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