DS80C390_00 Datasheet, PDF (59/155 Pages) Dallas Semiconductor – High-Speed Microcontroller User’s Guide Supplement

CTM
Bit 4
4X/ 2X
Bit 3
ALEOFF
Bit 2
Bits 1-0
DS80C390 High-Speed Microcontroller User’s Guide Supplement
Crystal Multiplier Enable. The CTM bit enables/disables the Crystal Clock
Multiplier. The CTM bit can be changed only when the CD1 and CD0 bits are set
to divide by 4 mode and the RGMD is cleared to 0. When cleared this bit disables
the Crystal Clock Multiplier to save energy. Setting this bit enables the Crystal
Clock Multiplier, permitting the use of the 1 or 2 clock per machine cycle speeds.
The following procedure must be performed when setting the CTM bit.
1. Select the desired clock rate via the 4X/ 2X bit. (4X/ 2X =1, 1 clock per cycle,
4X/ 2X =0, 2 clocks per cycle).
2. Set the CTM bit. At this point the CKRY bit (EXIF.3) will be cleared,
indicating the internal clock stabilization period has commenced. Software is
prohibited from modifying the CD1, CD0 bits while the CKRY bit is cleared.
3. Poll the CKRY bit until it is set.
4. Change CD0, CD1 bits to 00b.
CTM cannot be changed from a 1 to a 0 while the Crystal Clock Multiplier option
is selected via the CD1 and CD0 clock control bits. The CTM is also
automatically cleared to a logic 0 when the processor enters into a Stop mode.
System Clock Multiplier. This bit selects the internal crystal oscillator
multiplier setting, which in turn establishes a speed of one or two clocks per
machine cycle. This bit can only be altered when the CTM bit is cleared to
prevent the corruption of the system clock.
0 = The device operates at a rate of two clocks per machine cycle.
1 = The device operates at a rate of one clock per machine cycle.
ALE Disable. This bit disables the expression of the ALE signal on the device
pin during all on-board program and data memory accesses. External memory
accesses will automatically enable ALE independent of the ALEOFF bit.
0 = ALE expression is enabled.
1 = ALE expression is disabled.
Reserved. These bits will read 1
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