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706 lines
30 KiB
706 lines
30 KiB
/* Name: usbdrvasm18.inc 

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

* Author: Lukas Schrittwieser (based on 20 MHz usbdrvasm20.inc by Jeroen Benschop) 

* Creation Date: 20090120 

* Tabsize: 4 

* Copyright: (c) 2008 by Lukas Schrittwieser and 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 18 MHz version of the asssembler part of the USB driver. It 

requires a 18 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, [sofError], bitcnt(x5), shift, x1, x2, x3, x4, cnt, ZL, ZH] = 14 bytes 

;nominal frequency: 18 MHz > 12 cycles per bit 

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

; when instruction starts 

;register use in receive loop to receive the data bytes: 

; shift assembles the byte currently being received 

; x1 holds the D+ and D line state 

; x2 holds the previous line state 

; cnt holds the number of bytes left in the receive buffer 

; x3 holds the higher crc byte (see algorithm below) 

; x4 is used as temporary register for the crc algorithm 

; x5 is used for unstuffing: when unstuffing the last received bit is inverted in shift (to prevent further 

; unstuffing calls. In the same time the corresponding bit in x5 is cleared to mark the bit as beening iverted 

; zl lower crc value and crc table index 

; zh used for crc table accesses 



; 

; CRC mods: 

; table driven crc checker, Z points to table in prog space 

; ZL is the lower crc byte, x3 is the higher crc byte 

; x4 is used as temp register to store different results 

; the initialization of the crc register is not 0xFFFF but 0xFE54. This is because during the receipt of the 

; first data byte an virtual zero data byte is added to the crc register, this results in the correct initial 

; value of 0xFFFF at beginning of the second data byte before the first data byte is added to the crc. 

; The magic number 0xFE54 results form the crc table: At tabH[0x54] = 0xFF = crcH (required) and 

; tabL[0x54] = 0x01 > crcL = 0x01 xor 0xFE = 0xFF 

; bitcnt is renamed to x5 and is used for unstuffing purposes, the unstuffing works like in the 12MHz version 

; 

; CRC algorithm: 

; The crc register is formed by x3 (higher byte) and ZL (lower byte). The algorithm uses a 'reversed' form 

; i.e. that it takes the least significant bit first and shifts to the right. So in fact the highest order 

; bit seen from the polynomial devision point of view is the lsb of ZL. (If this sounds strange to you i 

; propose a research on CRC :) ) 

; Each data byte received is xored to ZL, the lower crc byte. This byte now builds the crc 

; table index. Next the new high byte is loaded from the table and stored in x4 until we have space in x3 

; (its destination). 

; Afterwards the lower table is loaded from the table and stored in ZL (the old index is overwritten as 

; we don't need it anymore. In fact this is a right shift by 8 bits.) Now the old crc high value is xored 

; to ZL, this is the second shift of the old crc value. Now x4 (the temp reg) is moved to x3 and the crc 

; calculation is done. 

; Prior to the first byte the two CRC register have to be initialized to 0xFFFF (as defined in usb spec) 

; however the crc engine also runs during the receipt of the first byte, therefore x3 and zl are initialized 

; to a magic number which results in a crc value of 0xFFFF after the first complete byte. 

; 

; This algorithm is split into the extra cycles of the different bits: 

; bit7: XOR the received byte to ZL 

; bit5: load the new high byte to x4 

; bit6: load the lower xor byte from the table, xor zl and x3, store result in zl (=the new crc low value) 

; move x4 (the new high byte) to x3, the crc value is ready 

; 





macro POP_STANDARD ; 18 cycles 

pop ZH 

pop ZL 

pop cnt 

pop x5 

pop x3 

pop x2 

pop x1 

pop shift 

pop x4 

endm 

macro POP_RETI ; 7 cycles 

pop YH 

pop YL 

out SREG, YL 

pop YL 

endm 



macro CRC_CLEANUP_AND_CHECK 

; the last byte has already been xored with the lower crc byte, we have to do the table lookup and xor 

; x3 is the higher crc byte, zl the lower one 

ldi ZH, hi8(usbCrcTableHigh);[+1] get the new high byte from the table 

lpm x2, Z ;[+2][+3][+4] 

ldi ZH, hi8(usbCrcTableLow);[+5] get the new low xor byte from the table 

lpm ZL, Z ;[+6][+7][+8] 

eor ZL, x3 ;[+7] xor the old high byte with the value from the table, x2:ZL now holds the crc value 

cpi ZL, 0x01 ;[+8] if the crc is ok we have a fixed remainder value of 0xb001 in x2:ZL (see usb spec) 

brne ignorePacket ;[+9] detected a crc fault > paket is ignored and retransmitted by the host 

cpi x2, 0xb0 ;[+10] 

brne ignorePacket ;[+11] detected a crc fault > paket is ignored and retransmitted by the host 

endm 





USB_INTR_VECTOR: 

;order of registers pushed: YL, SREG, YH, [sofError], x4, shift, x1, x2, x3, x5, cnt, ZL, ZH 

push YL ;[28] push only what is necessary to sync with edge ASAP 

in YL, SREG ;[26] 

push YL ;[25] 

push YH ;[23] 

; 

; 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 ;[17] 

rjmp foundK ;[16] 

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 

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: ;[15] 

;{3, 5} after falling D edge, average delay: 4 cycles 

;bit0 should be at 30 (2.5 bits) for center sampling. Currently at 4 so 26 cylces till bit 0 sample 

;use 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 x4 ;[14] 

; [] ;[13] 

lds YL, usbInputBufOffset;[12] used to toggle the two usb receive buffers 

; [] ;[11] 

clr YH ;[10] 

subi YL, lo8((usbRxBuf));[9] [rx loop init] 

sbci YH, hi8((usbRxBuf));[8] [rx loop init] 

push shift ;[7] 

; [] ;[6] 

ldi shift, 0x80 ;[5] the last bit is the end of byte marker for the pid receiver loop 

clc ;[4] the carry has to be clear for receipt of pid bit 0 

sbis USBIN, USBMINUS ;[3] we want two bits K (sample 3 cycles too early) 

rjmp haveTwoBitsK ;[2] 

pop shift ;[1] undo the push from before 

pop x4 ;[1] 

rjmp waitForK ;[3] this was not the end of sync, retry 

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

; bit times (= 24 cycles). 



; 

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

; 

haveTwoBitsK: 

push x1 ;[0] 

push x2 ;[2] 

push x3 ;[4] crc high byte 

ldi x2, 1<<USBPLUS ;[6] [rx loop init] current line state is K state. D+=="1", D=="0" 

push x5 ;[7] 

push cnt ;[9] 

ldi cnt, USB_BUFSIZE ;[11] 





; 

; receives the pid byte 

; there is no real unstuffing algorithm implemented here as a stuffing bit is impossible in the pid byte. 

; That's because the last four bits of the byte are the inverted of the first four bits. If we detect a 

; unstuffing condition something went wrong and abort 

; shift has to be initialized to 0x80 

; 



; pid bit 0  used for even more register saving (we need the z pointer) 

in x1, USBIN ;[0] sample line state 

andi x1, USBMASK ;[1] filter only D+ and D bits 

eor x2, x1 ;[2] generate inverted of actual bit 

sbrc x2, USBMINUS ;[3] if the bit is set we received a zero 

sec ;[4] 

ror shift ;[5] we perform no unstuffing check here as this is the first bit 

mov x2, x1 ;[6] 

push ZL ;[7] 

;[8] 

push ZH ;[9] 

;[10] 

ldi x3, 0xFE ;[11] x3 is the high order crc value 





bitloopPid: 

in x1, USBIN ;[0] sample line state 

andi x1, USBMASK ;[1] filter only D+ and D bits 

breq nse0 ;[2] both lines are low so handle se0 

eor x2, x1 ;[3] generate inverted of actual bit 

sbrc x2, USBMINUS ;[4] set the carry if we received a zero 

sec ;[5] 

ror shift ;[6] 

ldi ZL, 0x54 ;[7] ZL is the low order crc value 

ser x4 ;[8] the is no bit stuffing check here as the pid bit can't be stuffed. if so 

; some error occured. In this case the paket is discarded later on anyway. 

mov x2, x1 ;[9] prepare for the next cycle 

brcc bitloopPid ;[10] while 0s drop out of shift we get the next bit 

eor x4, shift ;[11] invert all bits in shift and store result in x4 



; 

; receives data bytes and calculates the crc 

; the last USBIN state has to be in x2 

; this is only the first half, due to branch distanc limitations the second half of the loop is near the end 

; of this asm file 

; 



rxDataStart: 

in x1, USBIN ;[0] sample line state (note: a se0 check is not useful due to bit dribbling) 

ser x5 ;[1] prepare the unstuff marker register 

eor x2, x1 ;[2] generates the inverted of the actual bit 

bst x2, USBMINUS ;[3] copy the bit from x2 

bld shift, 0 ;[4] and store it in shift 

mov x2, shift ;[5] make a copy of shift for unstuffing check 

andi x2, 0xF9 ;[6] mask the last six bits, if we got six zeros (which are six ones in fact) 

breq unstuff0 ;[7] then Z is set now and we branch to the unstuffing handler 

didunstuff0: 

subi cnt, 1 ;[8] cannot use dec because it doesn't affect the carry flag 

brcs nOverflow ;[9] Too many bytes received. Ignore packet 

st Y+, x4 ;[10] store the last received byte 

;[11] st needs two cycles 



; bit1 

in x2, USBIN ;[0] sample line state 

andi x1, USBMASK ;[1] check for se0 during bit 0 

breq nse0 ;[2] 

andi x2, USBMASK ;[3] check se0 during bit 1 

breq nse0 ;[4] 

eor x1, x2 ;[5] 

bst x1, USBMINUS ;[6] 

bld shift, 1 ;[7] 

mov x1, shift ;[8] 

andi x1, 0xF3 ;[9] 

breq unstuff1 ;[10] 

didunstuff1: 

nop ;[11] 



; bit2 

in x1, USBIN ;[0] sample line state 

andi x1, USBMASK ;[1] check for se0 (as there is nothing else to do here 

breq nOverflow ;[2] 

eor x2, x1 ;[3] generates the inverted of the actual bit 

bst x2, USBMINUS ;[4] 

bld shift, 2 ;[5] store the bit 

mov x2, shift ;[6] 

andi x2, 0xE7 ;[7] if we have six zeros here (which means six 1 in the stream) 

breq unstuff2 ;[8] the next bit is a stuffing bit 

didunstuff2: 

nop2 ;[9] 

;[10] 

nop ;[11] 



; bit3 

in x2, USBIN ;[0] sample line state 

andi x2, USBMASK ;[1] check for se0 

breq nOverflow ;[2] 

eor x1, x2 ;[3] 

bst x1, USBMINUS ;[4] 

bld shift, 3 ;[5] 

mov x1, shift ;[6] 

andi x1, 0xCF ;[7] 

breq unstuff3 ;[8] 

didunstuff3: 

nop ;[9] 

rjmp rxDataBit4 ;[10] 

;[11] 



; the avr branch instructions allow an offset of +63 insturction only, so we need this 

; 'local copy' of se0 

nse0: 

rjmp se0 ;[4] 

;[5] 

; the same same as for se0 is needed for overflow and StuffErr 

nOverflow: 

stuffErr: 

rjmp overflow 





unstuff0: ;[8] this is the branch delay of breq unstuffX 

andi x1, USBMASK ;[9] do an se0 check here (if the last crc byte ends with 5 one's we might end up here 

breq didunstuff0 ;[10] event tough the message is complete > jump back and store the byte 

ori shift, 0x01 ;[11] invert the last received bit to prevent furhter unstuffing 

in x2, USBIN ;[0] we have some free cycles so we could check for bit stuffing errors 

andi x5, 0xFE ;[1] mark this bit as inverted (will be corrected before storing shift) 

eor x1, x2 ;[2] x1 and x2 have to be different because the stuff bit is always a zero 

andi x1, USBMASK ;[3] mask the interesting bits 

breq stuffErr ;[4] if the stuff bit is a 1bit something went wrong 

mov x1, x2 ;[5] the next bit expects the last state to be in x1 

rjmp didunstuff0 ;[6] 

;[7] jump delay of rjmp didunstuffX 



unstuff1: ;[11] this is the jump delay of breq unstuffX 

in x1, USBIN ;[0] we have some free cycles so we could check for bit stuffing errors 

ori shift, 0x02 ;[1] invert the last received bit to prevent furhter unstuffing 

andi x5, 0xFD ;[2] mark this bit as inverted (will be corrected before storing shift) 

eor x2, x1 ;[3] x1 and x2 have to be different because the stuff bit is always a zero 

andi x2, USBMASK ;[4] mask the interesting bits 

breq stuffErr ;[5] if the stuff bit is a 1bit something went wrong 

mov x2, x1 ;[6] the next bit expects the last state to be in x2 

nop2 ;[7] 

;[8] 

rjmp didunstuff1 ;[9] 

;[10] jump delay of rjmp didunstuffX 



unstuff2: ;[9] this is the jump delay of breq unstuffX 

ori shift, 0x04 ;[10] invert the last received bit to prevent furhter unstuffing 

andi x5, 0xFB ;[11] mark this bit as inverted (will be corrected before storing shift) 

in x2, USBIN ;[0] we have some free cycles so we could check for bit stuffing errors 

eor x1, x2 ;[1] x1 and x2 have to be different because the stuff bit is always a zero 

andi x1, USBMASK ;[2] mask the interesting bits 

breq stuffErr ;[3] if the stuff bit is a 1bit something went wrong 

mov x1, x2 ;[4] the next bit expects the last state to be in x1 

nop2 ;[5] 

;[6] 

rjmp didunstuff2 ;[7] 

;[8] jump delay of rjmp didunstuffX 



unstuff3: ;[9] this is the jump delay of breq unstuffX 

ori shift, 0x08 ;[10] invert the last received bit to prevent furhter unstuffing 

andi x5, 0xF7 ;[11] mark this bit as inverted (will be corrected before storing shift) 

in x1, USBIN ;[0] we have some free cycles so we could check for bit stuffing errors 

eor x2, x1 ;[1] x1 and x2 have to be different because the stuff bit is always a zero 

andi x2, USBMASK ;[2] mask the interesting bits 

breq stuffErr ;[3] if the stuff bit is a 1bit something went wrong 

mov x2, x1 ;[4] the next bit expects the last state to be in x2 

nop2 ;[5] 

;[6] 

rjmp didunstuff3 ;[7] 

;[8] jump delay of rjmp didunstuffX 







; the include has to be here due to branch distance restirctions 

#define __USE_CRC__ 

#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 

; 7.5 bit times is 90 cycles. ...there is plenty of time 





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 



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 ;[6] < acquire bus 

ldi x2, 0 ;[6] init x2 (bitstuff history) because sync starts with 0 

ldi x4, USBMASK ;[5] exor mask 

ldi shift, 0x80 ;[4] sync byte is first byte sent 

txByteLoop: 

ldi bitcnt, 0x40 ;[3]=[9] binary 01000000 

txBitLoop: ; the loop sends the first 7 bits of the byte 

sbrs shift, 0 ;[2]=[10] if we have to send a 1 don't change the line state 

eor x1, x4 ;[1]=[11] 

out USBOUT, x1 ;[0] 

ror shift ;[1] 

ror x2 ;[2] transfers the last sent bit to the stuffing history 

didStuffN: 

nop ;[3] 

nop ;[4] 

cpi x2, 0xfc ;[5] if we sent six consecutive ones 

brcc bitstuffN ;[6] 

lsr bitcnt ;[7] 

brne txBitLoop ;[8] restart the loop while the 1 is still in the bitcount 



; transmit bit 7 

sbrs shift, 0 ;[9] 

eor x1, x4 ;[10] 

didStuff7: 

ror shift ;[11] 

out USBOUT, x1 ;[0] transfer bit 7 to the pins 

ror x2 ;[1] move the bit into the stuffing history 

cpi x2, 0xfc ;[2] 

brcc bitstuff7 ;[3] 

ld shift, y+ ;[4] get next byte to transmit 

dec cnt ;[5] decrement byte counter 

brne txByteLoop ;[7] if we have more bytes start next one 

;[8] branch delay 



;make SE0: 

cbr x1, USBMASK ;[8] prepare SE0 [spec says EOP may be 25 to 30 cycles] 

lds x2, usbNewDeviceAddr;[9] 

lsl x2 ;[11] we compare with left shifted address 

out USBOUT, x1 ;[0] < out SE0  from now 2 bits = 24 cycles until bus idle 

subi YL, 20 + 2 ;[1] Only assign address on data packets, not ACK/NAK in x3 

sbci YH, 0 ;[2] 

;20060306: moved transfer of new address to usbDeviceAddr from CCode to asm: 

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

breq skipAddrAssign ;[3] 

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

skipAddrAssign: 

;end of usbDeviceAddress transfer 

ldi x2, 1<<USB_INTR_PENDING_BIT;[5] int0 occurred during TX  clear pending flag 

USB_STORE_PENDING(x2) ;[6] 

ori x1, USBIDLE ;[7] 

in x2, USBDDR ;[8] 

cbr x2, USBMASK ;[9] set both pins to input 

mov x3, x1 ;[10] 

cbr x3, USBMASK ;[11] configure no pullup on both pins 

ldi x4, 4 ;[12] 

se0Delay: 

dec x4 ;[13] [16] [19] [22] 

brne se0Delay ;[14] [17] [20] [23] 

out USBOUT, x1 ;[24] < out J (idle)  end of SE0 (EOP signal) 

out USBDDR, x2 ;[25] < release bus now 

out USBOUT, x3 ;[26] < ensure no pullup resistors are active 

rjmp doReturn 



bitstuffN: 

eor x1, x4 ;[8] generate a zero 

ldi x2, 0 ;[9] reset the bit stuffing history 

nop2 ;[10] 

out USBOUT, x1 ;[0] < send the stuffing bit 

rjmp didStuffN ;[1] 



bitstuff7: 

eor x1, x4 ;[5] 

ldi x2, 0 ;[6] reset bit stuffing history 

clc ;[7] fill a zero into the shift register 

rol shift ;[8] compensate for ror shift at branch destination 

rjmp didStuff7 ;[9] 

;[10] jump delay 



; 

; receives data bytes and calculates the crc 

; second half of the data byte receiver loop 

; most parts of the crc algorithm are here 

; 



nOverflow2: 

rjmp overflow 



rxDataBit4: 

in x1, USBIN ;[0] sample line state 

andi x1, USBMASK ;[1] check for se0 

breq nOverflow2 ;[2] 

eor x2, x1 ;[3] 

bst x2, USBMINUS ;[4] 

bld shift, 4 ;[5] 

mov x2, shift ;[6] 

andi x2, 0x9F ;[7] 

breq unstuff4 ;[8] 

didunstuff4: 

nop2 ;[9][10] 

nop ;[11] 



; bit5 

in x2, USBIN ;[0] sample line state 

ldi ZH, hi8(usbCrcTableHigh);[1] use the table for the higher byte 

eor x1, x2 ;[2] 

bst x1, USBMINUS ;[3] 

bld shift, 5 ;[4] 

mov x1, shift ;[5] 

andi x1, 0x3F ;[6] 

breq unstuff5 ;[7] 

didunstuff5: 

lpm x4, Z ;[8] load the higher crc xorbyte and store it for later use 

;[9] lpm needs 3 cycles 

;[10] 

ldi ZH, hi8(usbCrcTableLow);[11] load the lower crc xor byte adress 



; bit6 

in x1, USBIN ;[0] sample line state 

eor x2, x1 ;[1] 

bst x2, USBMINUS ;[2] 

bld shift, 6 ;[3] 

mov x2, shift ;[4] 

andi x2, 0x7E ;[5] 

breq unstuff6 ;[6] 

didunstuff6: 

lpm ZL, Z ;[7] load the lower xor crc byte 

;[8] lpm needs 3 cycles 

;[9] 

eor ZL, x3 ;[10] xor the old high crc byte with the low xorbyte 

mov x3, x4 ;[11] move the new high order crc value from temp to its destination 



; bit7 

in x2, USBIN ;[0] sample line state 

eor x1, x2 ;[1] 

bst x1, USBMINUS ;[2] 

bld shift, 7 ;[3] now shift holds the complete but inverted data byte 

mov x1, shift ;[4] 

andi x1, 0xFC ;[5] 

breq unstuff7 ;[6] 

didunstuff7: 

eor x5, shift ;[7] x5 marks all bits which have not been inverted by the unstuffing subs 

mov x4, x5 ;[8] keep a copy of the data byte it will be stored during next bit0 

eor ZL, x4 ;[9] feed the actual byte into the crc algorithm 

rjmp rxDataStart ;[10] next byte 

;[11] during the reception of the next byte this one will be fed int the crc algorithm 



unstuff4: ;[9] this is the jump delay of rjmp unstuffX 

ori shift, 0x10 ;[10] invert the last received bit to prevent furhter unstuffing 

andi x5, 0xEF ;[11] mark this bit as inverted (will be corrected before storing shift) 

in x2, USBIN ;[0] we have some free cycles so we could check for bit stuffing errors 

eor x1, x2 ;[1] x1 and x2 have to be different because the stuff bit is always a zero 

andi x1, USBMASK ;[2] mask the interesting bits 

breq stuffErr2 ;[3] if the stuff bit is a 1bit something went wrong 

mov x1, x2 ;[4] the next bit expects the last state to be in x1 

nop2 ;[5] 

;[6] 

rjmp didunstuff4 ;[7] 

;[8] jump delay of rjmp didunstuffX 



unstuff5: ;[8] this is the jump delay of rjmp unstuffX 

nop ;[9] 

ori shift, 0x20 ;[10] invert the last received bit to prevent furhter unstuffing 

andi x5, 0xDF ;[11] mark this bit as inverted (will be corrected before storing shift) 

in x1, USBIN ;[0] we have some free cycles so we could check for bit stuffing errors 

eor x2, x1 ;[1] x1 and x2 have to be different because the stuff bit is always a zero 

andi x2, USBMASK ;[2] mask the interesting bits 

breq stuffErr2 ;[3] if the stuff bit is a 1bit something went wrong 

mov x2, x1 ;[4] the next bit expects the last state to be in x2 

nop ;[5] 

rjmp didunstuff5 ;[6] 

;[7] jump delay of rjmp didunstuffX 



unstuff6: ;[7] this is the jump delay of rjmp unstuffX 

nop2 ;[8] 

;[9] 

ori shift, 0x40 ;[10] invert the last received bit to prevent furhter unstuffing 

andi x5, 0xBF ;[11] mark this bit as inverted (will be corrected before storing shift) 

in x2, USBIN ;[0] we have some free cycles so we could check for bit stuffing errors 

eor x1, x2 ;[1] x1 and x2 have to be different because the stuff bit is always a zero 

andi x1, USBMASK ;[2] mask the interesting bits 

breq stuffErr2 ;[3] if the stuff bit is a 1bit something went wrong 

mov x1, x2 ;[4] the next bit expects the last state to be in x1 

rjmp didunstuff6 ;[5] 

;[6] jump delay of rjmp didunstuffX 



unstuff7: ;[7] this is the jump delay of rjmp unstuffX 

nop ;[8] 

nop ;[9] 

ori shift, 0x80 ;[10] invert the last received bit to prevent furhter unstuffing 

andi x5, 0x7F ;[11] mark this bit as inverted (will be corrected before storing shift) 

in x1, USBIN ;[0] we have some free cycles so we could check for bit stuffing errors 

eor x2, x1 ;[1] x1 and x2 have to be different because the stuff bit is always a zero 

andi x2, USBMASK ;[2] mask the interesting bits 

breq stuffErr2 ;[3] if the stuff bit is a 1bit something went wrong 

mov x2, x1 ;[4] the next bit expects the last state to be in x2 

rjmp didunstuff7 ;[5] 

;[6] jump delay of rjmp didunstuff7 



; local copy of the stuffErr desitnation for the second half of the receiver loop 

stuffErr2: 

rjmp stuffErr 



; 

; The crc table follows. It has to be aligned to enable a fast loading of the needed bytes. 

; There are two tables of 256 entries each, the low and the high byte table. 

; Table values were generated with the following C code: 

/* 

#include <stdio.h> 

int main (int argc, char **argv) 

{ 

int i, j; 

for (i=0; i<512; i++){ 

unsigned short crc = i & 0xff; 

for(j=0; j<8; j++) crc = (crc >> 1) ^ ((crc & 1) ? 0xa001 : 0); 

if((i & 7) == 0) printf("\n.byte "); 

printf("0x%02x, ", (i > 0xff ? (crc >> 8) : crc) & 0xff); 

if(i == 255) printf("\n"); 

} 

return 0; 

} 



// Use the following algorithm to compute CRC values: 

ushort computeCrc(uchar *msg, uchar msgLen) 

{ 

uchar i; 

ushort crc = 0xffff; 

for(i = 0; i < msgLen; i++) 

crc = usbCrcTable16[lo8(crc) ^ msg[i]] ^ hi8(crc); 

return crc; 

} 

*/ 



.balign 256 

usbCrcTableLow: 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41 

.byte 0x01, 0xC0, 0x80, 0x41, 0x00, 0xC1, 0x81, 0x40 



; .balign 256 

usbCrcTableHigh: 

.byte 0x00, 0xC0, 0xC1, 0x01, 0xC3, 0x03, 0x02, 0xC2 

.byte 0xC6, 0x06, 0x07, 0xC7, 0x05, 0xC5, 0xC4, 0x04 

.byte 0xCC, 0x0C, 0x0D, 0xCD, 0x0F, 0xCF, 0xCE, 0x0E 

.byte 0x0A, 0xCA, 0xCB, 0x0B, 0xC9, 0x09, 0x08, 0xC8 

.byte 0xD8, 0x18, 0x19, 0xD9, 0x1B, 0xDB, 0xDA, 0x1A 

.byte 0x1E, 0xDE, 0xDF, 0x1F, 0xDD, 0x1D, 0x1C, 0xDC 

.byte 0x14, 0xD4, 0xD5, 0x15, 0xD7, 0x17, 0x16, 0xD6 

.byte 0xD2, 0x12, 0x13, 0xD3, 0x11, 0xD1, 0xD0, 0x10 

.byte 0xF0, 0x30, 0x31, 0xF1, 0x33, 0xF3, 0xF2, 0x32 

.byte 0x36, 0xF6, 0xF7, 0x37, 0xF5, 0x35, 0x34, 0xF4 

.byte 0x3C, 0xFC, 0xFD, 0x3D, 0xFF, 0x3F, 0x3E, 0xFE 

.byte 0xFA, 0x3A, 0x3B, 0xFB, 0x39, 0xF9, 0xF8, 0x38 

.byte 0x28, 0xE8, 0xE9, 0x29, 0xEB, 0x2B, 0x2A, 0xEA 

.byte 0xEE, 0x2E, 0x2F, 0xEF, 0x2D, 0xED, 0xEC, 0x2C 

.byte 0xE4, 0x24, 0x25, 0xE5, 0x27, 0xE7, 0xE6, 0x26 

.byte 0x22, 0xE2, 0xE3, 0x23, 0xE1, 0x21, 0x20, 0xE0 

.byte 0xA0, 0x60, 0x61, 0xA1, 0x63, 0xA3, 0xA2, 0x62 

.byte 0x66, 0xA6, 0xA7, 0x67, 0xA5, 0x65, 0x64, 0xA4 

.byte 0x6C, 0xAC, 0xAD, 0x6D, 0xAF, 0x6F, 0x6E, 0xAE 

.byte 0xAA, 0x6A, 0x6B, 0xAB, 0x69, 0xA9, 0xA8, 0x68 

.byte 0x78, 0xB8, 0xB9, 0x79, 0xBB, 0x7B, 0x7A, 0xBA 

.byte 0xBE, 0x7E, 0x7F, 0xBF, 0x7D, 0xBD, 0xBC, 0x7C 

.byte 0xB4, 0x74, 0x75, 0xB5, 0x77, 0xB7, 0xB6, 0x76 

.byte 0x72, 0xB2, 0xB3, 0x73, 0xB1, 0x71, 0x70, 0xB0 

.byte 0x50, 0x90, 0x91, 0x51, 0x93, 0x53, 0x52, 0x92 

.byte 0x96, 0x56, 0x57, 0x97, 0x55, 0x95, 0x94, 0x54 

.byte 0x9C, 0x5C, 0x5D, 0x9D, 0x5F, 0x9F, 0x9E, 0x5E 

.byte 0x5A, 0x9A, 0x9B, 0x5B, 0x99, 0x59, 0x58, 0x98 

.byte 0x88, 0x48, 0x49, 0x89, 0x4B, 0x8B, 0x8A, 0x4A 

.byte 0x4E, 0x8E, 0x8F, 0x4F, 0x8D, 0x4D, 0x4C, 0x8C 

.byte 0x44, 0x84, 0x85, 0x45, 0x87, 0x47, 0x46, 0x86 

.byte 0x82, 0x42, 0x43, 0x83, 0x41, 0x81, 0x80, 0x40 



