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1.. SPDX-License-Identifier: GPL-2.0
2
3====================================================================
4Notes on Oxford Semiconductor PCIe (Tornado) 950 serial port devices
5====================================================================
6
7Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
8by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.
9
10The baud rate produced by the baud generator is obtained from this input
11frequency by dividing it by the clock prescaler, which can be set to any
12value from 1 to 63.875 in increments of 0.125, and then the usual 16-bit
13divisor is used as with the original 8250, to divide the frequency by a
14value from 1 to 65535. Finally a programmable oversampling rate is used
15that can take any value from 4 to 16 to divide the frequency further and
16determine the actual baud rate used. Baud rates from 15625000bps down
17to 0.933bps can be obtained this way.
18
19By default the oversampling rate is set to 16 and the clock prescaler is
20set to 33.875, meaning that the frequency to be used as the reference
21for the usual 16-bit divisor is 115313.653, which is close enough to the
22frequency of 115200 used by the original 8250 for the same values to be
23used for the divisor to obtain the requested baud rates by software that
24is unaware of the extra clock controls available.
25
26The oversampling rate is programmed with the TCR register and the clock
27prescaler is programmed with the CPR/CPR2 register pair [OX200]_ [OX952]_
28[OX954]_ [OX958]_. To switch away from the default value of 33.875 for
29the prescaler the enhanced mode has to be explicitly enabled though, by
30setting bit 4 of the EFR. In that mode setting bit 7 in the MCR enables
31the prescaler or otherwise it is bypassed as if the value of 1 was used.
32Additionally writing any value to CPR clears CPR2 for compatibility with
33old software written for older conventional PCI Oxford Semiconductor
34devices that do not have the extra prescaler's 9th bit in CPR2, so the
35CPR/CPR2 register pair has to be programmed in the right order.
36
37By using these parameters rates from 15625000bps down to 1bps can be
38obtained, with either exact or highly-accurate actual bit rates for
39standard and many non-standard rates.
40
41Here are the figures for the standard and some non-standard baud rates
42(including those quoted in Oxford Semiconductor documentation), giving
43the requested rate (r), the actual rate yielded (a) and its deviation
44from the requested rate (d), and the values of the oversampling rate
45(tcr), the clock prescaler (cpr) and the divisor (div) produced by the
46new ``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
88With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
89limitation imposed by ``serial8250_get_baud_rate`` standard baud rates
90below 300bps become unavailable in the regular way, e.g. the rate of
91200bps requires the baud base to be divided by 78125 and that is beyond
92the unsigned 16-bit range. The historic spd_cust feature can still be
93used by encoding the values for, the prescaler, the oversampling rate
94and the clock divisor (DLM/DLL) as follows to obtain such rates if so
95required:
96
97::
98
99 31 29 28 20 19 16 15 0
100 +-----+-----------------+-------+-------------------------------+
101 |0 0 0| CPR2:CPR | TCR | DLM:DLL |
102 +-----+-----------------+-------+-------------------------------+
103
104Use a value such encoded for the ``custom_divisor`` field along with the
105ASYNC_SPD_CUST flag set in the ``flags`` field in ``struct serial_struct``
106passed with the TIOCSSERIAL ioctl(2), such as with the setserial(8)
107utility and its ``divisor`` and ``spd_cust`` parameters, and then select
108the baud rate of 38400bps. Note that the value of 0 in TCR sets the
109oversampling rate to 16 and prescaler values below 1 in CPR2/CPR are
110clamped by the driver to 1.
111
112For example the value of 0x1f4004e2 will set CPR2/CPR, TCR and DLM/DLL
113respectively to 0x1f4, 0x0 and 0x04e2, choosing the prescaler value,
114the oversampling rate and the clock divisor of 62.500, 16 and 1250
115respectively. These parameters will set the baud rate for the serial
116port to 62500000 / 62.500 / 1250 / 16 = 50bps.
117
118Maciej 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