2 * Copyright (C) 2009 by Bart Hartgers (bart.hartgers+ark3116@gmail.com)
5 * Simon Schulz (ark3116_driver <at> auctionant.de)
8 * - implements a driver for the arkmicro ark3116 chipset (vendor=0x6547,
9 * productid=0x0232) (used in a datacable called KQ-U8A)
11 * Supports full modem status lines, break, hardware flow control. Does not
12 * support software flow control, since I do not know how to enable it in hw.
14 * This driver is a essentially new implementation. I initially dug
15 * into the old ark3116.c driver and suddenly realized the ark3116 is
16 * a 16450 with a USB interface glued to it. See comments at the
17 * bottom of this file.
19 * This program is free software; you can redistribute it and/or modify it
20 * under the terms of the GNU General Public License as published by the
21 * Free Software Foundation; either version 2 of the License, or (at your
22 * option) any later version.
25 #include <linux/kernel.h>
26 #include <linux/init.h>
27 #include <linux/ioctl.h>
28 #include <linux/tty.h>
29 #include <linux/slab.h>
30 #include <linux/tty_flip.h>
31 #include <linux/module.h>
32 #include <linux/usb.h>
33 #include <linux/usb/serial.h>
34 #include <linux/serial.h>
35 #include <linux/serial_reg.h>
36 #include <linux/uaccess.h>
37 #include <linux/mutex.h>
38 #include <linux/spinlock.h>
44 #define DRIVER_VERSION "v0.7"
45 #define DRIVER_AUTHOR "Bart Hartgers <bart.hartgers+ark3116@gmail.com>"
46 #define DRIVER_DESC "USB ARK3116 serial/IrDA driver"
47 #define DRIVER_DEV_DESC "ARK3116 RS232/IrDA"
48 #define DRIVER_NAME "ark3116"
50 /* usb timeout of 1 second */
51 #define ARK_TIMEOUT (1*HZ)
53 static const struct usb_device_id id_table[] = {
54 { USB_DEVICE(0x6547, 0x0232) },
55 { USB_DEVICE(0x18ec, 0x3118) }, /* USB to IrDA adapter */
58 MODULE_DEVICE_TABLE(usb, id_table);
60 static int is_irda(struct usb_serial *serial)
62 struct usb_device *dev = serial->dev;
63 if (le16_to_cpu(dev->descriptor.idVendor) == 0x18ec &&
64 le16_to_cpu(dev->descriptor.idProduct) == 0x3118)
69 struct ark3116_private {
70 wait_queue_head_t delta_msr_wait;
71 struct async_icount icount;
72 int irda; /* 1 for irda device */
74 /* protects hw register updates */
77 int quot; /* baudrate divisor */
78 __u32 lcr; /* line control register value */
79 __u32 hcr; /* handshake control register (0x8)
81 __u32 mcr; /* modem contol register value */
83 /* protects the status values below */
84 spinlock_t status_lock;
85 __u32 msr; /* modem status register value */
86 __u32 lsr; /* line status register value */
89 static int ark3116_write_reg(struct usb_serial *serial,
90 unsigned reg, __u8 val)
93 /* 0xfe 0x40 are magic values taken from original driver */
94 result = usb_control_msg(serial->dev,
95 usb_sndctrlpipe(serial->dev, 0),
97 NULL, 0, ARK_TIMEOUT);
101 static int ark3116_read_reg(struct usb_serial *serial,
102 unsigned reg, unsigned char *buf)
105 /* 0xfe 0xc0 are magic values taken from original driver */
106 result = usb_control_msg(serial->dev,
107 usb_rcvctrlpipe(serial->dev, 0),
109 buf, 1, ARK_TIMEOUT);
116 static inline int calc_divisor(int bps)
118 /* Original ark3116 made some exceptions in rounding here
119 * because windows did the same. Assume that is not really
121 * Crystal is 12MHz, probably because of USB, but we divide by 4?
123 return (12000000 + 2*bps) / (4*bps);
126 static int ark3116_attach(struct usb_serial *serial)
128 /* make sure we have our end-points */
129 if ((serial->num_bulk_in == 0) ||
130 (serial->num_bulk_out == 0) ||
131 (serial->num_interrupt_in == 0)) {
132 dev_err(&serial->dev->dev,
133 "%s - missing endpoint - "
134 "bulk in: %d, bulk out: %d, int in %d\n",
137 serial->num_bulk_out,
138 serial->num_interrupt_in);
145 static int ark3116_port_probe(struct usb_serial_port *port)
147 struct usb_serial *serial = port->serial;
148 struct ark3116_private *priv;
150 priv = kzalloc(sizeof(*priv), GFP_KERNEL);
154 init_waitqueue_head(&priv->delta_msr_wait);
155 mutex_init(&priv->hw_lock);
156 spin_lock_init(&priv->status_lock);
158 priv->irda = is_irda(serial);
160 usb_set_serial_port_data(port, priv);
162 /* setup the hardware */
163 ark3116_write_reg(serial, UART_IER, 0);
165 ark3116_write_reg(serial, UART_FCR, 0);
166 /* handshake control */
168 ark3116_write_reg(serial, 0x8 , 0);
171 ark3116_write_reg(serial, UART_MCR, 0);
174 ark3116_write_reg(serial, 0xb , 0);
176 ark3116_write_reg(serial, 0xb , 1);
177 ark3116_write_reg(serial, 0xc , 0);
178 ark3116_write_reg(serial, 0xd , 0x41);
179 ark3116_write_reg(serial, 0xa , 1);
183 ark3116_write_reg(serial, UART_LCR, UART_LCR_DLAB);
185 /* setup for 9600 8N1 */
186 priv->quot = calc_divisor(9600);
187 ark3116_write_reg(serial, UART_DLL, priv->quot & 0xff);
188 ark3116_write_reg(serial, UART_DLM, (priv->quot>>8) & 0xff);
190 priv->lcr = UART_LCR_WLEN8;
191 ark3116_write_reg(serial, UART_LCR, UART_LCR_WLEN8);
193 ark3116_write_reg(serial, 0xe, 0);
196 ark3116_write_reg(serial, 0x9, 0);
198 dev_info(&serial->dev->dev,
199 "%s using %s mode\n",
201 priv->irda ? "IrDA" : "RS232");
205 static int ark3116_port_remove(struct usb_serial_port *port)
207 struct ark3116_private *priv = usb_get_serial_port_data(port);
209 /* device is closed, so URBs and DMA should be down */
210 mutex_destroy(&priv->hw_lock);
216 static void ark3116_init_termios(struct tty_struct *tty)
218 struct ktermios *termios = &tty->termios;
219 *termios = tty_std_termios;
220 termios->c_cflag = B9600 | CS8
221 | CREAD | HUPCL | CLOCAL;
222 termios->c_ispeed = 9600;
223 termios->c_ospeed = 9600;
226 static void ark3116_set_termios(struct tty_struct *tty,
227 struct usb_serial_port *port,
228 struct ktermios *old_termios)
230 struct usb_serial *serial = port->serial;
231 struct ark3116_private *priv = usb_get_serial_port_data(port);
232 struct ktermios *termios = &tty->termios;
233 unsigned int cflag = termios->c_cflag;
234 int bps = tty_get_baud_rate(tty);
238 /* set data bit count */
239 switch (cflag & CSIZE) {
241 lcr = UART_LCR_WLEN5;
244 lcr = UART_LCR_WLEN6;
247 lcr = UART_LCR_WLEN7;
251 lcr = UART_LCR_WLEN8;
255 lcr |= UART_LCR_STOP;
257 lcr |= UART_LCR_PARITY;
258 if (!(cflag & PARODD))
259 lcr |= UART_LCR_EPAR;
262 lcr |= UART_LCR_SPAR;
264 /* handshake control */
265 hcr = (cflag & CRTSCTS) ? 0x03 : 0x00;
268 dev_dbg(&port->dev, "%s - setting bps to %d\n", __func__, bps);
272 quot = calc_divisor(9600);
275 if ((bps < 75) || (bps > 3000000))
277 quot = calc_divisor(bps);
281 quot = calc_divisor(bps);
285 quot = calc_divisor(bps);
289 /* Update state: synchronize */
290 mutex_lock(&priv->hw_lock);
292 /* keep old LCR_SBC bit */
293 lcr |= (priv->lcr & UART_LCR_SBC);
295 dev_dbg(&port->dev, "%s - setting hcr:0x%02x,lcr:0x%02x,quot:%d\n",
296 __func__, hcr, lcr, quot);
298 /* handshake control */
299 if (priv->hcr != hcr) {
301 ark3116_write_reg(serial, 0x8, hcr);
305 if (priv->quot != quot) {
307 priv->lcr = lcr; /* need to write lcr anyway */
309 /* disable DMA since transmit/receive is
310 * shadowed by UART_DLL
312 ark3116_write_reg(serial, UART_FCR, 0);
314 ark3116_write_reg(serial, UART_LCR,
316 ark3116_write_reg(serial, UART_DLL, quot & 0xff);
317 ark3116_write_reg(serial, UART_DLM, (quot>>8) & 0xff);
320 ark3116_write_reg(serial, UART_LCR, lcr);
321 /* magic baudrate thingy: not sure what it does,
322 * but windows does this as well.
324 ark3116_write_reg(serial, 0xe, eval);
327 ark3116_write_reg(serial, UART_FCR, UART_FCR_DMA_SELECT);
328 } else if (priv->lcr != lcr) {
330 ark3116_write_reg(serial, UART_LCR, lcr);
333 mutex_unlock(&priv->hw_lock);
335 /* check for software flow control */
336 if (I_IXOFF(tty) || I_IXON(tty)) {
337 dev_warn(&serial->dev->dev,
338 "%s: don't know how to do software flow control\n",
342 /* Don't rewrite B0 */
343 if (tty_termios_baud_rate(termios))
344 tty_termios_encode_baud_rate(termios, bps, bps);
347 static void ark3116_close(struct usb_serial_port *port)
349 struct usb_serial *serial = port->serial;
353 ark3116_write_reg(serial, UART_FCR, 0);
355 /* deactivate interrupts */
356 ark3116_write_reg(serial, UART_IER, 0);
358 usb_serial_generic_close(port);
359 if (serial->num_interrupt_in)
360 usb_kill_urb(port->interrupt_in_urb);
365 static int ark3116_open(struct tty_struct *tty, struct usb_serial_port *port)
367 struct ark3116_private *priv = usb_get_serial_port_data(port);
368 struct usb_serial *serial = port->serial;
372 buf = kmalloc(1, GFP_KERNEL);
376 result = usb_serial_generic_open(tty, port);
379 "%s - usb_serial_generic_open failed: %d\n",
384 /* remove any data still left: also clears error state */
385 ark3116_read_reg(serial, UART_RX, buf);
387 /* read modem status */
388 priv->msr = ark3116_read_reg(serial, UART_MSR, buf);
389 /* read line status */
390 priv->lsr = ark3116_read_reg(serial, UART_LSR, buf);
392 result = usb_submit_urb(port->interrupt_in_urb, GFP_KERNEL);
394 dev_err(&port->dev, "submit irq_in urb failed %d\n",
400 /* activate interrupts */
401 ark3116_write_reg(port->serial, UART_IER, UART_IER_MSI|UART_IER_RLSI);
404 ark3116_write_reg(port->serial, UART_FCR, UART_FCR_DMA_SELECT);
408 ark3116_set_termios(tty, port, NULL);
415 static int ark3116_get_icount(struct tty_struct *tty,
416 struct serial_icounter_struct *icount)
418 struct usb_serial_port *port = tty->driver_data;
419 struct ark3116_private *priv = usb_get_serial_port_data(port);
420 struct async_icount cnow = priv->icount;
421 icount->cts = cnow.cts;
422 icount->dsr = cnow.dsr;
423 icount->rng = cnow.rng;
424 icount->dcd = cnow.dcd;
425 icount->rx = cnow.rx;
426 icount->tx = cnow.tx;
427 icount->frame = cnow.frame;
428 icount->overrun = cnow.overrun;
429 icount->parity = cnow.parity;
430 icount->brk = cnow.brk;
431 icount->buf_overrun = cnow.buf_overrun;
435 static int ark3116_ioctl(struct tty_struct *tty,
436 unsigned int cmd, unsigned long arg)
438 struct usb_serial_port *port = tty->driver_data;
439 struct ark3116_private *priv = usb_get_serial_port_data(port);
440 struct serial_struct serstruct;
441 void __user *user_arg = (void __user *)arg;
445 /* XXX: Some of these values are probably wrong. */
446 memset(&serstruct, 0, sizeof(serstruct));
447 serstruct.type = PORT_16654;
448 serstruct.line = port->serial->minor;
449 serstruct.port = port->number;
450 serstruct.custom_divisor = 0;
451 serstruct.baud_base = 460800;
453 if (copy_to_user(user_arg, &serstruct, sizeof(serstruct)))
458 if (copy_from_user(&serstruct, user_arg, sizeof(serstruct)))
463 struct async_icount prev = priv->icount;
464 interruptible_sleep_on(&priv->delta_msr_wait);
465 /* see if a signal did it */
466 if (signal_pending(current))
468 if ((prev.rng == priv->icount.rng) &&
469 (prev.dsr == priv->icount.dsr) &&
470 (prev.dcd == priv->icount.dcd) &&
471 (prev.cts == priv->icount.cts))
473 if ((arg & TIOCM_RNG &&
474 (prev.rng != priv->icount.rng)) ||
476 (prev.dsr != priv->icount.dsr)) ||
478 (prev.dcd != priv->icount.dcd)) ||
480 (prev.cts != priv->icount.cts)))
489 static int ark3116_tiocmget(struct tty_struct *tty)
491 struct usb_serial_port *port = tty->driver_data;
492 struct ark3116_private *priv = usb_get_serial_port_data(port);
497 mutex_lock(&priv->hw_lock);
499 mutex_unlock(&priv->hw_lock);
501 spin_lock_irqsave(&priv->status_lock, flags);
503 spin_unlock_irqrestore(&priv->status_lock, flags);
505 return (status & UART_MSR_DSR ? TIOCM_DSR : 0) |
506 (status & UART_MSR_CTS ? TIOCM_CTS : 0) |
507 (status & UART_MSR_RI ? TIOCM_RI : 0) |
508 (status & UART_MSR_DCD ? TIOCM_CD : 0) |
509 (ctrl & UART_MCR_DTR ? TIOCM_DTR : 0) |
510 (ctrl & UART_MCR_RTS ? TIOCM_RTS : 0) |
511 (ctrl & UART_MCR_OUT1 ? TIOCM_OUT1 : 0) |
512 (ctrl & UART_MCR_OUT2 ? TIOCM_OUT2 : 0);
515 static int ark3116_tiocmset(struct tty_struct *tty,
516 unsigned set, unsigned clr)
518 struct usb_serial_port *port = tty->driver_data;
519 struct ark3116_private *priv = usb_get_serial_port_data(port);
521 /* we need to take the mutex here, to make sure that the value
522 * in priv->mcr is actually the one that is in the hardware
525 mutex_lock(&priv->hw_lock);
528 priv->mcr |= UART_MCR_RTS;
530 priv->mcr |= UART_MCR_DTR;
531 if (set & TIOCM_OUT1)
532 priv->mcr |= UART_MCR_OUT1;
533 if (set & TIOCM_OUT2)
534 priv->mcr |= UART_MCR_OUT2;
536 priv->mcr &= ~UART_MCR_RTS;
538 priv->mcr &= ~UART_MCR_DTR;
539 if (clr & TIOCM_OUT1)
540 priv->mcr &= ~UART_MCR_OUT1;
541 if (clr & TIOCM_OUT2)
542 priv->mcr &= ~UART_MCR_OUT2;
544 ark3116_write_reg(port->serial, UART_MCR, priv->mcr);
546 mutex_unlock(&priv->hw_lock);
551 static void ark3116_break_ctl(struct tty_struct *tty, int break_state)
553 struct usb_serial_port *port = tty->driver_data;
554 struct ark3116_private *priv = usb_get_serial_port_data(port);
556 /* LCR is also used for other things: protect access */
557 mutex_lock(&priv->hw_lock);
560 priv->lcr |= UART_LCR_SBC;
562 priv->lcr &= ~UART_LCR_SBC;
564 ark3116_write_reg(port->serial, UART_LCR, priv->lcr);
566 mutex_unlock(&priv->hw_lock);
569 static void ark3116_update_msr(struct usb_serial_port *port, __u8 msr)
571 struct ark3116_private *priv = usb_get_serial_port_data(port);
574 spin_lock_irqsave(&priv->status_lock, flags);
576 spin_unlock_irqrestore(&priv->status_lock, flags);
578 if (msr & UART_MSR_ANY_DELTA) {
579 /* update input line counters */
580 if (msr & UART_MSR_DCTS)
582 if (msr & UART_MSR_DDSR)
584 if (msr & UART_MSR_DDCD)
586 if (msr & UART_MSR_TERI)
588 wake_up_interruptible(&priv->delta_msr_wait);
592 static void ark3116_update_lsr(struct usb_serial_port *port, __u8 lsr)
594 struct ark3116_private *priv = usb_get_serial_port_data(port);
597 spin_lock_irqsave(&priv->status_lock, flags);
600 spin_unlock_irqrestore(&priv->status_lock, flags);
602 if (lsr&UART_LSR_BRK_ERROR_BITS) {
603 if (lsr & UART_LSR_BI)
605 if (lsr & UART_LSR_FE)
606 priv->icount.frame++;
607 if (lsr & UART_LSR_PE)
608 priv->icount.parity++;
609 if (lsr & UART_LSR_OE)
610 priv->icount.overrun++;
614 static void ark3116_read_int_callback(struct urb *urb)
616 struct usb_serial_port *port = urb->context;
617 int status = urb->status;
618 const __u8 *data = urb->transfer_buffer;
625 /* this urb is terminated, clean up */
626 dev_dbg(&port->dev, "%s - urb shutting down with status: %d\n",
630 dev_dbg(&port->dev, "%s - nonzero urb status received: %d\n",
633 case 0: /* success */
634 /* discovered this by trail and error... */
635 if ((urb->actual_length == 4) && (data[0] == 0xe8)) {
636 const __u8 id = data[1]&UART_IIR_ID;
637 dev_dbg(&port->dev, "%s: iir=%02x\n", __func__, data[1]);
638 if (id == UART_IIR_MSI) {
639 dev_dbg(&port->dev, "%s: msr=%02x\n",
641 ark3116_update_msr(port, data[3]);
643 } else if (id == UART_IIR_RLSI) {
644 dev_dbg(&port->dev, "%s: lsr=%02x\n",
646 ark3116_update_lsr(port, data[2]);
651 * Not sure what this data meant...
653 usb_serial_debug_data(&port->dev, __func__,
655 urb->transfer_buffer);
659 result = usb_submit_urb(urb, GFP_ATOMIC);
661 dev_err(&urb->dev->dev,
662 "%s - Error %d submitting interrupt urb\n",
667 /* Data comes in via the bulk (data) URB, erors/interrupts via the int URB.
668 * This means that we cannot be sure which data byte has an associated error
669 * condition, so we report an error for all data in the next bulk read.
671 * Actually, there might even be a window between the bulk data leaving the
672 * ark and reading/resetting the lsr in the read_bulk_callback where an
673 * interrupt for the next data block could come in.
674 * Without somekind of ordering on the ark, we would have to report the
675 * error for the next block of data as well...
676 * For now, let's pretend this can't happen.
678 static void ark3116_process_read_urb(struct urb *urb)
680 struct usb_serial_port *port = urb->context;
681 struct ark3116_private *priv = usb_get_serial_port_data(port);
682 struct tty_struct *tty;
683 unsigned char *data = urb->transfer_buffer;
684 char tty_flag = TTY_NORMAL;
688 /* update line status */
689 spin_lock_irqsave(&priv->status_lock, flags);
691 priv->lsr &= ~UART_LSR_BRK_ERROR_BITS;
692 spin_unlock_irqrestore(&priv->status_lock, flags);
694 if (!urb->actual_length)
697 tty = tty_port_tty_get(&port->port);
701 if (lsr & UART_LSR_BRK_ERROR_BITS) {
702 if (lsr & UART_LSR_BI)
703 tty_flag = TTY_BREAK;
704 else if (lsr & UART_LSR_PE)
705 tty_flag = TTY_PARITY;
706 else if (lsr & UART_LSR_FE)
707 tty_flag = TTY_FRAME;
709 /* overrun is special, not associated with a char */
710 if (lsr & UART_LSR_OE)
711 tty_insert_flip_char(tty, 0, TTY_OVERRUN);
713 tty_insert_flip_string_fixed_flag(tty, data, tty_flag,
715 tty_flip_buffer_push(tty);
719 static struct usb_serial_driver ark3116_device = {
721 .owner = THIS_MODULE,
724 .id_table = id_table,
726 .attach = ark3116_attach,
727 .port_probe = ark3116_port_probe,
728 .port_remove = ark3116_port_remove,
729 .set_termios = ark3116_set_termios,
730 .init_termios = ark3116_init_termios,
731 .ioctl = ark3116_ioctl,
732 .tiocmget = ark3116_tiocmget,
733 .tiocmset = ark3116_tiocmset,
734 .get_icount = ark3116_get_icount,
735 .open = ark3116_open,
736 .close = ark3116_close,
737 .break_ctl = ark3116_break_ctl,
738 .read_int_callback = ark3116_read_int_callback,
739 .process_read_urb = ark3116_process_read_urb,
742 static struct usb_serial_driver * const serial_drivers[] = {
743 &ark3116_device, NULL
746 module_usb_serial_driver(serial_drivers, id_table);
748 MODULE_LICENSE("GPL");
750 MODULE_AUTHOR(DRIVER_AUTHOR);
751 MODULE_DESCRIPTION(DRIVER_DESC);
754 * The following describes what I learned from studying the old
755 * ark3116.c driver, disassembling the windows driver, and some lucky
756 * guesses. Since I do not have any datasheet or other
757 * documentation, inaccuracies are almost guaranteed.
759 * Some specs for the ARK3116 can be found here:
760 * http://web.archive.org/web/20060318000438/
761 * www.arkmicro.com/en/products/view.php?id=10
762 * On that page, 2 GPIO pins are mentioned: I assume these are the
763 * OUT1 and OUT2 pins of the UART, so I added support for those
764 * through the MCR. Since the pins are not available on my hardware,
765 * I could not verify this.
766 * Also, it states there is "on-chip hardware flow control". I have
767 * discovered how to enable that. Unfortunately, I do not know how to
768 * enable XON/XOFF (software) flow control, which would need support
769 * from the chip as well to work. Because of the wording on the web
770 * page there is a real possibility the chip simply does not support
771 * software flow control.
773 * I got my ark3116 as part of a mobile phone adapter cable. On the
774 * PCB, the following numbered contacts are present:
787 * On my chip, all signals seem to be 3.3V, but 5V tolerant. But that
788 * may be different for the one you have ;-).
790 * The windows driver limits the registers to 0-F, so I assume there
791 * are actually 16 present on the device.
793 * On an UART interrupt, 4 bytes of data come in on the interrupt
794 * endpoint. The bytes are 0xe8 IIR LSR MSR.
796 * The baudrate seems to be generated from the 12MHz crystal, using
797 * 4-times subsampling. So quot=12e6/(4*baud). Also see description
801 * These seem to be the same as for a regular 16450. The FCR is set
802 * to UART_FCR_DMA_SELECT (0x8), I guess to enable transfers between
803 * the UART and the USB bridge/DMA engine.
806 * By trial and error, I found out that bit 0 enables hardware CTS,
807 * stopping TX when CTS is +5V. Bit 1 does the same for RTS, making
808 * RTS +5V when the 3116 cannot transfer the data to the USB bus
809 * (verified by disabling the reading URB). Note that as far as I can
810 * tell, the windows driver does NOT use this, so there might be some
811 * hardware bug or something.
813 * According to a patch provided here
814 * (http://lkml.org/lkml/2009/7/26/56), the ARK3116 can also be used
815 * as an IrDA dongle. Since I do not have such a thing, I could not
816 * investigate that aspect. However, I can speculate ;-).
818 * - IrDA encodes data differently than RS232. Most likely, one of
819 * the bits in registers 9..E enables the IR ENDEC (encoder/decoder).
820 * - Depending on the IR transceiver, the input and output need to be
821 * inverted, so there are probably bits for that as well.
822 * - IrDA is half-duplex, so there should be a bit for selecting that.
824 * This still leaves at least two registers unaccounted for. Perhaps
825 * The chip can do XON/XOFF or CRC in HW?
828 * Set to 0x00 for IrDA, when the baudrate is initialised.
831 * Set to 0x01 for IrDA, at init.
834 * Set to 0x01 for IrDA, 0x00 for RS232, at init.
837 * Set to 00 for IrDA, at init.
840 * Set to 0x41 for IrDA, at init.
843 * Somekind of baudrate override. The windows driver seems to set
844 * this to 0x00 for normal baudrates, 0x01 for 460800, 0x02 for 921600.
845 * Since 460800 and 921600 cannot be obtained by dividing 3MHz by an integer,
846 * it could be somekind of subdivisor thingy.
847 * However,it does not seem to do anything: selecting 921600 (divisor 3,
848 * reg E=2), still gets 1 MHz. I also checked if registers 9, C or F would
849 * work, but they don't.
851 * Register F: unknown