usbdrvasm12.inc

USBasp

/* Name: usbdrvasm12.inc
 * 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), GNU GPL v3 or proprietary (CommercialLicense.txt)
 * This Revision: $Id: usbdrvasm12.inc 692 2008-11-07 15:07:40Z cs $
 */

/* 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
;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
    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:
;{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