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