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Tool/software:
MCU:
External crystal oscillator: 10MHz
The above combination is used and the PLL setting is (OSCCLK * 12)/4.
Q1:
Regarding crystal oscillator signal input to square wave conversion within the MCU
(1) Is the square wave shaped by the rising edge detected by threshold judgment and 0V (mid-amplitude potential)? www.ti.com/.../tms320f280039c.pdf
(2) The above link (listed on p.91/276 Figure 6-19) is a document for a different type of microcontroller, but is it correct to assume that the TMS320F28030 has similar specifications? Are there any publicly available documents?
Q2:
If there is a problem with the crystal oscillator and the frequency fluctuates, how will the PLL period be affected?
(1) The overall period of the PLL multiplication part will be extended
(2) The period of the final square wave of the PLL multiplication part will be extended
(3) Factors other than (1) and (2)
Hi Yasuhiro,
(1) Is the square wave shaped by the rising edge detected by threshold judgment and 0V (mid-amplitude potential)? www.ti.com/.../tms320f280039c.pdf
JC: Internal clock is triggered by the clock transition level on X1, which is VIL (less than 0.2*1.8v) and VIH (greater than 0.8*1.8v)
(2) The above link (listed on p.91/276 Figure 6-19) is a document for a different type of microcontroller, but is it correct to assume that the TMS320F28030 has similar specifications? Are there any publicly available documents?
JC: No, the crystal circuit in F28020(older device) is different from F280039(newer device). There obviously are more improvements on F280039 crystal oscillator design.
If there is a problem with the crystal oscillator and the frequency fluctuates, how will the PLL period be affected?
JC:PLL should lock to the target frequency. If the crystal oscillator frequency fluctuates, that issue has to be addressed first, Please check that the external crystal chosen meets the specs of the F2802x crystal specs below:
A deviation to the above specs may cause the resonant frequency to shift or may cause the XTAL to not start up at all.
Regards,
Joseph
