Digital Signal Controllersのトピック56F82746VLF - pin capacitances?

56F82746VLF - pin capacitances?

matteomagistra — Wed, 02 Dec 2020 13:25:31 GMT

Hello all,

I am having some difficulties while searching specific information about the input capacitance of the following 56F82746VLF pins (LQFP-48):
- pin 3, configured as CLKIN0
- pin 4, configured as GPIOC1

Can anyone help me?

Thanks, Matteo

Re: 56F82746VLF - pin capacitances?

xiangjun_rong — Thu, 03 Dec 2020 01:59:39 GMT

Hi, Matteo,

I am not clear about your question. Anyway, as you know that the MC56F82746 can connect to crystal and clock source. The crystal component is a passive, it has two pins, you can connect the crystal to the Xtal(pin4) and EXTAL(pin3) together two 22pF capacitors and one 1Mohm parallel resistor(optional) as the followng fig.

For the crystal, there is internal oscillator for the MC56F827xx so that a clock signal can be generated.

If you connect a clock source which is active device(it has independent power system can output clock signal), you can connect the clock signal to CLKIN0(pin3) directly, no capacitor is required in the case.

Hope it can help you

XiangJun Rong

Re: 56F82746VLF - pin capacitances?

matteomagistra — Thu, 03 Dec 2020 08:08:01 GMT

Hi Xiangjun,

thank you for your reply.

Actually, we are going to use a crystal oscillator with independent power supply, which goes directly on CLKIN0 as a clock source.

The specs of this oscillator say: Maximum output load capacitance = 15pF.

For technical reasons, we had to connect both pins 3 and 4 of the DSC together, so we want to be sure that the end capacitance of these pins doesn't exceed 15pF.

Can you help me?

Best regards, Matteo

Re: 56F82746VLF - pin capacitances?

xiangjun_rong — Thu, 03 Dec 2020 09:00:21 GMT

Hi, Matteo,

The 15pF capacitor means that if the pin parasitic capacitances exceeds 15pF, the clock source signal will distort, it does not mean that you are required to connect an external capacitor. The less the pin capacitances is, the better the clock source signal quality is.

In conclusion, pls just connect the clock source output signal to the CLKIN0 pin(pin 3 for 48pins LQFP of LPC82746) directly, float the pin4, or use it as GPIO.

But DO NOT connect the pin3 and pin4 together.

Hope it can help you

XiangJun Rong

Re: 56F82746VLF - pin capacitances?

matteomagistra — Thu, 03 Dec 2020 09:45:20 GMT

Hi Xiangjun,

thanks for the reply, There is no additional external capacitor.

We have a batch of pcbs with pin 3 and pin 4 connected together (cannot delete connection).
Pin 3 is set as CLKIN0
Pin 4 is set as GPIOC1

My question is:
(Internal input capacitance of pin3 + Internal input capacitance of pin4) <= 15pF?

Best regards, Matteo

Re: 56F82746VLF - pin capacitances?

xiangjun_rong — Fri, 04 Dec 2020 01:21:02 GMT

Hi, Matteo,

Now I see your issue, you connect the pin3&4 together by mistake on PCB.

Generally, the pin parasitic capacitance is 1~2pF, so there is not any problem if you connect pin3&4 together.

In default, the pin4 is in GPIO input mode, it is okay. Do not configure it as GPIO output mode or XTAL mode.

Hope it can help you

XiangJun Rong

Re: 56F82746VLF - pin capacitances?

matteomagistra — Fri, 04 Dec 2020 07:52:48 GMT

Hi Xiangjun,

perfect, thank you very much for you help!

Best regards, Matteo