Documentation/misc-devices/oxsemi-tornado.rst

   1 .. SPDX-License-Identifier: GPL-2.0
   2
   3 ====================================================================
   4 Notes on Oxford Semiconductor PCIe (Tornado) 950 serial port devices
   5 ====================================================================
   6
   7 Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
   8 by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.
   9
  10 The baud rate produced by the baud generator is obtained from this input
  11 frequency by dividing it by the clock prescaler, which can be set to any
  12 value from 1 to 63.875 in increments of 0.125, and then the usual 16-bit
  13 divisor is used as with the original 8250, to divide the frequency by a
  14 value from 1 to 65535.  Finally a programmable oversampling rate is used
  15 that can take any value from 4 to 16 to divide the frequency further and
  16 determine the actual baud rate used.  Baud rates from 15625000bps down
  17 to 0.933bps can be obtained this way.
  18
  19 By default the oversampling rate is set to 16 and the clock prescaler is
  20 set to 33.875, meaning that the frequency to be used as the reference
  21 for the usual 16-bit divisor is 115313.653, which is close enough to the
  22 frequency of 115200 used by the original 8250 for the same values to be
  23 used for the divisor to obtain the requested baud rates by software that
  24 is unaware of the extra clock controls available.
  25
  26 The oversampling rate is programmed with the TCR register and the clock
  27 prescaler is programmed with the CPR/CPR2 register pair [OX200]_ [OX952]_
  28 [OX954]_ [OX958]_.  To switch away from the default value of 33.875 for
  29 the prescaler the enhanced mode has to be explicitly enabled though, by
  30 setting bit 4 of the EFR.  In that mode setting bit 7 in the MCR enables
  31 the prescaler or otherwise it is bypassed as if the value of 1 was used.
  32 Additionally writing any value to CPR clears CPR2 for compatibility with
  33 old software written for older conventional PCI Oxford Semiconductor
  34 devices that do not have the extra prescaler's 9th bit in CPR2, so the
  35 CPR/CPR2 register pair has to be programmed in the right order.
  36
  37 By using these parameters rates from 15625000bps down to 1bps can be
  38 obtained, with either exact or highly-accurate actual bit rates for
  39 standard and many non-standard rates.
  40
  41 Here are the figures for the standard and some non-standard baud rates
  42 (including those quoted in Oxford Semiconductor documentation), giving
  43 the requested rate (r), the actual rate yielded (a) and its deviation
  44 from the requested rate (d), and the values of the oversampling rate
  45 (tcr), the clock prescaler (cpr) and the divisor (div) produced by the
  46 new ``get_divisor`` handler:
  47
  48 ::
  49
  50  r: 15625000, a: 15625000.00, d:  0.0000%, tcr:  4, cpr:  1.000, div:     1
  51  r: 12500000, a: 12500000.00, d:  0.0000%, tcr:  5, cpr:  1.000, div:     1
  52  r: 10416666, a: 10416666.67, d:  0.0000%, tcr:  6, cpr:  1.000, div:     1
  53  r:  8928571, a:  8928571.43, d:  0.0000%, tcr:  7, cpr:  1.000, div:     1
  54  r:  7812500, a:  7812500.00, d:  0.0000%, tcr:  8, cpr:  1.000, div:     1
  55  r:  4000000, a:  4000000.00, d:  0.0000%, tcr:  5, cpr:  3.125, div:     1
  56  r:  3686400, a:  3676470.59, d: -0.2694%, tcr:  8, cpr:  2.125, div:     1
  57  r:  3500000, a:  3496503.50, d: -0.0999%, tcr: 13, cpr:  1.375, div:     1
  58  r:  3000000, a:  2976190.48, d: -0.7937%, tcr: 14, cpr:  1.500, div:     1
  59  r:  2500000, a:  2500000.00, d:  0.0000%, tcr: 10, cpr:  2.500, div:     1
  60  r:  2000000, a:  2000000.00, d:  0.0000%, tcr: 10, cpr:  3.125, div:     1
  61  r:  1843200, a:  1838235.29, d: -0.2694%, tcr: 16, cpr:  2.125, div:     1
  62  r:  1500000, a:  1492537.31, d: -0.4975%, tcr:  5, cpr:  8.375, div:     1
  63  r:  1152000, a:  1152073.73, d:  0.0064%, tcr: 14, cpr:  3.875, div:     1
  64  r:   921600, a:   919117.65, d: -0.2694%, tcr: 16, cpr:  2.125, div:     2
  65  r:   576000, a:   576036.87, d:  0.0064%, tcr: 14, cpr:  3.875, div:     2
  66  r:   460800, a:   460829.49, d:  0.0064%, tcr:  7, cpr:  3.875, div:     5
  67  r:   230400, a:   230414.75, d:  0.0064%, tcr: 14, cpr:  3.875, div:     5
  68  r:   115200, a:   115207.37, d:  0.0064%, tcr: 14, cpr:  1.250, div:    31
  69  r:    57600, a:    57603.69, d:  0.0064%, tcr:  8, cpr:  3.875, div:    35
  70  r:    38400, a:    38402.46, d:  0.0064%, tcr: 14, cpr:  3.875, div:    30
  71  r:    19200, a:    19201.23, d:  0.0064%, tcr:  8, cpr:  3.875, div:   105
  72  r:     9600, a:     9600.06, d:  0.0006%, tcr:  9, cpr:  1.125, div:   643
  73  r:     4800, a:     4799.98, d: -0.0004%, tcr:  7, cpr:  2.875, div:   647
  74  r:     2400, a:     2400.02, d:  0.0008%, tcr:  9, cpr:  2.250, div:  1286
  75  r:     1200, a:     1200.00, d:  0.0000%, tcr: 14, cpr:  2.875, div:  1294
  76  r:      300, a:      300.00, d:  0.0000%, tcr: 11, cpr:  2.625, div:  7215
  77  r:      200, a:      200.00, d:  0.0000%, tcr: 16, cpr:  1.250, div: 15625
  78  r:      150, a:      150.00, d:  0.0000%, tcr: 13, cpr:  2.250, div: 14245
  79  r:      134, a:      134.00, d:  0.0000%, tcr: 11, cpr:  2.625, div: 16153
  80  r:      110, a:      110.00, d:  0.0000%, tcr: 12, cpr:  1.000, div: 47348
  81  r:       75, a:       75.00, d:  0.0000%, tcr:  4, cpr:  5.875, div: 35461
  82  r:       50, a:       50.00, d:  0.0000%, tcr: 16, cpr:  1.250, div: 62500
  83  r:       25, a:       25.00, d:  0.0000%, tcr: 16, cpr:  2.500, div: 62500
  84  r:        4, a:        4.00, d:  0.0000%, tcr: 16, cpr: 20.000, div: 48828
  85  r:        2, a:        2.00, d:  0.0000%, tcr: 16, cpr: 40.000, div: 48828
  86  r:        1, a:        1.00, d:  0.0000%, tcr: 16, cpr: 63.875, div: 61154
  87
  88 With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
  89 limitation imposed by ``serial8250_get_baud_rate`` standard baud rates
  90 below 300bps become unavailable in the regular way, e.g. the rate of
  91 200bps requires the baud base to be divided by 78125 and that is beyond
  92 the unsigned 16-bit range.  The historic spd_cust feature can still be
  93 used by encoding the values for, the prescaler, the oversampling rate
  94 and the clock divisor (DLM/DLL) as follows to obtain such rates if so
  95 required:
  96
  97 ::
  98
  99   31 29 28             20 19   16 15                            0
 100  +-----+-----------------+-------+-------------------------------+
 101  |0 0 0|    CPR2:CPR     |  TCR  |            DLM:DLL            |
 102  +-----+-----------------+-------+-------------------------------+
 103
 104 Use a value such encoded for the ``custom_divisor`` field along with the
 105 ASYNC_SPD_CUST flag set in the ``flags`` field in ``struct serial_struct``
 106 passed with the TIOCSSERIAL ioctl(2), such as with the setserial(8)
 107 utility and its ``divisor`` and ``spd_cust`` parameters, and then select
 108 the baud rate of 38400bps.  Note that the value of 0 in TCR sets the
 109 oversampling rate to 16 and prescaler values below 1 in CPR2/CPR are
 110 clamped by the driver to 1.
 111
 112 For example the value of 0x1f4004e2 will set CPR2/CPR, TCR and DLM/DLL
 113 respectively to 0x1f4, 0x0 and 0x04e2, choosing the prescaler value,
 114 the oversampling rate and the clock divisor of 62.500, 16 and 1250
 115 respectively.  These parameters will set the baud rate for the serial
 116 port to 62500000 / 62.500 / 1250 / 16 = 50bps.
 117
 118 Maciej W. Rozycki  <macro@orcam.me.uk>
 119
 120 .. [OX200] "OXPCIe200 PCI Express Multi-Port Bridge", Oxford Semiconductor,
 121    Inc., DS-0045, 10 Nov 2008, Section "950 Mode", pp. 64-65
 122
 123 .. [OX952] "OXPCIe952 PCI Express Bridge to Dual Serial & Parallel Port",
 124    Oxford Semiconductor, Inc., DS-0046, Mar 06 08, Section "950 Mode",
 125    p. 20
 126
 127 .. [OX954] "OXPCIe954 PCI Express Bridge to Quad Serial Port", Oxford
 128    Semiconductor, Inc., DS-0047, Feb 08, Section "950 Mode", p. 20
 129
 130 .. [OX958] "OXPCIe958 PCI Express Bridge to Octal Serial Port", Oxford
 131    Semiconductor, Inc., DS-0048, Feb 08, Section "950 Mode", p. 20