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[1][2][3][4]. 28To switch away from the default value of 33.875 for the prescaler the 29the enhanced mode has to be explicitly enabled though, by setting bit 4 30of the EFR. In that mode setting bit 7 in the MCR enables the prescaler 31or otherwise it is bypassed as if the value of 1 was used. Additionally 32writing any value to CPR clears CPR2 for compatibility with old software 33written for older conventional PCI Oxford Semiconductor devices that do 34not have the extra prescaler's 9th bit in CPR2, so the CPR/CPR2 register 35pair 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 48r: 15625000, a: 15625000.00, d: 0.0000%, tcr: 4, cpr: 1.000, div: 1 49r: 12500000, a: 12500000.00, d: 0.0000%, tcr: 5, cpr: 1.000, div: 1 50r: 10416666, a: 10416666.67, d: 0.0000%, tcr: 6, cpr: 1.000, div: 1 51r: 8928571, a: 8928571.43, d: 0.0000%, tcr: 7, cpr: 1.000, div: 1 52r: 7812500, a: 7812500.00, d: 0.0000%, tcr: 8, cpr: 1.000, div: 1 53r: 4000000, a: 4000000.00, d: 0.0000%, tcr: 5, cpr: 3.125, div: 1 54r: 3686400, a: 3676470.59, d: -0.2694%, tcr: 8, cpr: 2.125, div: 1 55r: 3500000, a: 3496503.50, d: -0.0999%, tcr: 13, cpr: 1.375, div: 1 56r: 3000000, a: 2976190.48, d: -0.7937%, tcr: 14, cpr: 1.500, div: 1 57r: 2500000, a: 2500000.00, d: 0.0000%, tcr: 10, cpr: 2.500, div: 1 58r: 2000000, a: 2000000.00, d: 0.0000%, tcr: 10, cpr: 3.125, div: 1 59r: 1843200, a: 1838235.29, d: -0.2694%, tcr: 16, cpr: 2.125, div: 1 60r: 1500000, a: 1492537.31, d: -0.4975%, tcr: 5, cpr: 8.375, div: 1 61r: 1152000, a: 1152073.73, d: 0.0064%, tcr: 14, cpr: 3.875, div: 1 62r: 921600, a: 919117.65, d: -0.2694%, tcr: 16, cpr: 2.125, div: 2 63r: 576000, a: 576036.87, d: 0.0064%, tcr: 14, cpr: 3.875, div: 2 64r: 460800, a: 460829.49, d: 0.0064%, tcr: 7, cpr: 3.875, div: 5 65r: 230400, a: 230414.75, d: 0.0064%, tcr: 14, cpr: 3.875, div: 5 66r: 115200, a: 115207.37, d: 0.0064%, tcr: 14, cpr: 1.250, div: 31 67r: 57600, a: 57603.69, d: 0.0064%, tcr: 8, cpr: 3.875, div: 35 68r: 38400, a: 38402.46, d: 0.0064%, tcr: 14, cpr: 3.875, div: 30 69r: 19200, a: 19201.23, d: 0.0064%, tcr: 8, cpr: 3.875, div: 105 70r: 9600, a: 9600.06, d: 0.0006%, tcr: 9, cpr: 1.125, div: 643 71r: 4800, a: 4799.98, d: -0.0004%, tcr: 7, cpr: 2.875, div: 647 72r: 2400, a: 2400.02, d: 0.0008%, tcr: 9, cpr: 2.250, div: 1286 73r: 1200, a: 1200.00, d: 0.0000%, tcr: 14, cpr: 2.875, div: 1294 74r: 300, a: 300.00, d: 0.0000%, tcr: 11, cpr: 2.625, div: 7215 75r: 200, a: 200.00, d: 0.0000%, tcr: 16, cpr: 1.250, div: 15625 76r: 150, a: 150.00, d: 0.0000%, tcr: 13, cpr: 2.250, div: 14245 77r: 134, a: 134.00, d: 0.0000%, tcr: 11, cpr: 2.625, div: 16153 78r: 110, a: 110.00, d: 0.0000%, tcr: 12, cpr: 1.000, div: 47348 79r: 75, a: 75.00, d: 0.0000%, tcr: 4, cpr: 5.875, div: 35461 80r: 50, a: 50.00, d: 0.0000%, tcr: 16, cpr: 1.250, div: 62500 81r: 25, a: 25.00, d: 0.0000%, tcr: 16, cpr: 2.500, div: 62500 82r: 4, a: 4.00, d: 0.0000%, tcr: 16, cpr: 20.000, div: 48828 83r: 2, a: 2.00, d: 0.0000%, tcr: 16, cpr: 40.000, div: 48828 84r: 1, a: 1.00, d: 0.0000%, tcr: 16, cpr: 63.875, div: 61154 85 86With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX 87limitation imposed by `serial8250_get_baud_rate' standard baud rates 88below 300bps become unavailable in the regular way, e.g. the rate of 89200bps requires the baud base to be divided by 78125 and that is beyond 90the unsigned 16-bit range. The historic spd_cust feature can still be 91used by encoding the values for, the prescaler, the oversampling rate 92and the clock divisor (DLM/DLL) as follows to obtain such rates if so 93required: 94 95 31 29 28 20 19 16 15 0 96+-----+-----------------+-------+-------------------------------+ 97|0 0 0| CPR2:CPR | TCR | DLM:DLL | 98+-----+-----------------+-------+-------------------------------+ 99 100Use a value such encoded for the `custom_divisor' field along with the 101ASYNC_SPD_CUST flag set in the `flags' field in `struct serial_struct' 102passed with the TIOCSSERIAL ioctl(2), such as with the setserial(8) 103utility and its `divisor' and `spd_cust' parameters, and the select 104the baud rate of 38400bps. Note that the value of 0 in TCR sets the 105oversampling rate to 16 and prescaler values below 1 in CPR2/CPR are 106clamped by the driver to 1. 107 108For example the value of 0x1f4004e2 will set CPR2/CPR, TCR and DLM/DLL 109respectively to 0x1f4, 0x0 and 0x04e2, choosing the prescaler value, 110the oversampling rate and the clock divisor of 62.500, 16 and 1250 111respectively. These parameters will set the baud rate for the serial 112port to 62500000 / 62.500 / 1250 / 16 = 50bps. 113 114References: 115 116[1] "OXPCIe200 PCI Express Multi-Port Bridge", Oxford Semiconductor, 117 Inc., DS-0045, 10 Nov 2008, Section "950 Mode", pp. 64-65 118 119[2] "OXPCIe952 PCI Express Bridge to Dual Serial & Parallel Port", 120 Oxford Semiconductor, Inc., DS-0046, Mar 06 08, Section "950 Mode", 121 p. 20 122 123[3] "OXPCIe954 PCI Express Bridge to Quad Serial Port", Oxford 124 Semiconductor, Inc., DS-0047, Feb 08, Section "950 Mode", p. 20 125 126[4] "OXPCIe958 PCI Express Bridge to Octal Serial Port", Oxford 127 Semiconductor, Inc., DS-0048, Feb 08, Section "950 Mode", p. 20 128 129Maciej W. Rozycki <macro@orcam.me.uk> 130