I'm designing an industrial product with the TLK110. Because of the very limited board space I can only use the RMII mode. The microcontroller is STM32F407, so I must provide two different clock signals: 50 MHz required for RMII (TLK110 and STM32F4), and a separate clock for the STM32F4 core. I considered the following solutions:
1. One 25 MHz oscillator (ASEMB-25.000MHZ-LY-T from Abracon) and a clock buffer with PLL (NB2304AI2). The PLL would generate 50 MHz ( 2*25 MHz) and the buffer would distribute it to both TLK110 and STM32F4, as well as 25 MHz to STM32F4. In this way all the clocks would be from the same source and synchronized, but separately buffered. However, I dislike the jitter of the NB2304AI2, which is up to 400 ps.
2. Two different oscillators: one for STM32F4 (e.g. 12 MHz) and one 50 MHz oscillator (ASEMB-50.000MHZ-LY-T) for RMII (goes to both STM32F4 and TLK110). A problem here could be the loading of the 50 MHz oscillator. I failed to find in the available documentation the capacitance of the XI input pin of the TLK110 when an external oscillator is used. The 50 MHz oscillator (ASEMB-50.000MHZ-LY-T) can handle the total load capacitance up to 15 pF. In this case the total capacitance would be that of TLK110's XI pin and a microcontroller's input pin (about 5 pf) in parallel. Is there a risk of overloading the oscillator's output under these circumstances?
3. Two separate oscillators, but the 50 MHz oscillator would supply the clock signal only to the TLK110, and the STM32F4 would obtain its 50 MHz clock from TLK110's CLKOUT pin. The TLK110 datasheet does not explicitly suggest using the CLKOUT pin in this way, so I don't know if this would work reliably. The propagation delay of the CLKOUT signal can be a problem here.
Your comments and suggestions will be highly appreciated.
TLK110 CLKOUT preparation delay will violate the STM32F4 timing requirements on the RMII Interface so it is not recommended.
Solution 2 seem to be the best, we check the capacitance value parameter and will send the answer in few days.
About solution 1, it feasible, but not recommended due to the jitter problem you mentioned.
regarding option number 2: the max input load capacitance of the Xi pin at 50Mhz is 2pF. it seems The 50 MHz oscillator (ASEMB-50.000MHZ-LY-T) can handle the total load capacitance of TLK110's XI pin and a microcontroller's input pin (about 5 pf) in parallel.
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