1 /* $Id: loop.s,v 1.23 2000/03/20 09:49:06 warner Exp $
3 * Firmware for the Keyspan PDA Serial Adapter, a USB serial port based on
4 * the EzUSB microcontroller.
6 * (C) Copyright 2000 Brian Warner <warner@lothar.com>
8 * This program is free software; you can redistribute it and/or modify
9 * it under the terms of the GNU General Public License as published by
10 * the Free Software Foundation; either version 2 of the License, or
11 * (at your option) any later version.
13 * "Keyspan PDA Serial Adapter" is probably a copyright of Keyspan, the
16 * This serial adapter is basically an EzUSB chip and an RS-232 line driver
17 * in a little widget that has a DB-9 on one end and a USB plug on the other.
18 * It uses the EzUSB's internal UART0 (using the pins from Port C) and timer2
19 * as a baud-rate generator. The wiring is:
20 * PC0/RxD0 <- rxd (DB9 pin 2) PC4 <- dsr pin 6
21 * PC1/TxD0 -> txd pin 3 PC5 <- ri pin 9
22 * PC2 -> rts pin 7 PC6 <- dcd pin 1
23 * PC3 <- cts pin 8 PC7 -> dtr pin 4
24 * PB1 -> line driver standby
26 * The EzUSB register constants below come from their excellent documentation
27 * and sample code (which used to be available at www.anchorchips.com, but
28 * that has now been absorbed into Cypress' site and the CD-ROM contents
29 * don't appear to be available online anymore). If we get multiple
30 * EzUSB-based drivers into the kernel, it might be useful to pull them out
31 * into a separate .h file.
33 * THEORY OF OPERATION:
35 * There are two 256-byte ring buffers, one for tx, one for rx.
37 * EP2out is pure tx data. When it appears, the data is copied into the tx
38 * ring and serial transmission is started if it wasn't already running. The
39 * "tx buffer empty" interrupt may kick off another character if the ring
40 * still has data. If the host is tx-blocked because the ring filled up,
41 * it will request a "tx unthrottle" interrupt. If sending a serial character
42 * empties the ring below the desired threshold, we set a bit that will send
43 * up the tx unthrottle message as soon as the rx buffer becomes free.
45 * EP2in (interrupt) is used to send both rx chars and rx status messages
46 * (only "tx unthrottle" at this time) back up to the host. The first byte
47 * of the rx message indicates data (0) or status msg (1). Status messages
48 * are sent before any data.
50 * Incoming serial characters are put into the rx ring by the serial
51 * interrupt, and the EP2in buffer sent if it wasn't already in transit.
52 * When the EP2in buffer returns, the interrupt prompts us to send more
53 * rx chars (or status messages) if they are pending.
55 * Device control happens through "vendor specific" control messages on EP0.
56 * All messages are destined for the "Interface" (with the index always 0,
57 * so that if their two-port device might someday use similar firmware, we
58 * can use index=1 to refer to the second port). The messages defined are:
60 * bRequest = 0 : set baud/bits/parity
62 * 2 : reserved for setting HW flow control (CTSRTS)
63 * 3 : get/set "modem info" (pin states: DTR, RTS, DCD, RI, etc)
64 * 4 : set break (on/off)
65 * 5 : reserved for requesting interrupts on pin state change
66 * 6 : query buffer room or chars in tx buffer
67 * 7 : request tx unthrottle interrupt
69 * The host-side driver is set to recognize the device ID values stashed in
70 * serial EEPROM (0x06cd, 0x0103), program this firmware into place, then
71 * start it running. This firmware will use EzUSB's "renumeration" trick by
72 * simulating a bus disconnect, then reconnect with a different device ID
73 * (encoded in the desc_device descriptor below). The host driver then
74 * recognizes the new device ID and glues it to the real serial driver code.
77 * EzUSB Technical Reference Manual: <http://www.anchorchips.com>
78 * 8051 manuals: everywhere, but try www.dalsemi.com because the EzUSB is
79 * basically the Dallas enhanced 8051 code. Remember that the EzUSB IO ports
80 * use totally different registers!
81 * USB 1.1 spec: www.usb.org
84 * gcc -x assembler-with-cpp -P -E -o keyspan_pda.asm keyspan_pda.s
85 * as31 -l keyspan_pda.asm
86 * mv keyspan_pda.obj keyspan_pda.hex
87 * perl ezusb_convert.pl keyspan_pda < keyspan_pda.hex > keyspan_pda_fw.h
88 * Get as31 from <http://www.pjrc.com/tech/8051/index.html>, and hack on it
89 * a bit to make it build.
92 * Greg Kroah-Hartman, for coordinating the whole usb-serial thing.
93 * AnchorChips, for making such an incredibly useful little microcontroller.
94 * KeySpan, for making a handy, cheap ($40) widget that was so easy to take
95 * apart and trace with an ohmmeter.
98 * lots. grep for TODO. Interrupt safety needs stress-testing. Better flow
99 * control. Interrupting host upon change in DCD, etc, counting transitions.
100 * Need to find a safe device id to use (the one used by the Keyspan firmware
101 * under Windows would be ideal.. can anyone figure out what it is?). Parity.
102 * More baud rates. Oh, and the string-descriptor-length silicon bug
103 * workaround should be implemented, but I'm lazy, and the consequence is
104 * that the device name strings that show up in your kernel log will have
105 * lots of trailing binary garbage in them (appears as ????). Device strings
106 * should be made more accurate.
108 * Questions, bugs, patches to Brian.
110 * -Brian Warner <warner@lothar.com>
114 #define HIGH(x) (((x) & 0xff00) / 256)
115 #define LOW(x) ((x) & 0xff)
121 ;;; our bit assignments
123 #define DO_TX_UNTHROTTLE 1
125 ;; stack from 0x60 to 0x7f: should really set SP to 0x60-1, not 0x60
126 #define STACK #0x60-1
133 #define EP0CS #0x7fb4
134 #define EP0STALLbit #0x01
135 #define IN0BUF #0x7f00
136 #define IN0BC #0x7fb5
137 #define OUT0BUF #0x7ec0
138 #define OUT0BC #0x7fc5
139 #define IN2BUF #0x7e00
140 #define IN2BC #0x7fb9
141 #define IN2CS #0x7fb8
142 #define OUT2BC #0x7fc9
143 #define OUT2CS #0x7fc8
144 #define OUT2BUF #0x7dc0
145 #define IN4BUF #0x7d00
146 #define IN4BC #0x7fbd
147 #define IN4CS #0x7fbc
152 #define PINSC #0x7f9b
153 #define PORTCCFG #0x7f95
154 #define IN07IRQ #0x7fa9
155 #define OUT07IRQ #0x7faa
156 #define IN07IEN #0x7fac
157 #define OUT07IEN #0x7fad
158 #define USBIRQ #0x7fab
159 #define USBIEN #0x7fae
160 #define USBBAV #0x7faf
161 #define USBCS #0x7fd6
162 #define SUDPTRH #0x7fd4
163 #define SUDPTRL #0x7fd5
164 #define SETUPDAT #0x7fe8
166 ;; usb interrupt : enable is EIE.0 (0xe8), flag is EXIF.4 (0x91)
177 .byte 0 ; filled in by the USB core
179 ;;; local variables. These are not initialized properly: do it by hand.
185 tx_unthrottle_threshold: .byte 0
187 .org 0x100H ; wants to be on a page boundary
189 ljmp ISR_Sudav ; Setup Data Available
191 ljmp 0 ; Start of Frame
193 ljmp 0 ; Setup Data Loading
195 ljmp 0 ; Global Suspend
201 ljmp 0 ; End Point 0 In
203 ljmp 0 ; End Point 0 Out
205 ljmp 0 ; End Point 1 In
207 ljmp 0 ; End Point 1 Out
217 start: mov SP,STACK-1 ; set stack
218 ;; clear local variables
224 mov tx_unthrottle_threshold, a
228 ;; clear fifo with "fe"
235 djnz r1, clear_tx_ring_loop
242 djnz r1, clear_rx_ring_loop
244 ;;; turn on the RS-232 driver chip (bring the STANDBY pin low)
257 ;; set PORTCCFG.[01] to route TxD0,RxD0 to serial port
262 ;; set up interrupts, autovectoring
265 setb acc.0 ; AVEN bit to 0
268 mov a,#0x01 ; enable SUDAV: setup data available (for ep0)
270 movx @dptr, a ; clear SUDAVI
275 mov a,#0x04 ; enable IN2 int
279 mov a,#0x04 ; enable OUT2 int
282 movx @dptr, a ; arm OUT2
284 mov a, #0x84 ; turn on RTS, DTR
287 ;; setup the serial port. 9600 8N1.
288 ;; Original source had:
289 ;;mov a,#01010011 ; mode 1, enable rx, clear int
290 ;; This was presumably meant to be a binary constant, but it's
291 ;; really decimal and out of 8-bit range. as31 used to treat
292 ;; it as 0 and that seems to have worked, so carry on with 0.
295 ;; using timer2, in 16-bit baud-rate-generator mode
296 ;; (xtal 12MHz, internal fosc 24MHz)
297 ;; RCAP2H,RCAP2L = 65536 - fosc/(32*baud)
298 ;; 57600: 0xFFF2.F, say 0xFFF3
299 ;; 9600: 0xFFB1.E, say 0xFFB2
302 #define BAUD_TIMEOUT(rate) (65536 - (24 * 1000 * 1000) / (32 * rate))
303 #define BAUD_HIGH(rate) HIGH(BAUD_TIMEOUT(rate))
304 #define BAUD_LOW(rate) LOW(BAUD_TIMEOUT(rate))
306 mov T2CON, #030h ; rclk=1,tclk=1,cp=0,tr2=0(enable later)
334 ;; hey, what say we RENUMERATE! (TRM p.62)
338 mov a, #0x02 ; DISCON=0, DISCOE=0, RENUM=1
340 ;; now presence pin is floating, simulating disconnect. wait 0.5s
349 djnz r1, renum_wait1 ; wait about n*(256^2) 6MHz clocks
350 mov a, #0x06 ; DISCON=0, DISCOE=1, RENUM=1
352 ;; we are back online. the host device will now re-query us
368 mov EXIF,a ; clear INT2 first
369 mov dptr, USBIRQ ; clear USB int
376 mov r1, a ; r1 = bmRequestType
379 mov r2, a ; r2 = bRequest
382 mov r3, a ; r3 = wValueL
385 mov r4, a ; r4 = wValueH
387 ;; main switch on bmRequest.type: standard or vendor
390 cjne a, #0x00, setup_bmreq_type_not_standard
391 ;; standard request: now main switch is on bRequest
392 ljmp setup_bmreq_is_standard
394 setup_bmreq_type_not_standard:
395 ;; a still has bmreq&0x60
396 cjne a, #0x40, setup_bmreq_type_not_vendor
397 ;; Anchor reserves bRequest 0xa0-0xaf, we use small ones
398 ;; switch on bRequest. bmRequest will always be 0x41 or 0xc1
399 cjne r2, #0x00, setup_ctrl_not_00
400 ;; 00 is set baud, wValue[0] has baud rate index
401 lcall set_baud ; index in r3, carry set if error
402 jc setup_bmreq_type_not_standard__do_stall
404 setup_bmreq_type_not_standard__do_stall:
407 cjne r2, #0x01, setup_ctrl_not_01
408 ;; 01 is reserved for set bits (parity). TODO
411 cjne r2, #0x02, setup_ctrl_not_02
412 ;; 02 is set HW flow control. TODO
415 cjne r2, #0x03, setup_ctrl_not_03
416 ;; 03 is control pins (RTS, DTR).
417 ljmp control_pins ; will jump to setup_done_ack,
418 ; or setup_return_one_byte
420 cjne r2, #0x04, setup_ctrl_not_04
421 ;; 04 is send break (really "turn break on/off"). TODO
422 cjne r3, #0x00, setup_ctrl_do_break_on
423 ;; do break off: restore PORTCCFG.1 to reconnect TxD0 to serial port
429 setup_ctrl_do_break_on:
430 ;; do break on: clear PORTCCFG.0, set TxD high(?) (b1 low)
441 cjne r2, #0x05, setup_ctrl_not_05
442 ;; 05 is set desired interrupt bitmap. TODO
445 cjne r2, #0x06, setup_ctrl_not_06
447 cjne r3, #0x00, setup_ctrl_06_not_00
448 ;; 06, wValue[0]=0 is query write_room
451 subb a, tx_ring_in ; out-1-in = 255 - (in-out)
452 ljmp setup_return_one_byte
453 setup_ctrl_06_not_00:
454 cjne r3, #0x01, setup_ctrl_06_not_01
455 ;; 06, wValue[0]=1 is query chars_in_buffer
458 subb a, tx_ring_out ; in-out
459 ljmp setup_return_one_byte
460 setup_ctrl_06_not_01:
463 cjne r2, #0x07, setup_ctrl_not_07
464 ;; 07 is request tx unthrottle interrupt
465 mov tx_unthrottle_threshold, r3; wValue[0] is threshold value
470 setup_bmreq_type_not_vendor:
474 setup_bmreq_is_standard:
475 cjne r2, #0x00, setup_breq_not_00
476 ;; 00: Get_Status (sub-switch on bmRequestType: device, ep, int)
477 cjne r1, #0x80, setup_Get_Status_not_device
478 ;; Get_Status(device)
479 ;; are we self-powered? no. can we do remote wakeup? no
480 ;; so return two zero bytes. This is reusable
481 setup_return_two_zero_bytes:
491 setup_Get_Status_not_device:
492 cjne r1, #0x82, setup_Get_Status_not_endpoint
493 ;; Get_Status(endpoint)
494 ;; must get stall bit for ep[wIndexL], return two bytes, bit in lsb 0
495 ;; for now: cheat. TODO
496 sjmp setup_return_two_zero_bytes
497 setup_Get_Status_not_endpoint:
498 cjne r1, #0x81, setup_Get_Status_not_interface
499 ;; Get_Status(interface): return two zeros
500 sjmp setup_return_two_zero_bytes
501 setup_Get_Status_not_interface:
505 cjne r2, #0x01, setup_breq_not_01
506 ;; 01: Clear_Feature (sub-switch on wValueL: stall, remote wakeup)
507 cjne r3, #0x00, setup_Clear_Feature_not_stall
508 ;; Clear_Feature(stall). should clear a stall bit. TODO
510 setup_Clear_Feature_not_stall:
511 cjne r3, #0x01, setup_Clear_Feature_not_rwake
512 ;; Clear_Feature(remote wakeup). ignored.
514 setup_Clear_Feature_not_rwake:
518 cjne r2, #0x03, setup_breq_not_03
519 ;; 03: Set_Feature (sub-switch on wValueL: stall, remote wakeup)
520 cjne r3, #0x00, setup_Set_Feature_not_stall
521 ;; Set_Feature(stall). Should set a stall bit. TODO
523 setup_Set_Feature_not_stall:
524 cjne r3, #0x01, setup_Set_Feature_not_rwake
525 ;; Set_Feature(remote wakeup). ignored.
527 setup_Set_Feature_not_rwake:
531 cjne r2, #0x06, setup_breq_not_06
532 ;; 06: Get_Descriptor (s-switch on wValueH: dev, config[n], string[n])
533 cjne r4, #0x01, setup_Get_Descriptor_not_device
534 ;; Get_Descriptor(device)
536 mov a, #HIGH(desc_device)
539 mov a, #LOW(desc_device)
542 setup_Get_Descriptor_not_device:
543 cjne r4, #0x02, setup_Get_Descriptor_not_config
544 ;; Get_Descriptor(config[n])
545 cjne r3, #0x00, setup_stall; only handle n==0
546 ;; Get_Descriptor(config[0])
548 mov a, #HIGH(desc_config1)
551 mov a, #LOW(desc_config1)
554 setup_Get_Descriptor_not_config:
555 cjne r4, #0x03, setup_Get_Descriptor_not_string
556 ;; Get_Descriptor(string[wValueL])
557 ;; if (wValueL >= maxstrings) stall
558 mov a, #((desc_strings_end-desc_strings)/2)
560 subb a,r3 ; a=4, r3 = 0..3 . if a<=0 then stall
564 add a, r3 ; a = 2*wValueL
565 mov dptr, #desc_strings
570 mov dph, a ; dph = desc_strings[a]. big endian! (handy)
571 ;; it looks like my adapter uses a revision of the EZUSB that
572 ;; contains "rev D errata number 8", as hinted in the EzUSB example
573 ;; code. I cannot find an actual errata description on the Cypress
574 ;; web site, but from the example code it looks like this bug causes
575 ;; the length of string descriptors to be read incorrectly, possibly
576 ;; sending back more characters than the descriptor has. The workaround
577 ;; is to manually send out all of the data. The consequence of not
578 ;; using the workaround is that the strings gathered by the kernel
579 ;; driver are too long and are filled with trailing garbage (including
580 ;; leftover strings). Writing this out by hand is a nuisance, so for
581 ;; now I will just live with the bug.
596 setup_Get_Descriptor_not_string:
600 cjne r2, #0x08, setup_breq_not_08
601 ;; Get_Configuration. always 1. return one byte.
604 setup_return_one_byte:
612 cjne r2, #0x09, setup_breq_not_09
613 ;; 09: Set_Configuration. ignored.
616 cjne r2, #0x0a, setup_breq_not_0a
617 ;; 0a: Get_Interface. get the current altsetting for int[wIndexL]
618 ;; since we only have one interface, ignore wIndexL, return a 0
620 ljmp setup_return_one_byte
622 cjne r2, #0x0b, setup_breq_not_0b
623 ;; 0b: Set_Interface. set altsetting for interface[wIndexL]. ignored
653 ;;; ==============================================================
655 set_baud: ; baud index in r3
658 jb ACC.7, set_baud__badbaud
661 jnc set_baud__badbaud
664 add a, #LOW(baud_table)
666 mov a, #HIGH(baud_table)
669 ;; TODO: shut down xmit/receive
670 ;; TODO: wait for current xmit char to leave
671 ;; TODO: shut down timer to avoid partial-char glitch
672 movx a,@dptr ; BAUD_HIGH
676 movx a,@dptr ; BAUD_LOW
679 ;; TODO: restart xmit/receive
680 ;; TODO: reenable interrupts, resume tx if pending
684 setb c ; c=1: failure
687 ;;; ==================================================
689 cjne r1, #0x41, control_pins_in
691 mov a, r3 ; wValue[0] holds new bits: b7 is new DTR, b2 is new RTS
692 xrl a, #0xff ; 1 means active, 0V, +12V ?
696 movx a, @dptr ; only change bits 7 and 2
699 movx @dptr, a ; other pins are inputs, bits ignored
705 ljmp setup_return_one_byte
707 ;;; ========================================
718 mov EXIF,a ; clear INT2 first
719 mov dptr, IN07IRQ ; clear USB int
743 mov EXIF,a ; clear INT2 first
744 mov dptr, OUT07IRQ ; clear USB int
750 ;; copy data into buffer. for now, assume we will have enough space
751 mov dptr, OUT2BC ; get byte count
756 mov dptr, OUT2BUF ; load DPTR0 with source
757 mov dph1, #HIGH(tx_ring) ; load DPTR1 with target
761 inc dps ; switch to DPTR1: target
762 inc dpl1 ; target = tx_ring_in+1
765 cjne a, tx_ring_out, OUT_no_overflow
768 inc tx_ring_in ; tx_ring_in++
769 inc dps ; switch to DPTR0: source
794 ;; fill in EP4in with a debugging message:
795 ;; tx_ring_in, tx_ring_out, rx_ring_in, rx_ring_out
805 jb acc.1, dump_stat__done; busy: cannot dump, old one still pending
823 jnb TX_RUNNING, dump_stat__no_tx_running
825 dump_stat__no_tx_running:
830 mov dptr, #tx_ring ; DPTR1: source
832 dump_stat__tx_ring_loop:
839 djnz r1, dump_stat__tx_ring_loop
848 mov dptr, #rx_ring ; DPTR1: source
850 dump_stat__rx_ring_loop:
857 djnz r1, dump_stat__rx_ring_loop
868 ;;; ============================================================
871 ;; make sure the tx process is running.
872 jb TX_RUNNING, start_tx_done
874 ;; is there work to be done?
876 cjne a,tx_ring_out, start_tx__work
879 ;; tx was not running. send the first character, setup the TI int
880 inc tx_ring_out ; [++tx_ring_out]
881 mov dph, #HIGH(tx_ring)
887 ;; can we unthrottle the host tx process?
888 ;; step 1: do we care?
890 cjne a, tx_unthrottle_threshold, start_tx__maybe_unthrottle_tx
892 start_tx_really_done:
894 start_tx__maybe_unthrottle_tx:
895 ;; step 2: is there now room?
899 ;; a is now write_room. If thresh >= a, we can unthrottle
901 subb a, tx_unthrottle_threshold
902 jc start_tx_really_done ; nope
903 ;; yes, we can unthrottle. remove the threshold and mark a request
904 mov tx_unthrottle_threshold, #0
905 setb DO_TX_UNTHROTTLE
906 ;; prod rx, which will actually send the message when in2 becomes free
917 jnb TI, serial_int__not_tx
918 ;; tx finished. send another character if we have one
923 jnb RI, serial_int__not_rx
937 mov dph, #HIGH(rx_ring)
939 inc dpl ; target = rx_ring_in+1
942 ;; check for overflow before incrementing rx_ring_in
944 cjne a, rx_ring_out, get_rx_char__no_overflow
947 get_rx_char__no_overflow:
949 ;; kick off USB INpipe
954 ;; check if the inpipe is already running.
957 jb acc.1, start_in__done; int will handle it
958 jb DO_TX_UNTHROTTLE, start_in__do_tx_unthrottle
959 ;; see if there is any work to do. a serial interrupt might occur
960 ;; during this sequence?
962 cjne a, rx_ring_out, start_in__have_work
965 ;; now copy as much data as possible into the pipe. 63 bytes max.
968 mov dph, #HIGH(rx_ring) ; load DPTR0 with source
970 mov dptr, IN2BUF ; load DPTR1 with target
971 movx @dptr, a ; in[0] signals that rest of IN is rx data
974 ;; loop until we run out of data, or we have copied 64 bytes
975 mov r1, #1 ; INbuf size counter
978 cjne a, rx_ring_out, start_inlocal_irq_enablell_copying
980 start_inlocal_irq_enablell_copying:
985 movx @dptr, a ; write into IN buffer
989 cjne r1, #64, start_in__loop; loop
991 ;; either we ran out of data, or we copied 64 bytes. r1 has byte count
1001 start_in__do_tx_unthrottle:
1002 ;; special sequence: send a tx unthrottle message
1003 clr DO_TX_UNTHROTTLE
1020 jnb TI, putchar_wait
1025 baud_table: ; baud_high, then baud_low
1027 .byte BAUD_HIGH(110)
1030 .byte BAUD_HIGH(300)
1033 .byte BAUD_HIGH(1200)
1034 .byte BAUD_LOW(1200)
1036 .byte BAUD_HIGH(2400)
1037 .byte BAUD_LOW(2400)
1039 .byte BAUD_HIGH(4800)
1040 .byte BAUD_LOW(4800)
1042 .byte BAUD_HIGH(9600)
1043 .byte BAUD_LOW(9600)
1045 .byte BAUD_HIGH(19200)
1046 .byte BAUD_LOW(19200)
1048 .byte BAUD_HIGH(38400)
1049 .byte BAUD_LOW(38400)
1051 .byte BAUD_HIGH(57600)
1052 .byte BAUD_LOW(57600)
1054 .byte BAUD_HIGH(115200)
1055 .byte BAUD_LOW(115200)
1058 .byte 0x12, 0x01, 0x00, 0x01, 0xff, 0xff, 0xff, 0x40
1059 .byte 0xcd, 0x06, 0x04, 0x01, 0x89, 0xab, 1, 2, 3, 0x01
1060 ;;; The "real" device id, which must match the host driver, is that
1061 ;;; "0xcd 0x06 0x04 0x01" sequence, which is 0x06cd, 0x0104
1064 .byte 0x09, 0x02, 0x20, 0x00, 0x01, 0x01, 0x00, 0x80, 0x32
1065 .byte 0x09, 0x04, 0x00, 0x00, 0x02, 0xff, 0xff, 0xff, 0x00
1066 .byte 0x07, 0x05, 0x82, 0x03, 0x40, 0x00, 0x01
1067 .byte 0x07, 0x05, 0x02, 0x02, 0x40, 0x00, 0x00
1070 .word string_langids, string_mfg, string_product, string_serial
1073 string_langids: .byte string_langids_end-string_langids
1078 ;; sigh. These strings are Unicode, meaning UTF16? 2 bytes each. Now
1079 ;; *that* is a pain in the ass to encode. And they are little-endian
1080 ;; too. Use this perl snippet to get the bytecodes:
1084 printf("0x%02x, 0x00, ", ord($c));
1089 string_mfg: .byte string_mfg_end-string_mfg
1091 ; .byte "ACME usb widgets"
1092 .byte 0x41, 0x00, 0x43, 0x00, 0x4d, 0x00, 0x45, 0x00, 0x20, 0x00, 0x75, 0x00, 0x73, 0x00, 0x62, 0x00, 0x20, 0x00, 0x77, 0x00, 0x69, 0x00, 0x64, 0x00, 0x67, 0x00, 0x65, 0x00, 0x74, 0x00, 0x73, 0x00
1095 string_product: .byte string_product_end-string_product
1097 ; .byte "ACME USB serial widget"
1098 .byte 0x41, 0x00, 0x43, 0x00, 0x4d, 0x00, 0x45, 0x00, 0x20, 0x00, 0x55, 0x00, 0x53, 0x00, 0x42, 0x00, 0x20, 0x00, 0x73, 0x00, 0x65, 0x00, 0x72, 0x00, 0x69, 0x00, 0x61, 0x00, 0x6c, 0x00, 0x20, 0x00, 0x77, 0x00, 0x69, 0x00, 0x64, 0x00, 0x67, 0x00, 0x65, 0x00, 0x74, 0x00
1101 string_serial: .byte string_serial_end-string_serial
1104 .byte 0x34, 0x00, 0x37, 0x00
1107 ;;; ring buffer memory
1108 ;; tx_ring_in+1 is where the next input byte will go
1109 ;; [tx_ring_out] has been sent
1110 ;; if tx_ring_in == tx_ring_out, theres no work to do
1111 ;; there are (tx_ring_in - tx_ring_out) chars to be written
1112 ;; dont let _in lap _out
1113 ;; cannot inc if tx_ring_in+1 == tx_ring_out
1114 ;; write [tx_ring_in+1] then tx_ring_in++
1115 ;; if (tx_ring_in+1 == tx_ring_out), overflow
1116 ;; else tx_ring_in++
1117 ;; read/send [tx_ring_out+1], then tx_ring_out++
1119 ;; rx_ring_in works the same way
1123 .skip 0x100 ; 256 bytes
1125 .skip 0x100 ; 256 bytes