[PATCH] clk: si570: Add a driver for SI570 oscillators
Sören Brinkmann
soren.brinkmann at xilinx.com
Fri Sep 13 13:26:04 EDT 2013
On Fri, Sep 13, 2013 at 10:00:05AM -0700, Guenter Roeck wrote:
> On Thu, Sep 12, 2013 at 05:55:37PM -0700, Soren Brinkmann wrote:
> > Add a driver for SILabs 570, 571, 598, 599 programmable oscillators.
> > The devices generate low-jitter clock signals and are reprogrammable via
> > an I2C interface.
> >
> > Cc: Guenter Roeck <linux at roeck-us.net>
> > Signed-off-by: Soren Brinkmann <soren.brinkmann at xilinx.com>
> > ---
> > .../devicetree/bindings/clock/silabs,si570.txt | 31 ++
> > drivers/clk/Kconfig | 10 +
> > drivers/clk/Makefile | 1 +
> > drivers/clk/clk-si570.c | 546 +++++++++++++++++++++
> > 4 files changed, 588 insertions(+)
> > create mode 100644 Documentation/devicetree/bindings/clock/silabs,si570.txt
> > create mode 100644 drivers/clk/clk-si570.c
> >
> > diff --git a/Documentation/devicetree/bindings/clock/silabs,si570.txt b/Documentation/devicetree/bindings/clock/silabs,si570.txt
> > new file mode 100644
> > index 0000000..099f0ee
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/clock/silabs,si570.txt
> > @@ -0,0 +1,31 @@
> > +Binding for Silicon Labs 570, 571, 598 and 599 programmable
> > +I2C clock generators.
> > +
> > +Reference
> > +This binding uses the common clock binding[1]. Details about the devices can be
> > +found in the data sheets[2][3].
> > +
> > +[1] Documentation/devicetree/bindings/clock/clock-bindings.txt
> > +[2] Si57x Data Sheet
> > + http://www.silabs.com/Support%20Documents/TechnicalDocs/si570.pdf
> > +[3] Si59x Data Sheet
> > + http://www.silabs.com/Support%20Documents/TechnicalDocs/si598-99.pdf
> > +
> > +Required properties:
> > + - compatible: Shall be one of "silabs,si57x", "silabs,si59x".
>
> This should probably explicitly list all supported devices, not "x".
> After all, there might at some point be an incompatible device which
> still matches the "x".
Okay, I'll make it explicitly list 570, 571, 598 and 599.
>
> > + - reg: I2C device address.
> > + - #clock-cells: From common clock bindings: Shall be 0.
> > + - factory-fout: Factory set default frequency
> > +
> > +Optional properties:
> > + - initial-fout: Initial output frequency to set during probe
> > + - temperature-stability-7ppm: Indicate a device with a temperature stability
> > + of 7ppm
> > +
> > +Example:
> > + si570: clock-generator at 5d {
> > + #clock-cells = <0>;
> > + compatible = "silabs,si570";
> > + reg = <0x5d>;
> > + factory-fout = <156250000>;
> > + };
> > diff --git a/drivers/clk/Kconfig b/drivers/clk/Kconfig
> > index 279407a..f5afabc 100644
> > --- a/drivers/clk/Kconfig
> > +++ b/drivers/clk/Kconfig
> > @@ -64,6 +64,16 @@ config COMMON_CLK_SI5351
> > This driver supports Silicon Labs 5351A/B/C programmable clock
> > generators.
> >
> > +config COMMON_CLK_SI570
> > + tristate "Clock driver for SiLabs 57x/59x"
>
> Better state something like "SiLabs Si570 and compatible chips"
Sounds good to me. I'll change this.
>
> > + depends on I2C
> > + depends on OF
> > + select REGMAP_I2C
> > + help
> > + ---help---
> > + This driver supports Silicon Labs 570/571/598/599 programmable
> > + clock generators.
> > +
> > config COMMON_CLK_S2MPS11
> > tristate "Clock driver for S2MPS11 MFD"
> > depends on MFD_SEC_CORE
> > diff --git a/drivers/clk/Makefile b/drivers/clk/Makefile
> > index 7b11106..c0e94b3 100644
> > --- a/drivers/clk/Makefile
> > +++ b/drivers/clk/Makefile
> > @@ -40,6 +40,7 @@ obj-$(CONFIG_COMMON_CLK_AXI_CLKGEN) += clk-axi-clkgen.o
> > obj-$(CONFIG_COMMON_CLK_WM831X) += clk-wm831x.o
> > obj-$(CONFIG_COMMON_CLK_MAX77686) += clk-max77686.o
> > obj-$(CONFIG_COMMON_CLK_SI5351) += clk-si5351.o
> > +obj-$(CONFIG_COMMON_CLK_SI570) += clk-si570.o
> > obj-$(CONFIG_COMMON_CLK_S2MPS11) += clk-s2mps11.o
> > obj-$(CONFIG_CLK_TWL6040) += clk-twl6040.o
> > obj-$(CONFIG_CLK_PPC_CORENET) += clk-ppc-corenet.o
> > diff --git a/drivers/clk/clk-si570.c b/drivers/clk/clk-si570.c
> > new file mode 100644
> > index 0000000..960d689
> > --- /dev/null
> > +++ b/drivers/clk/clk-si570.c
> > @@ -0,0 +1,546 @@
> > +/*
> > + * Driver for Silicon Labs Si570/Si571 Programmable XO/VCXO
> > + *
> > + * Copyright (C) 2010, 2011 Ericsson AB.
> > + * Copyright (C) 2011 Guenter Roeck.
> > + * Copyright (C) 2011 - 2013 Xilinx Inc.
> > + *
> > + * Author: Guenter Roeck <guenter.roeck at ericsson.com>
> > + * Sören Brinkmann <soren.brinkmann at xilinx.com>
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License as published by
> > + * the Free Software Foundation; either version 2 of the License, or
> > + * (at your option) any later version.
> > + *
> > + * This program is distributed in the hope that it will be useful,
> > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> > + * GNU General Public License for more details.
> > + */
> > +
> > +#include <linux/clk-provider.h>
> > +#include <linux/delay.h>
> > +#include <linux/module.h>
> > +#include <linux/i2c.h>
> > +#include <linux/regmap.h>
> > +#include <linux/slab.h>
> > +
> > +/* Si570 registers */
> > +#define SI570_REG_HS_N1 7
> > +#define SI570_REG_N1_RFREQ0 8
> > +#define SI570_REG_RFREQ1 9
> > +#define SI570_REG_RFREQ2 10
> > +#define SI570_REG_RFREQ3 11
> > +#define SI570_REG_RFREQ4 12
> > +#define SI570_REG_CONTROL 135
> > +#define SI570_REG_FREEZE_DCO 137
> > +#define SI570_DIV_OFFSET_7PPM 6
> > +
> > +#define HS_DIV_SHIFT 5
> > +#define HS_DIV_MASK 0xe0
> > +#define HS_DIV_OFFSET 4
> > +#define N1_6_2_MASK 0x1f
> > +#define N1_1_0_MASK 0xc0
> > +#define RFREQ_37_32_MASK 0x3f
> > +
> > +#define SI570_FOUT_FACTORY_DFLT 156250000LL
> > +#define SI598_FOUT_FACTORY_DFLT 10000000LL
> > +
> > +#define SI570_MIN_FREQ 10000000L
> > +#define SI570_MAX_FREQ 1417500000L
> > +#define SI598_MAX_FREQ 525000000L
> > +
> > +#define FDCO_MIN 4850000000LL
> > +#define FDCO_MAX 5670000000LL
> > +
> > +#define SI570_CNTRL_RECALL (1 << 0)
> > +#define SI570_CNTRL_FREEZE_M (1 << 5)
> > +#define SI570_CNTRL_NEWFREQ (1 << 6)
> > +
> > +#define SI570_FREEZE_DCO (1 << 4)
> > +
> > +/**
> > + * struct clk_si570:
> > + * @hw: Clock hw struct
> > + * @regmap: Device's regmap
> > + * @div_offset: Rgister offset for dividers
> > + * @max_freq: Maximum frequency for this device
> > + * @fxtal: Factory xtal frequency
> > + * @n1: Clock divider N1
> > + * @hs_div: Clock divider HSDIV
> > + * @rfreq: Clock multiplier RFREQ
> > + * @frequency: Current output frequency
> > + * @i2c_client: I2C client pointer
> > + */
> > +struct clk_si570 {
> > + struct clk_hw hw;
> > + struct regmap *regmap;
> > + unsigned int div_offset;
> > + u64 max_freq;
> > + u64 fxtal;
> > + unsigned int n1;
> > + unsigned int hs_div;
> > + u64 rfreq;
> > + u64 frequency;
> > + struct i2c_client *i2c_client;
> > +};
> > +#define to_clk_si570(_hw) container_of(_hw, struct clk_si570, hw)
> > +
> > +/**
> > + * struct si570_device_data:
> > + * @max_freq: Maximum output frequency
> > + * @default_fout: Default factory output frequency value
> > + */
> > +struct si570_device_data {
> > + u64 max_freq;
> > + u32 default_fout;
> > +};
> > +
> > +static const struct si570_device_data si57x_ddata = {
> > + .max_freq = SI570_MAX_FREQ,
> > + .default_fout = SI570_FOUT_FACTORY_DFLT
> > +};
> > +
> > +static const struct si570_device_data si59x_ddata = {
> > + .max_freq = SI598_MAX_FREQ,
> > + .default_fout = SI598_FOUT_FACTORY_DFLT
> > +};
> > +
> > +/**
> > + * si570_get_divs() - Read clock dividers from HW
> > + * @data: Pointer to struct clk_si570
> > + * @rfreq: Fractional multiplier (output)
> > + * @n1: Divider N1 (output)
> > + * @hs_div: Divider HSDIV (output)
> > + * Returns 0 on success, negative errno otherwise.
> > + *
> > + * Retrieve clock dividers and multipliers from the HW.
> > + */
> > +static int si570_get_divs(struct clk_si570 *data, u64 *rfreq,
> > + unsigned int *n1, unsigned int *hs_div)
>
> I would suggest to align continuation lines with the opening (,
> though that is really up to you (and the maintainer).
Hmm, I don't really like that since it is high maintenance and is easily
messed up by search and replace operations and similar. Therefore I rely
on vim's smart indent.
>
> > +{
> > + int err;
> > + u8 reg[6];
> > + u64 tmp;
> > +
> > + err = regmap_bulk_read(data->regmap, SI570_REG_HS_N1 + data->div_offset,
> > + reg, ARRAY_SIZE(reg));
> > + if (err)
> > + return err;
> > +
> > + *hs_div = ((reg[0] & HS_DIV_MASK) >> HS_DIV_SHIFT) + HS_DIV_OFFSET;
> > + *n1 = ((reg[0] & N1_6_2_MASK) << 2) + ((reg[1] & N1_1_0_MASK) >> 6) + 1;
> > + /* Handle invalid cases */
> > + if (*n1 > 1)
> > + *n1 &= ~1;
> > +
> > + tmp = reg[1] & RFREQ_37_32_MASK;
> > + tmp = (tmp << 8) + reg[2];
> > + tmp = (tmp << 8) + reg[3];
> > + tmp = (tmp << 8) + reg[4];
> > + tmp = (tmp << 8) + reg[5];
> > + *rfreq = tmp;
> > +
> > + return 0;
> > +}
> > +
> > +/**
> > + * si570_get_defaults() - Get default values
> > + * @data: Driver data structure
> > + * @fout: Factory frequency output
> > + * Returns 0 on success, negative errno otherwise.
> > + */
> > +static int si570_get_defaults(struct clk_si570 *data, u64 fout)
> > +{
> > + int err;
> > + u64 fdco;
> > +
> > + regmap_write(data->regmap, SI570_REG_CONTROL, SI570_CNTRL_RECALL);
> > +
> > + err = si570_get_divs(data, &data->rfreq, &data->n1, &data->hs_div);
> > + if (err)
> > + return err;
> > +
> > + /*
> > + * Accept optional precision loss to avoid arithmetic overflows.
> > + * Acceptable per Silicon Labs Application Note AN334.
> > + */
> > + fdco = fout * data->n1 * data->hs_div;
> > + if (fdco >= (1LL << 36))
> > + data->fxtal = div64_u64(fdco << 24, data->rfreq >> 4);
> > + else
> > + data->fxtal = div64_u64(fdco << 28, data->rfreq);
> > +
> > + data->frequency = fout;
> > +
> > + return 0;
> > +}
> > +
> > +/**
> > + * si570_update_rfreq() - Update clock multiplier
> > + * @data: Driver data structure
> > + * Passes on regmap_bulk_write() return value.
> > + */
> > +static int si570_update_rfreq(struct clk_si570 *data)
> > +{
> > + u8 reg[5];
> > +
> > + reg[0] = ((data->n1 - 1) << 6) |
> > + ((data->rfreq >> 32) & RFREQ_37_32_MASK);
> > + reg[1] = (data->rfreq >> 24) & 0xff;
> > + reg[2] = (data->rfreq >> 16) & 0xff;
> > + reg[3] = (data->rfreq >> 8) & 0xff;
> > + reg[4] = data->rfreq & 0xff;
> > +
> > + return regmap_bulk_write(data->regmap, SI570_REG_N1_RFREQ0 +
> > + data->div_offset, reg, ARRAY_SIZE(reg));
> > +}
> > +
> > +/**
> > + * si570_calc_divs() - Caluclate clock dividers
> > + * @frequency: Target frequency
> > + * @data: Driver data structure
> > + * @out_rfreq: RFREG fractional multiplier (output)
> > + * @out_n1: Clock divider N1 (output)
> > + * @out_hs_div: Clock divider HSDIV (output)
> > + * Returns 0 on success, negative errno otherwise.
> > + *
> > + * Calculate the clock dividers (@out_hs_div, @out_n1) and clock multiplier
> > + * (@out_rfreq) for a given target @frequency.
> > + */
> > +static int si570_calc_divs(unsigned long frequency, struct clk_si570 *data,
> > + u64 *out_rfreq, unsigned int *out_n1, unsigned int *out_hs_div)
> > +{
> > + int i;
> > + unsigned int n1, hs_div;
> > + u64 fdco, best_fdco = ULLONG_MAX;
> > + static const uint8_t si570_hs_div_values[] = { 11, 9, 7, 6, 5, 4 };
> > +
> > + for (i = 0; i < ARRAY_SIZE(si570_hs_div_values); i++) {
> > + hs_div = si570_hs_div_values[i];
> > + /* Calculate lowest possible value for n1 */
> > + n1 = div_u64(div_u64(FDCO_MIN, hs_div), frequency);
> > + if (!n1 || (n1 & 1))
> > + n1++;
> > + while (n1 <= 128) {
> > + fdco = (u64)frequency * (u64)hs_div * (u64)n1;
> > + if (fdco > FDCO_MAX)
> > + break;
> > + if (fdco >= FDCO_MIN && fdco < best_fdco) {
> > + *out_n1 = n1;
> > + *out_hs_div = hs_div;
> > + *out_rfreq = div64_u64(fdco << 28, data->fxtal);
> > + best_fdco = fdco;
> > + }
> > + n1 += (n1 == 1 ? 1 : 2);
> > + }
> > + }
> > +
> > + if (best_fdco == ULLONG_MAX)
> > + return -EINVAL;
> > +
> > + return 0;
> > +}
> > +
> > +static unsigned long si570_recalc_rate(struct clk_hw *hw,
> > + unsigned long parent_rate)
> > +{
> > + int err;
> > + u64 rfreq, rate;
> > + unsigned int n1, hs_div;
> > + struct clk_si570 *data = to_clk_si570(hw);
> > +
> > + err = si570_get_divs(data, &rfreq, &n1, &hs_div);
> > + if (err) {
> > + dev_err(&data->i2c_client->dev, "unable to recalc rate\n");
> > + return data->frequency;
> > + }
> > +
> > + rfreq = div_u64(rfreq, hs_div * n1);
> > + rate = (data->fxtal * rfreq) >> 28;
> > +
> > + return rate;
> > +}
> > +
> > +static long si570_round_rate(struct clk_hw *hw, unsigned long rate,
> > + unsigned long *parent_rate)
> > +{
> > + int err;
> > + u64 rfreq;
> > + unsigned int n1, hs_div;
> > + struct clk_si570 *data = to_clk_si570(hw);
> > +
> > + if (!rate)
> > + return 0;
> > +
> > + if (div64_u64(abs(rate - data->frequency) * 10000LL,
> > + data->frequency) < 35) {
> > + rfreq = div64_u64((data->rfreq * rate) +
> > + div64_u64(data->frequency, 2), data->frequency);
> > + n1 = data->n1;
> > + hs_div = data->hs_div;
> > +
> > + } else {
> > + err = si570_calc_divs(rate, data, &rfreq, &n1, &hs_div);
> > + if (err) {
> > + dev_err(&data->i2c_client->dev,
> > + "unable to round rate\n");
> > + return 0;
> > + }
> > + }
> > +
> > + return rate;
> > +}
> > +
> > +/**
> > + * si570_set_frequency() - Adjust output frequency
> > + * @data: Driver data structure
> > + * @frequency: Target frequency
> > + * Returns 0 on success.
> > + *
> > + * Update output frequency for big frequency changes (> 3,500 ppm).
> > + */
> > +static int si570_set_frequency(struct clk_si570 *data, unsigned long frequency)
> > +{
> > + int err;
> > +
> > + err = si570_calc_divs(frequency, data, &data->rfreq, &data->n1,
> > + &data->hs_div);
> > + if (err)
> > + return err;
> > +
> > + /*
> > + * The DCO reg should be accessed with a read-modify-write operation
> > + * per AN334
> > + */
> > + regmap_write(data->regmap, SI570_REG_FREEZE_DCO, SI570_FREEZE_DCO);
> > + regmap_write(data->regmap, SI570_REG_HS_N1 + data->div_offset,
> > + ((data->hs_div - HS_DIV_OFFSET) <<
> > + HS_DIV_SHIFT)
> > + | (((data->n1 - 1) >> 2) & N1_6_2_MASK));
>
> This is an example where the alignment to ( would really help.
>
> regmap_write(data->regmap, SI570_REG_HS_N1 + data->div_offset,
> ((data->hs_div - HS_DIV_OFFSET) << HS_DIV_SHIFT)
> | (((data->n1 - 1) >> 2) & N1_6_2_MASK));
I agree on this one. I'll look into making this look a little prettier :)
>
> > + si570_update_rfreq(data);
> > + regmap_write(data->regmap, SI570_REG_FREEZE_DCO, 0);
> > + regmap_write(data->regmap, SI570_REG_CONTROL, SI570_CNTRL_NEWFREQ);
> > +
> > + /* Applying a new frequency can take up to 10ms */
> > + usleep_range(10000, 10001);
>
> One microsecond range doesn't really buy anything besides forcing the compiler
> to generate extra code. Why not just use 10000 ?
That's due to checkpatch. I originally had 'msleep(10)' and checkpatch
suggested to use 'usleep_range()'. Then I put
'usleep_range(10000, 10000)' and checkpatch suggested to not use the
same values for MIN and MAX, so I added the 1 to the MAX value.
If it is considered to be safe and okay to put in MIN=MAX=10000, I'll
change it accordingly.
>
> > +
> > + return 0;
> > +}
> > +
> > +/**
> > + * si570_set_frequency_small() - Adjust output frequency
> > + * @data: Driver data structure
> > + * @frequency: Target frequency
> > + * Returns 0 on success.
> > + *
> > + * Update output frequency for small frequency changes (< 3,500 ppm).
> > + */
> > +static int si570_set_frequency_small(struct clk_si570 *data,
> > + unsigned long frequency)
> > +{
> > + /*
> > + * This is a re-implementation of DIV_ROUND_CLOSEST
> > + * using the div64_u64 function lieu of letting the compiler
> > + * insert EABI calls
> > + */
> > + data->rfreq = div64_u64((data->rfreq * frequency) +
> > + div_u64(data->frequency, 2), data->frequency);
> > + regmap_write(data->regmap, SI570_REG_CONTROL, SI570_CNTRL_FREEZE_M);
> > + si570_update_rfreq(data);
> > + regmap_write(data->regmap, SI570_REG_CONTROL, 0);
> > +
> > + return 0;
> > +}
> > +
> > +static int si570_set_rate(struct clk_hw *hw, unsigned long rate,
> > + unsigned long parent_rate)
> > +{
> > + struct clk_si570 *data = to_clk_si570(hw);
> > + struct i2c_client *client = data->i2c_client;
> > + int err;
> > +
> > + if (rate < SI570_MIN_FREQ || rate > data->max_freq) {
> > + dev_warn(&client->dev,
> > + "requested frequency %lu Hz is out of range\n", rate);
> > + return -EINVAL;
> > + }
> > +
> > + if (div64_u64(abs(rate - data->frequency) * 10000LL,
> > + data->frequency) < 35)
> > + err = si570_set_frequency_small(data, rate);
> > + else
> > + err = si570_set_frequency(data, rate);
> > +
> > + if (err)
> > + return err;
> > +
> > + data->frequency = rate;
> > +
> > + return 0;
> > +}
> > +
> > +static const struct clk_ops si570_clk_ops = {
> > + .recalc_rate = si570_recalc_rate,
> > + .round_rate = si570_round_rate,
> > + .set_rate = si570_set_rate,
> > +};
> > +
> > +static const struct of_device_id clk_si570_of_match[] = {
> > + { .compatible = "silabs,si57x", .data = &si57x_ddata },
> > + { .compatible = "silabs,si59x", .data = &si59x_ddata },
> > + { },
> > +};
> > +MODULE_DEVICE_TABLE(of, clk_si570_of_match);
> > +
> > +static bool si570_regmap_is_volatile(struct device *dev, unsigned int reg)
> > +{
> > + switch (reg) {
> > + case 135:
> > + return true;
> > + default:
> > + return false;
> > + }
> > +}
> > +
> > +static bool si570_regmap_is_writeable(struct device *dev, unsigned int reg)
> > +{
> > + switch (reg) {
> > + case 7 ... 18:
> > + case 135:
> > + case 137:
> > + return true;
> > + default:
> > + return false;
> > + }
> > +}
> > +
> > +static struct regmap_config si570_regmap_config = {
> > + .reg_bits = 8,
> > + .val_bits = 8,
> > + .cache_type = REGCACHE_RBTREE,
> > + .max_register = 137,
> > + .writeable_reg = si570_regmap_is_writeable,
> > + .volatile_reg = si570_regmap_is_volatile,
> > +};
> > +
> > +static int si570_probe(struct i2c_client *client,
> > + const struct i2c_device_id *id)
> > +{
> > + struct clk_si570 *data;
> > + struct clk_init_data init;
> > + struct clk *clk;
> > + u32 initial_fout, factory_fout;
> > + int err;
> > + const struct of_device_id *match;
> > + const struct si570_device_data *ddata;
> > +
> > + data = devm_kzalloc(&client->dev, sizeof(*data), GFP_KERNEL);
> > + if (!data)
> > + return -ENOMEM;
> > +
> > + match = of_match_node(clk_si570_of_match, client->dev.of_node);
> > + if (!match)
> > + return -EINVAL;
>
> Seems unusual. Is this really needed ? It precludes the driver from being used
> in a non-devicetree environment, for example. I would guess that there is a match
> if client->dev.of_node is set. Otherwise, this code would be needed in every
> driver supporting devicetree, and I don't recall seeing that.
>
> > + ddata = match->data;
> > +
> You should be able to get this information (ie the pointer to si570_device_data)
> from id->driver_data. That would be more consistent with other i2c devices.
I think I copied this approach from the other clk-si... driver. I'll
do some research on your suggestion and change it. Could you point me to
an example for your proposal?
>
> > + init.ops = &si570_clk_ops;
> > + init.flags = CLK_IS_ROOT;
> > + init.num_parents = 0;
> > + data->hw.init = &init;
> > + data->max_freq = ddata->max_freq;
> > + data->i2c_client = client;
> > +
> > + if (of_property_read_bool(client->dev.of_node,
> > + "temperature-stability-7ppm"))
> > + data->div_offset = SI570_DIV_OFFSET_7PPM;
> > +
> > + if (of_property_read_string(client->dev.of_node, "clock-output-names",
> > + &init.name))
> > + init.name = client->dev.of_node->name;
> > +
> > + if (of_property_read_u32(client->dev.of_node, "factory-fout",
> > + &factory_fout)) {
> > + dev_warn(&client->dev,
> > + "DTS does not contain factory-fout, using default\n");
>
> Is that really worth a warning ?
My understanding is, that the default output frequency is part-specific
and required to calculate the frequency of the internal crystal. Hence,
if you do not provide the correct default output frequency for your part, all
frequencies generated by this driver will be off, unless the here used
default matches your part. Please correct me if I'm wrong, otherwise, I
think it's worth a warning.
Sören
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