Constant: Pre-defined, one-time settable constant for specifying application-level clock mode settings.

CON
_CLKMODE = Expression

Expression is an integer expression made up of one or two Clock Mode Setting Constants shown in the table Clock Mode Setting Constants. This will be the clock mode upon application start-up.

Explanation

_CLKMODE is used to specify the desired nature of the System Clock. It is a pre-defined constant symbol whose value is determined by the top object file of an application. The clock mode setting is a byte whose value is described by a combination of the RCxxxx, XINPUT, XTALx and PLLxx constants at compile time. The table Clock Mode Setting Constants illustrates the clock mode setting constants. Note that not every combination is valid; the table Valid Clock Mode Expressions and CLK Register Values shows all valid combinations.

Clock Mode Setting Constants

Clock Mode Setting Constant¹	XO Resistance²	XI/XO Capacitance²	Description
RCFAST	Infinite	n/a	Internal fast oscillator (~12 MHz). May be 8 MHz to 20 MHz. (Default)
RCSLOW	Infinite	n/a	Internal slow oscillator (~20 KHz). May be 13 KHz to 33 KHz.
XINPUT	Infinite	6 pF (pad only)	External clock-oscillator (DC to 80 MHz); XI pin only, XO disconnected
XTAL1	2 kΩ	36 pF	External low-speed crystal (4 MHz to 16 MHz)
XTAL2	1 kΩ	26 pF	External medium-speed crystal (8 MHz to 32 MHz)
XTAL3	500 Ω	16 pF	External high-speed crystal (20 MHz to 60 MHz)
PLL1X	n/a	n/a	Multiply external frequency times 1
PLL2X	n/a	n/a	Multiply external frequency times 2
PLL4X	n/a	n/a	Multiply external frequency times 4
PLL8X	n/a	n/a	Multiply external frequency times 8
PLL16X	n/a	n/a	Multiply external frequency times 16

¹All constants are also available in Propeller Assembly.
²All necessary resistors/capacitors are included in Propeller chip.

Valid Clock Mode Expressions and CLK Register Values

Valid Expression	CLK Register Value	Valid Expression	CLK Register Value
RCFAST	0_0_0_00_000	XTAL1 + PLL1X XTAL1 + PLL2X XTAL1 + PLL4X XTAL1 + PLL8X XTAL1 + PLL16X	0_1_1_01_011 0_1_1_01_100 0_1_1_01_101 0_1_1_01_110 0_1_1_01_111
RCSLOW	0_0_0_00_001
XINPUT	0_0_1_00_010
XTAL1 XTAL2 XTAL3	0_0_1_01_010 0_0_1_10_010 0_0_1_11_010	XTAL2 + PLL1X XTAL2 + PLL2X XTAL2 + PLL4X XTAL2 + PLL8X XTAL2 + PLL16X	0_1_1_10_011 0_1_1_10_100 0_1_1_10_101 0_1_1_10_110 0_1_1_10_111
XINPUT + PLL1X XINPUT + PLL2X XINPUT + PLL4X XINPUT + PLL8X XINPUT + PLL16X	0_1_1_00_011 0_1_1_00_100 0_1_1_00_101 0_1_1_00_110 0_1_1_00_111	XTAL3 + PLL1X XTAL3 + PLL2X XTAL3 + PLL4X XTAL3 + PLL8X XTAL3 + PLL16X	0_1_1_11_011 0_1_1_11_100 0_1_1_11_101 0_1_1_11_110 0_1_1_11_111

The top object file in an application (the one where compilation starts from) can specify a setting for _CLKMODE in its CON block. This defines the initial clock mode setting for the application and is the mode that the System Clock will switch to as soon as the application is booted up and execution begins. The following examples assume that they are contained within the top object file. Any _CLKMODE settings in child objects are simply ignored by the compiler. For example:

CON
  _CLKMODE = RCFAST

This sets the clock mode for the internal, fast RC Clock/Oscillator circuit. The System Clock would run at approximately 12 MHz with this setting. The RCFAST setting is the default setting, so if no _CLKMODE was actually defined, this is the setting that would be used. Note that the Clock PLL can not be used with the internal RC Clock/Oscillator. Here's an example with an external clock:

CON
  _CLKMODE = XTAL1 + PLL8X

This sets the clock mode for an external low-speed crystal (XTAL1), enables the Clock PLL circuit and sets the System Clock to use the 8x tap from the Clock PLL (PLL8X). If an external 4 MHz crystal was attached to XI and XO, for example, its signal would be multiplied by 16 (the Clock PLL always multiplies by 16) but the 8x bit result would be used; the System Clock would be 4 MHz * 8 = 32 MHz.

CON
  _CLKMODE = XINPUT + PLL2X

This sets the clock mode for an external clock-oscillator, connected to XI only, and enables the Clock PLL circuit and sets the System Clock to use the 2x result. If an external clock-oscillator pack of 8 MHz was attached to XI, the System clock would run at 16 MHz; that's 8 MHz * 2.

Note that the Clock PLL is not required and can be disabled by simply not specifying any multiplier setting, for example:

CON
  _CLKMODE = XTAL1

This sets the clock mode for an external low-speed crystal but leaves the Clock PLL disabled; the System Clock will be equal to the external crystal's frequency.

The _CLKFREQ and _XINFREQ Settings

For simplicity, the examples above only show _CLKMODE settings, but either a _CLKFREQ or _XINFREQ setting is required to follow it so that objects can determine their actual System Clock's frequency. The following is the full version of the second example, above, with an external crystal frequency (_XINFREQ) of 4 MHz.

CON
  _CLKMODE = XTAL1 + PLL8X       'low-speed crystal x 8
  _XINFREQ = 4_000_000           'external crystal of 4 MHz

This example is exactly like the second example above, but _XINFREQ indicates that the frequency of the external crystal is 4 MHz. The Propeller chip uses this value along with the _CLKMODE setting to determine the System Clock frequency (as reported by the CLKFREQ command) so that objects can properly adjust their timing. See _XINFREQ.

_CLKMODE vs.CLKMODE

_CLKMODE is related to, but not the same as, CLKMODE. _CLKMODE is an application-defined constant containing the requested clock mode at startup (in the form of clock setting constants that are OR'd together) whereas CLKMODE is a command that returns the current clock mode (in the form of the CLK register's bit pattern). Both may describe the same logical clock mode but their values are not equivalent.