usbdrvasm12.S
raphnet_saturn_usb/usbdrv/usbdrvasm12.S
/* Name: usbdrvasm12.S
 * Project: AVR USB driver
 * Author: Christian Starkjohann
 * Creation Date: 2004-12-29
 * Tabsize: 4
 * Copyright: (c) 2007 by OBJECTIVE DEVELOPMENT Software GmbH
 * License: GNU GPL v2 (see License.txt) or proprietary (CommercialLicense.txt)
 * This Revision: $Id: usbdrvasm12.S,v 1.1 2011-06-26 19:22:29 cvs Exp $
 */

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

/*
General Description:
This file is the 12 MHz version of the asssembler part of the USB driver. It
requires a 12 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!


Timing constraints according to spec (in bit times):
timing subject                                      min max    CPUcycles
---------------------------------------------------------------------------
EOP of OUT/SETUP to sync pattern of DATA0 (both rx) 2   16     16-128
EOP of IN to sync pattern of DATA0 (rx, then tx)    2   7.5    16-60
DATAx (rx) to ACK/NAK/STALL (tx)                    2   7.5    16-60
*/

;Software-receiver engine. Strict timing! Don't change unless you can preserve timing!
;interrupt response time: 4 cycles + insn running = 7 max if interrupts always enabled
;max allowable interrupt latency: 34 cycles -> max 25 cycles interrupt disable
;max stack usage: [ret(2), YL, SREG, YH, shift, x1, x2, x3, cnt, x4] = 11 bytes
;Numbers in brackets are maximum cycles since SOF.
USB_INTR_VECTOR:
;order of registers pushed: YL, SREG [sofError], YH, shift, x1, x2, x3, cnt
    push    YL              ;2 [35] push only what is necessary to sync with edge ASAP
    in      YL, SREG        ;1 [37]
    push    YL              ;2 [39]
;----------------------------------------------------------------------------
; 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
;first part has no timeout because it waits for IDLE or SE1 (== disconnected)
waitForJ:
    sbis    USBIN, USBMINUS ;1 [40] wait for D- == 1
    rjmp    waitForJ        ;2
waitForK:
;The following code results in a sampling window of 1/4 bit which meets the spec.
    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 */
    rjmp    sofError
foundK:
;{3, 5} after falling D- edge, average delay: 4 cycles [we want 4 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    YH                  ;2 [2]
    lds     YL, usbInputBufOffset;2 [4]
    clr     YH                  ;1 [5]
    subi    YL, lo8(-(usbRxBuf));1 [6]
    sbci    YH, hi8(-(usbRxBuf));1 [7]

    sbis    USBIN, USBMINUS ;1 [8] we want two bits K [sample 1 cycle too early]
    rjmp    haveTwoBitsK    ;2 [10]
    pop     YH              ;2 [11] undo the push from before
    rjmp    waitForK        ;2 [13] this was not the end of sync, retry
haveTwoBitsK:
;----------------------------------------------------------------------------
; push more registers and initialize values while we sample the first bits:
;----------------------------------------------------------------------------
    push    shift           ;2 [16]
    push    x1              ;2 [12]
    push    x2              ;2 [14]

    in      x1, USBIN       ;1 [17] <-- sample bit 0
    ldi     shift, 0xff     ;1 [18]
    bst     x1, USBMINUS    ;1 [19]
    bld     shift, 0        ;1 [20]
    push    x3              ;2 [22]
    push    cnt             ;2 [24]
    
    in      x2, USBIN       ;1 [25] <-- sample bit 1
    ser     x3              ;1 [26] [inserted init instruction]
    eor     x1, x2          ;1 [27]
    bst     x1, USBMINUS    ;1 [28]
    bld     shift, 1        ;1 [29]
    ldi     cnt, USB_BUFSIZE;1 [30] [inserted init instruction]
    rjmp    rxbit2          ;2 [32]

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

unstuff0:               ;1 (branch taken)
    andi    x3, ~0x01   ;1 [15]
    mov     x1, x2      ;1 [16] x2 contains last sampled (stuffed) bit
    in      x2, USBIN   ;1 [17] <-- sample bit 1 again
    ori     shift, 0x01 ;1 [18]
    rjmp    didUnstuff0 ;2 [20]

unstuff1:               ;1 (branch taken)
    mov     x2, x1      ;1 [21] x1 contains last sampled (stuffed) bit
    andi    x3, ~0x02   ;1 [22]
    ori     shift, 0x02 ;1 [23]
    nop                 ;1 [24]
    in      x1, USBIN   ;1 [25] <-- sample bit 2 again
    rjmp    didUnstuff1 ;2 [27]

unstuff2:               ;1 (branch taken)
    andi    x3, ~0x04   ;1 [29]
    ori     shift, 0x04 ;1 [30]
    mov     x1, x2      ;1 [31] x2 contains last sampled (stuffed) bit
    nop                 ;1 [32]
    in      x2, USBIN   ;1 [33] <-- sample bit 3
    rjmp    didUnstuff2 ;2 [35]

unstuff3:               ;1 (branch taken)
    in      x2, USBIN   ;1 [34] <-- sample stuffed bit 3 [one cycle too late]
    andi    x3, ~0x08   ;1 [35]
    ori     shift, 0x08 ;1 [36]
    rjmp    didUnstuff3 ;2 [38]

unstuff4:               ;1 (branch taken)
    andi    x3, ~0x10   ;1 [40]
    in      x1, USBIN   ;1 [41] <-- sample stuffed bit 4
    ori     shift, 0x10 ;1 [42]
    rjmp    didUnstuff4 ;2 [44]

unstuff5:               ;1 (branch taken)
    andi    x3, ~0x20   ;1 [48]
    in      x2, USBIN   ;1 [49] <-- sample stuffed bit 5
    ori     shift, 0x20 ;1 [50]
    rjmp    didUnstuff5 ;2 [52]

unstuff6:               ;1 (branch taken)
    andi    x3, ~0x40   ;1 [56]
    in      x1, USBIN   ;1 [57] <-- sample stuffed bit 6
    ori     shift, 0x40 ;1 [58]
    rjmp    didUnstuff6 ;2 [60]

; extra jobs done during bit interval:
; bit 0:    store, clear [SE0 is unreliable here due to bit dribbling in hubs]
; bit 1:    se0 check
; bit 2:    overflow check
; bit 3:    recovery from delay [bit 0 tasks took too long]
; bit 4:    none
; bit 5:    none
; bit 6:    none
; bit 7:    jump, eor
rxLoop:
    eor     x3, shift   ;1 [0] reconstruct: x3 is 0 at bit locations we changed, 1 at others
    in      x1, USBIN   ;1 [1] <-- sample bit 0
    st      y+, x3      ;2 [3] store data
    ser     x3          ;1 [4]
    nop                 ;1 [5]
    eor     x2, x1      ;1 [6]
    bst     x2, USBMINUS;1 [7]
    bld     shift, 0    ;1 [8]
    in      x2, USBIN   ;1 [9] <-- sample bit 1 (or possibly bit 0 stuffed)
    andi    x2, USBMASK ;1 [10]
    breq    se0         ;1 [11] SE0 check for bit 1
    andi    shift, 0xf9 ;1 [12]
didUnstuff0:
    breq    unstuff0    ;1 [13]
    eor     x1, x2      ;1 [14]
    bst     x1, USBMINUS;1 [15]
    bld     shift, 1    ;1 [16]
rxbit2:
    in      x1, USBIN   ;1 [17] <-- sample bit 2 (or possibly bit 1 stuffed)
    andi    shift, 0xf3 ;1 [18]
    breq    unstuff1    ;1 [19] do remaining work for bit 1
didUnstuff1:
    subi    cnt, 1      ;1 [20]
    brcs    overflow    ;1 [21] loop control
    eor     x2, x1      ;1 [22]
    bst     x2, USBMINUS;1 [23]
    bld     shift, 2    ;1 [24]
    in      x2, USBIN   ;1 [25] <-- sample bit 3 (or possibly bit 2 stuffed)
    andi    shift, 0xe7 ;1 [26]
    breq    unstuff2    ;1 [27]
didUnstuff2:
    eor     x1, x2      ;1 [28]
    bst     x1, USBMINUS;1 [29]
    bld     shift, 3    ;1 [30]
didUnstuff3:
    andi    shift, 0xcf ;1 [31]
    breq    unstuff3    ;1 [32]
    in      x1, USBIN   ;1 [33] <-- sample bit 4
    eor     x2, x1      ;1 [34]
    bst     x2, USBMINUS;1 [35]
    bld     shift, 4    ;1 [36]
didUnstuff4:
    andi    shift, 0x9f ;1 [37]
    breq    unstuff4    ;1 [38]
    nop2                ;2 [40]
    in      x2, USBIN   ;1 [41] <-- sample bit 5
    eor     x1, x2      ;1 [42]
    bst     x1, USBMINUS;1 [43]
    bld     shift, 5    ;1 [44]
didUnstuff5:
    andi    shift, 0x3f ;1 [45]
    breq    unstuff5    ;1 [46]
    nop2                ;2 [48]
    in      x1, USBIN   ;1 [49] <-- sample bit 6
    eor     x2, x1      ;1 [50]
    bst     x2, USBMINUS;1 [51]
    bld     shift, 6    ;1 [52]
didUnstuff6:
    cpi     shift, 0x02 ;1 [53]
    brlo    unstuff6    ;1 [54]
    nop2                ;2 [56]
    in      x2, USBIN   ;1 [57] <-- sample bit 7
    eor     x1, x2      ;1 [58]
    bst     x1, USBMINUS;1 [59]
    bld     shift, 7    ;1 [60]
didUnstuff7:
    cpi     shift, 0x04 ;1 [61]
    brsh    rxLoop      ;2 [63] loop control
unstuff7:
    andi    x3, ~0x80   ;1 [63]
    ori     shift, 0x80 ;1 [64]
    in      x2, USBIN   ;1 [65] <-- sample stuffed bit 7
    nop                 ;1 [66]
    rjmp    didUnstuff7 ;2 [68]


;----------------------------------------------------------------------------
; Processing of received packet (numbers in brackets are cycles after end of SE0)
;----------------------------------------------------------------------------
;This is the only non-error exit point for the software receiver loop
;we don't check any CRCs here because there is no time left.
#define token   x1
se0:                            ;  [0]
    subi    cnt, USB_BUFSIZE    ;1 [1]
    neg     cnt                 ;1 [2]
    cpi     cnt, 3              ;1 [3]
    ldi     x2, 1<<USB_INTR_PENDING_BIT ;1 [4]
    USB_STORE_PENDING(x2)       ;1 [5] clear pending intr and check flag later. SE0 should be over.
    brlo    doReturn            ;1 [6] this is probably an ACK, NAK or similar packet
    sub     YL, cnt             ;1 [7]
    sbci    YH, 0               ;1 [8]
    ld      token, y            ;2 [10]
    cpi     token, USBPID_DATA0 ;1 [11]
    breq    handleData          ;1 [12]
    cpi     token, USBPID_DATA1 ;1 [13]
    breq    handleData          ;1 [14]
    ldd     x2, y+1             ;2 [16] ADDR and 1 bit endpoint number
    mov     x3, x2              ;1 [17] store for endpoint number
    andi    x2, 0x7f            ;1 [18] x2 is now ADDR
    lds     shift, usbDeviceAddr;2 [20]
    cp      x2, shift           ;1 [21]
overflow:                       ; This is a hack: brcs overflow will never have Z flag set
    brne    ignorePacket        ;1 [22] packet for different address
    cpi     token, USBPID_IN    ;1 [23]
    breq    handleIn            ;1 [24]
    cpi     token, USBPID_SETUP ;1 [25]
    breq    handleSetupOrOut    ;1 [26]
    cpi     token, USBPID_OUT   ;1 [27]
    breq    handleSetupOrOut    ;1 [28]
;   rjmp    ignorePacket        ;fallthrough, should not happen anyway.

ignorePacket:
    clr     shift
    sts     usbCurrentTok, shift
doReturn:
    pop     cnt
    pop     x3
    pop     x2
    pop     x1
    pop     shift
    pop     YH
sofError:
    pop     YL
    out     SREG, YL
    pop     YL
    reti

#if USB_CFG_HAVE_INTRIN_ENDPOINT && USB_CFG_HAVE_INTRIN_ENDPOINT3
handleIn3:                      ;1 [38] (branch taken)
    lds     cnt, usbTxLen3      ;2 [40]
    sbrc    cnt, 4              ;2 [42]
    rjmp    sendCntAndReti      ;0 43 + 17 = 60 until SOP
    sts     usbTxLen3, x1       ;2 [44] x1 == USBPID_NAK from above
    ldi     YL, lo8(usbTxBuf3)  ;1 [45]
    ldi     YH, hi8(usbTxBuf3)  ;1 [46]
    rjmp    usbSendAndReti      ;2 [48] + 13 = 61 until SOP (violates the spec by 1 cycle)
#endif

;Setup and Out are followed by a data packet two bit times (16 cycles) after
;the end of SE0. The sync code allows up to 40 cycles delay from the start of
;the sync pattern until the first bit is sampled. That's a total of 56 cycles.
handleSetupOrOut:               ;1 [29] (branch taken)
#if USB_CFG_IMPLEMENT_FN_WRITEOUT   /* if we have data for second OUT endpoint, set usbCurrentTok to -1 */
    sbrc    x3, 7               ;1 [30] skip if endpoint 0
    ldi     token, -1           ;1 [31] indicate that this is endpoint 1 OUT
#endif
    sts     usbCurrentTok, token;2 [33]
    pop     cnt                 ;2 [35]
    pop     x3                  ;2 [37]
    pop     x2                  ;2 [39]
    pop     x1                  ;2 [41]
    pop     shift               ;2 [43]
    pop     YH                  ;2 [45]
    USB_LOAD_PENDING(YL)        ;1 [46]
    sbrc    YL, USB_INTR_PENDING_BIT;1 [47] check whether data is already arriving
    rjmp    waitForJ            ;2 [49] save the pops and pushes -- a new interrupt is aready pending
    rjmp    sofError            ;2 not an error, but it does the pops and reti we want


handleData:                     ;1 [15] (branch taken)
    lds     token, usbCurrentTok;2 [17]
    tst     token               ;1 [18]
    breq    doReturn            ;1 [19]
    lds     x2, usbRxLen        ;2 [21]
    tst     x2                  ;1 [22]
    brne    sendNakAndReti      ;1 [23]
; 2006-03-11: The following two lines fix a problem where the device was not
; recognized if usbPoll() was called less frequently than once every 4 ms.
    cpi     cnt, 4              ;1 [24] zero sized data packets are status phase only -- ignore and ack
    brmi    sendAckAndReti      ;1 [25] keep rx buffer clean -- we must not NAK next SETUP
    sts     usbRxLen, cnt       ;2 [27] store received data, swap buffers
    sts     usbRxToken, token   ;2 [29]
    lds     x2, usbInputBufOffset;2 [31] swap buffers
    ldi     cnt, USB_BUFSIZE    ;1 [32]
    sub     cnt, x2             ;1 [33]
    sts     usbInputBufOffset, cnt;2 [35] buffers now swapped
    rjmp    sendAckAndReti      ;2 [37] + 19 = 56 until SOP

handleIn:                       ;1 [25] (branch taken)
;We don't send any data as long as the C code has not processed the current
;input data and potentially updated the output data. That's more efficient
;in terms of code size than clearing the tx buffers when a packet is received.
    lds     x1, usbRxLen        ;2 [27]
    cpi     x1, 1               ;1 [28] negative values are flow control, 0 means "buffer free"
    brge    sendNakAndReti      ;1 [29] unprocessed input packet?
    ldi     x1, USBPID_NAK      ;1 [30] prepare value for usbTxLen
#if USB_CFG_HAVE_INTRIN_ENDPOINT
    sbrc    x3, 7               ;2 [33] x3 contains addr + endpoint
    rjmp    handleIn1           ;0
#endif
    lds     cnt, usbTxLen       ;2 [34]
    sbrc    cnt, 4              ;2 [36] all handshake tokens have bit 4 set
    rjmp    sendCntAndReti      ;0 37 + 17 = 54 until SOP
    sts     usbTxLen, x1        ;2 [38] x1 == USBPID_NAK from above
    ldi     YL, lo8(usbTxBuf)   ;1 [39]
    ldi     YH, hi8(usbTxBuf)   ;1 [40]
    rjmp    usbSendAndReti      ;2 [42] + 14 = 56 until SOP

; Comment about when to set usbTxLen to USBPID_NAK:
; We should set it back when we receive the ACK from the host. This would
; be simple to implement: One static variable which stores whether the last
; tx was for endpoint 0 or 1 and a compare in the receiver to distinguish the
; ACK. However, we set it back immediately when we send the package,
; assuming that no error occurs and the host sends an ACK. We save one byte
; RAM this way and avoid potential problems with endless retries. The rest of
; the driver assumes error-free transfers anyway.

#if USB_CFG_HAVE_INTRIN_ENDPOINT    /* placed here due to relative jump range */
handleIn1:                      ;1 [33] (branch taken)
#if USB_CFG_HAVE_INTRIN_ENDPOINT3
; 2006-06-10 as suggested by O.Tamura: support second INTR IN / BULK IN endpoint
    ldd     x2, y+2             ;2 [35]
    sbrc    x2, 0               ;2 [37]
    rjmp    handleIn3           ;0
#endif
    lds     cnt, usbTxLen1      ;2 [39]
    sbrc    cnt, 4              ;2 [41] all handshake tokens have bit 4 set
    rjmp    sendCntAndReti      ;0 42 + 17 = 59 until SOP
    sts     usbTxLen1, x1       ;2 [43] x1 == USBPID_NAK from above
    ldi     YL, lo8(usbTxBuf1)  ;1 [44]
    ldi     YH, hi8(usbTxBuf1)  ;1 [45]
    rjmp    usbSendAndReti      ;2 [47] + 13 = 60 until SOP
#endif


;----------------------------------------------------------------------------
; Transmitting data
;----------------------------------------------------------------------------

bitstuff0:                  ;1 (for branch taken)
    eor     x1, x4          ;1
    ldi     x2, 0           ;1
    out     USBOUT, x1      ;1 <-- out
    rjmp    didStuff0       ;2 branch back 2 cycles earlier
bitstuff1:                  ;1 (for branch taken)
    eor     x1, x4          ;1
    rjmp    didStuff1       ;2 we know that C is clear, jump back to do OUT and ror 0 into x2
bitstuff2:                  ;1 (for branch taken)
    eor     x1, x4          ;1
    rjmp    didStuff2       ;2 jump back 4 cycles earlier and do out and ror 0 into x2
bitstuff3:                  ;1 (for branch taken)
    eor     x1, x4          ;1
    rjmp    didStuff3       ;2 jump back earlier and ror 0 into x2
bitstuff4:                  ;1 (for branch taken)
    eor     x1, x4          ;1
    ldi     x2, 0           ;1
    out     USBOUT, x1      ;1 <-- out
    rjmp    didStuff4       ;2 jump back 2 cycles earlier

sendNakAndReti:                 ;0 [-19] 19 cycles until SOP
    ldi     x3, USBPID_NAK      ;1 [-18]
    rjmp    usbSendX3           ;2 [-16]
sendAckAndReti:                 ;0 [-19] 19 cycles until SOP
    ldi     x3, USBPID_ACK      ;1 [-18]
    rjmp    usbSendX3           ;2 [-16]
sendCntAndReti:                 ;0 [-17] 17 cycles until SOP
    mov     x3, cnt             ;1 [-16]
usbSendX3:                      ;0 [-16]
    ldi     YL, 20              ;1 [-15] 'x3' is R20
    ldi     YH, 0               ;1 [-14]
    ldi     cnt, 2              ;1 [-13]
;   rjmp    usbSendAndReti      fallthrough

; USB spec says:
; idle = J
; J = (D+ = 0), (D- = 1) or USBOUT = 0x01
; K = (D+ = 1), (D- = 0) or USBOUT = 0x02
; Spec allows 7.5 bit times from EOP to SOP for replies (= 60 cycles)

;usbSend:
;pointer to data in 'Y'
;number of bytes in 'cnt' -- including sync byte
;uses: x1...x4, shift, cnt, Y
;Numbers in brackets are time since first bit of sync pattern is sent
usbSendAndReti:             ;0 [-13] timing: 13 cycles until SOP
    in      x2, USBDDR      ;1 [-12]
    ori     x2, USBMASK     ;1 [-11]
    sbi     USBOUT, USBMINUS;2 [-9] prepare idle state; D+ and D- must have been 0 (no pullups)
    in      x1, USBOUT      ;1 [-8] port mirror for tx loop
    out     USBDDR, x2      ;1 [-7] <- acquire bus
; need not init x2 (bitstuff history) because sync starts with 0
    push    x4              ;2 [-5]
    ldi     x4, USBMASK     ;1 [-4] exor mask
    ldi     shift, 0x80     ;1 [-3] sync byte is first byte sent
txLoop:                     ;       [62]
    sbrs    shift, 0        ;1 [-2] [62]
    eor     x1, x4          ;1 [-1] [63]
    out     USBOUT, x1      ;1 [0] <-- out bit 0
    ror     shift           ;1 [1]
    ror     x2              ;1 [2]
didStuff0:
    cpi     x2, 0xfc        ;1 [3]
    brsh    bitstuff0       ;1 [4]
    sbrs    shift, 0        ;1 [5]
    eor     x1, x4          ;1 [6]
    ror     shift           ;1 [7]
didStuff1:
    out     USBOUT, x1      ;1 [8] <-- out bit 1
    ror     x2              ;1 [9]
    cpi     x2, 0xfc        ;1 [10]
    brsh    bitstuff1       ;1 [11]
    sbrs    shift, 0        ;1 [12]
    eor     x1, x4          ;1 [13]
    ror     shift           ;1 [14]
didStuff2:
    ror     x2              ;1 [15]
    out     USBOUT, x1      ;1 [16] <-- out bit 2
    cpi     x2, 0xfc        ;1 [17]
    brsh    bitstuff2       ;1 [18]
    sbrs    shift, 0        ;1 [19]
    eor     x1, x4          ;1 [20]
    ror     shift           ;1 [21]
didStuff3:
    ror     x2              ;1 [22]
    cpi     x2, 0xfc        ;1 [23]
    out     USBOUT, x1      ;1 [24] <-- out bit 3
    brsh    bitstuff3       ;1 [25]
    nop2                    ;2 [27]
    ld      x3, y+          ;2 [29]
    sbrs    shift, 0        ;1 [30]
    eor     x1, x4          ;1 [31]
    out     USBOUT, x1      ;1 [32] <-- out bit 4
    ror     shift           ;1 [33]
    ror     x2              ;1 [34]
didStuff4:
    cpi     x2, 0xfc        ;1 [35]
    brsh    bitstuff4       ;1 [36]
    sbrs    shift, 0        ;1 [37]
    eor     x1, x4          ;1 [38]
    ror     shift           ;1 [39]
didStuff5:
    out     USBOUT, x1      ;1 [40] <-- out bit 5
    ror     x2              ;1 [41]
    cpi     x2, 0xfc        ;1 [42]
    brsh    bitstuff5       ;1 [43]
    sbrs    shift, 0        ;1 [44]
    eor     x1, x4          ;1 [45]
    ror     shift           ;1 [46]
didStuff6:
    ror     x2              ;1 [47]
    out     USBOUT, x1      ;1 [48] <-- out bit 6
    cpi     x2, 0xfc        ;1 [49]
    brsh    bitstuff6       ;1 [50]
    sbrs    shift, 0        ;1 [51]
    eor     x1, x4          ;1 [52]
    ror     shift           ;1 [53]
didStuff7:
    ror     x2              ;1 [54]
    cpi     x2, 0xfc        ;1 [55]
    out     USBOUT, x1      ;1 [56] <-- out bit 7
    brsh    bitstuff7       ;1 [57]
    mov     shift, x3       ;1 [58]
    dec     cnt             ;1 [59]
    brne    txLoop          ;1/2 [60/61]
;make SE0:
    cbr     x1, USBMASK     ;1 [61] prepare SE0 [spec says EOP may be 15 to 18 cycles]
    pop     x4              ;2 [63]
;brackets are cycles from start of SE0 now
    out     USBOUT, x1      ;1 [0] <-- out SE0 -- from now 2 bits = 16 cycles until bus idle
    nop2                    ;2 [2]
;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
    lds     x2, usbNewDeviceAddr;2 [4]
    subi    YL, 20 + 2      ;1 [5]
    sbci    YH, 0           ;1 [6]
    breq    skipAddrAssign  ;2 [8]
    sts     usbDeviceAddr, x2;0  if not skipped: SE0 is one cycle longer
skipAddrAssign:
;end of usbDeviceAddress transfer
    ldi     x2, 1<<USB_INTR_PENDING_BIT;1 [9] int0 occurred during TX -- clear pending flag
    USB_STORE_PENDING(x2)   ;1 [10]
    ori     x1, USBIDLE     ;1 [11]
    in      x2, USBDDR      ;1 [12]
    cbr     x2, USBMASK     ;1 [13] set both pins to input
    mov     x3, x1          ;1 [14]
    cbr     x3, USBMASK     ;1 [15] configure no pullup on both pins
    out     USBOUT, x1      ;1 [16] <-- out J (idle) -- end of SE0 (EOP signal)
    out     USBDDR, x2      ;1 [17] <-- release bus now
    out     USBOUT, x3      ;1 [18] <-- ensure no pull-up resistors are active
    rjmp    doReturn

bitstuff5:                  ;1 (for branch taken)
    eor     x1, x4          ;1
    rjmp    didStuff5       ;2 same trick as above...
bitstuff6:                  ;1 (for branch taken)
    eor     x1, x4          ;1
    rjmp    didStuff6       ;2 same trick as above...
bitstuff7:                  ;1 (for branch taken)
    eor     x1, x4          ;1
    rjmp    didStuff7       ;2 same trick as above...

