参数资料
型号: MC68HC908RK2DW
厂商: FREESCALE SEMICONDUCTOR INC
元件分类: 微控制器/微处理器
英文描述: 8-BIT, FLASH, 4 MHz, MICROCONTROLLER, PDSO20
封装: PLASTIC, SOIC-20
文件页数: 133/158页
文件大小: 1020K
代理商: MC68HC908RK2DW
Internal Clock Generator Module (ICG)
MC68HC908RK2 Data Sheet, Rev. 5.1
76
Freescale Semiconductor
Figure 6-10. Code Example for Writing DDIV and DSTG
The method of changing the unadjusted operating point is by changing the size of the capacitor. This
capacitor is designed with 639 equally sized units. 384 of which are always connected. The remaining
255 units are put in by adjusting the ICG trim factor (TRIM). The default value for TRIM is $80, or 128
units, making the default capacitor size 512. Each unit added or removed will adjust the output frequency
by about
±0.195% of the unadjusted frequency (adding to TRIM will decrease frequency). Therefore, the
frequency of IBASE can be changed to
±25% of its unadjusted value, which is enough to cancel the
process variability mentioned before.
The best way to trim the internal clock is to use the timer to measure the width of an input pulse on an
input capture pin (this pulse must be supplied by the application and should be as long or wide as
possible). Considering the prescale value of the timer and the theoretical (zero error) frequency of the bus
(307.2 kHz *N/4), the error can be calculated. This error, expressed as a percentage, can be divided by
0.195% and the resultant factor added or subtracted from TRIM. This process should be repeated to
eliminate any residual error.
NOTE
It is recommended that the user preserve a copy of the contents of the ICG
trim register (ICGTR) in non-volitale memory.
Address $7FEF is reserved for an optional factory-determined value.
Consult with a local Freescale representative for more information and
availability of this option.
;DDIV and DSTG modification code example
;Changes DDIV and DSTG according to the initial and
; desired clock period values
;Requires ICGON to be clear (disabled)
;User must previously calculate DVFACT and STFACT by
; the equations listed in the specification
;Modifies X and A registers
start
lda
icgcr
;Verify ICGON clear (this will require
cmp
#13
; CMIE,CMF,CMON,ICGON,ICGS clear and CS,ECGON,ECGS set)
lda
#dvfact
;Add the DDIV factor (calculated before
add
icgdvr
; coding by the DDIV2 equation)
sta
icgdvr
lda
#stfact
;Load the DSTG factor (calculated before coding and
; multiplied by 128 to make it 0-255 for maximum precision
ldx
icgdsr
;Load current stage register contents
mul
;Multiply factor times current value
rola
;Since factor was multiplied by 128,
rolx
; result is x6-x0:a7, so put it all in X
bcc
store
;If result is >255, rolx will set carry
rorx
; so divide result by two and
inc
; add one to DDIV
store
stx
icgdsr
;Store value
lda
icgdvr
;Test to see if DDIV too high or low
cmp
#09
;Valid range 0-9; too low is FF/FE; too high is 0A/0B
bhi
exit
;If DDIV is 0-9, you’re almost done
lda
#09
;Otherwise, maximize period and execute error code
sta
icgdvr
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