

    subi    YL, lo8(-(usbRxBuf));[-7] [rx loop init]

    sbci    YH, hi8(-(usbRxBuf));[-6] [rx loop init]

    push    shift               ;[-5]

;   [---]                       ;[-4]

    ldi     bitcnt, 0x55        ;[-3] [rx loop init]

    sbis    USBIN, USBMINUS     ;[-2] we want two bits K (sample 2 cycles too early)

    rjmp    haveTwoBitsK        ;[-1]

    pop     shift               ;[0] undo the push from before

    pop     bitcnt              ;[2] undo the push from before

    rjmp    waitForK            ;[4] this was not the end of sync, retry

; The entire loop from waitForK until rjmp waitForK above must not exceed two

; bit times (= 21 cycles).

;----------------------------------------------------------------------------

; push more registers and initialize values while we sample the first bits:

;----------------------------------------------------------------------------

haveTwoBitsK:

    push    x1              ;[1]

    push    x2              ;[3]

    push    x3              ;[5]

    ldi     shift, 0        ;[7]

    ldi     x3, 1<<4        ;[8] [rx loop init] first sample is inverse bit, compensate that

    push    x4              ;[9] == leap

    in      x1, USBIN       ;[11] <-- sample bit 0

    andi    x1, USBMASK     ;[12]

    bst     x1, USBMINUS    ;[13]

    bld     shift, 7        ;[14]

    push    cnt             ;[15]

    ldi     leap, 0         ;[17] [rx loop init]

    ldi     cnt, USB_BUFSIZE;[18] [rx loop init]

    rjmp    rxbit1          ;[19] arrives at [21]

;----------------------------------------------------------------------------

; Receiver loop (numbers in brackets are cycles within byte after instr)

;----------------------------------------------------------------------------

; duration of unstuffing code should be 10.66666667 cycles. We adjust "leap"

; accordingly to approximate this value in the long run.

unstuff6:

    andi    x2, USBMASK ;[03]

    ori     x3, 1<<6    ;[04] will not be shifted any more

    andi    shift, ~0x80;[05]

    mov     x1, x2      ;[06] sampled bit 7 is actually re-sampled bit 6

    subi    leap, -1    ;[07] total duration = 11 bits -> subtract 1/3

    rjmp    didUnstuff6 ;[08]

unstuff7:

    ori     x3, 1<<7    ;[09] will not be shifted any more

    in      x2, USBIN   ;[00] [10]  re-sample bit 7

    andi    x2, USBMASK ;[01]

    andi    shift, ~0x80;[02]

    subi    leap, 2     ;[03] total duration = 10 bits -> add 1/3

    rjmp    didUnstuff7 ;[04]

unstuffEven:

    ori     x3, 1<<6    ;[09] will be shifted right 6 times for bit 0

    in      x1, USBIN   ;[00] [10]

    andi    shift, ~0x80;[01]

    andi    x1, USBMASK ;[02]

    breq    se0         ;[03]

    subi    leap, -1    ;[04] total duration = 11 bits -> subtract 1/3

    nop2                ;[05]

    rjmp    didUnstuffE ;[06]

unstuffOdd:

    ori     x3, 1<<5    ;[09] will be shifted right 4 times for bit 1

    in      x2, USBIN   ;[00] [10]

    andi    shift, ~0x80;[01]

    andi    x2, USBMASK ;[02]

    breq    se0         ;[03]

    subi    leap, -1    ;[04] total duration = 11 bits -> subtract 1/3

    nop2                ;[05]

    rjmp    didUnstuffO ;[06]

rxByteLoop:

    andi    x1, USBMASK ;[03]

    eor     x2, x1      ;[04]

    subi    leap, 1     ;[05]

    brpl    skipLeap    ;[06]

    subi    leap, -3    ;1 one leap cycle every 3rd byte -> 85 + 1/3 cycles per byte

    nop                 ;1

skipLeap:

    subi    x2, 1       ;[08]

    ror     shift       ;[09]

didUnstuff6:

    cpi     shift, 0xfc ;[10]

    in      x2, USBIN   ;[00] [11] <-- sample bit 7

    brcc    unstuff6    ;[01]

    andi    x2, USBMASK ;[02]

    eor     x1, x2      ;[03]

    subi    x1, 1       ;[04]

    ror     shift       ;[05]

didUnstuff7:

    cpi     shift, 0xfc ;[06]

    brcc    unstuff7    ;[07]

    eor     x3, shift   ;[08] reconstruct: x3 is 1 at bit locations we changed, 0 at others

    st      y+, x3      ;[09] store data

rxBitLoop:

    in      x1, USBIN   ;[00] [11] <-- sample bit 0/2/4

    andi    x1, USBMASK ;[01]

    eor     x2, x1      ;[02]

    andi    x3, 0x3f    ;[03] topmost two bits reserved for 6 and 7

    subi    x2, 1       ;[04]

    ror     shift       ;[05]

    cpi     shift, 0xfc ;[06]

    brcc    unstuffEven ;[07]

didUnstuffE:

    lsr     x3          ;[08]

    lsr     x3          ;[09]

rxbit1:

    in      x2, USBIN   ;[00] [10] <-- sample bit 1/3/5

    andi    x2, USBMASK ;[01]

    breq    se0         ;[02]

    eor     x1, x2      ;[03]

    subi    x1, 1       ;[04]

    ror     shift       ;[05]

    cpi     shift, 0xfc ;[06]

    brcc    unstuffOdd  ;[07]

didUnstuffO:

    subi    bitcnt, 0xab;[08] == addi 0x55, 0x55 = 0x100/3

    brcs    rxBitLoop   ;[09]

    subi    cnt, 1      ;[10]

    in      x1, USBIN   ;[00] [11] <-- sample bit 6

    brcc    rxByteLoop  ;[01]

    rjmp    overflow

macro POP_STANDARD ; 14 cycles

    pop     cnt

    pop     x4

    pop     x3

    pop     x2

    pop     x1

    pop     shift

    pop     bitcnt

    endm

macro POP_RETI     ; 7 cycles

    pop     YH

    pop     YL

    out     SREG, YL

    pop     YL

    endm

#include "asmcommon.inc"

; USB spec says:

; idle = J

; J = (D+ = 0), (D- = 1)

; K = (D+ = 1), (D- = 0)

; Spec allows 7.5 bit times from EOP to SOP for replies

bitstuffN:

    eor     x1, x4          ;[5]

    ldi     x2, 0           ;[6]

    nop2                    ;[7]

    nop                     ;[9]

    out     USBOUT, x1      ;[10] <-- out

    rjmp    didStuffN       ;[0]

bitstuff6:

    eor     x1, x4          ;[5]

    ldi     x2, 0           ;[6] Carry is zero due to brcc

    rol     shift           ;[7] compensate for ror shift at branch destination

    rjmp    didStuff6       ;[8]

bitstuff7:

    ldi     x2, 0           ;[2] Carry is zero due to brcc

    rjmp    didStuff7       ;[3]

sendNakAndReti:

    ldi     x3, USBPID_NAK  ;[-18]

    rjmp    sendX3AndReti   ;[-17]

sendAckAndReti:

    ldi     cnt, USBPID_ACK ;[-17]

sendCntAndReti:

    mov     x3, cnt         ;[-16]

sendX3AndReti:

    ldi     YL, 20          ;[-15] x3==r20 address is 20

    ldi     YH, 0           ;[-14]

    ldi     cnt, 2          ;[-13]

;   rjmp    usbSendAndReti      fallthrough

;usbSend:

;pointer to data in 'Y'

;number of bytes in 'cnt' -- including sync byte [range 2 ... 12]

;uses: x1...x4, btcnt, shift, cnt, Y

;Numbers in brackets are time since first bit of sync pattern is sent

;We don't match the transfer rate exactly (don't insert leap cycles every third

;byte) because the spec demands only 1.5% precision anyway.

usbSendAndReti:             ; 12 cycles until SOP

    in      x2, USBDDR      ;[-12]

    ori     x2, USBMASK     ;[-11]

    sbi     USBOUT, USBMINUS;[-10] prepare idle state; D+ and D- must have been 0 (no pullups)

    in      x1, USBOUT      ;[-8] port mirror for tx loop

    out     USBDDR, x2      ;[-7] <- acquire bus

; need not init x2 (bitstuff history) because sync starts with 0

    ldi     x4, USBMASK     ;[-6] exor mask

    ldi     shift, 0x80     ;[-5] sync byte is first byte sent

txByteLoop:

    ldi     bitcnt, 0x35    ;[-4] [6] binary 0011 0101

txBitLoop:

    sbrs    shift, 0        ;[-3] [7]

    eor     x1, x4          ;[-2] [8]

    out     USBOUT, x1      ;[-1] [9] <-- out N

    ror     shift           ;[0] [10]

    ror     x2              ;[1]

didStuffN:

    cpi     x2, 0xfc        ;[2]

    brcc    bitstuffN       ;[3]

    lsr     bitcnt          ;[4]

    brcc    txBitLoop       ;[5]

    brne    txBitLoop       ;[6]

    sbrs    shift, 0        ;[7]

    eor     x1, x4          ;[8]

didStuff6:

    out     USBOUT, x1      ;[-1] [9] <-- out 6

    ror     shift           ;[0] [10]

    ror     x2              ;[1]

    cpi     x2, 0xfc        ;[2]

    brcc    bitstuff6       ;[3]

    ror     shift           ;[4]

didStuff7:

    ror     x2              ;[5]

    sbrs    x2, 7           ;[6]

    eor     x1, x4          ;[7]

    nop                     ;[8]

    cpi     x2, 0xfc        ;[9]

    out     USBOUT, x1      ;[-1][10] <-- out 7

    brcc    bitstuff7       ;[0] [11]

    ld      shift, y+       ;[1]

    dec     cnt             ;[3]

    brne    txByteLoop      ;[4]

;make SE0:

    cbr     x1, USBMASK     ;[5] prepare SE0 [spec says EOP may be 21 to 25 cycles]

    lds     x2, usbNewDeviceAddr;[6]

    lsl     x2              ;[8] we compare with left shifted address

    subi    YL, 20 + 2      ;[9] Only assign address on data packets, not ACK/NAK in x3

    sbci    YH, 0           ;[10]

    out     USBOUT, x1      ;[11] <-- out SE0 -- from now 2 bits = 22 cycles until bus idle

;2006-03-06: moved transfer of new address to usbDeviceAddr from C-Code to asm:

;set address only after data packet was sent, not after handshake

    breq    skipAddrAssign  ;[0]

    sts     usbDeviceAddr, x2; if not skipped: SE0 is one cycle longer

skipAddrAssign:

;end of usbDeviceAddress transfer

    ldi     x2, 1<<USB_INTR_PENDING_BIT;[2] int0 occurred during TX -- clear pending flag

    USB_STORE_PENDING(x2)   ;[3]

    ori     x1, USBIDLE     ;[4]

    in      x2, USBDDR      ;[5]

    cbr     x2, USBMASK     ;[6] set both pins to input

    mov     x3, x1          ;[7]

    cbr     x3, USBMASK     ;[8] configure no pullup on both pins

    ldi     x4, 4           ;[9]

se0Delay:

    dec     x4              ;[10] [13] [16] [19]

    brne    se0Delay        ;[11] [14] [17] [20]

    out     USBOUT, x1      ;[21] <-- out J (idle) -- end of SE0 (EOP signal)

    out     USBDDR, x2      ;[22] <-- release bus now

    out     USBOUT, x3      ;[23] <-- ensure no pull-up resistors are active

    rjmp    doReturn