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. 20 132