SPI Header – Purpose of V_IO Pin

circuit-designpcbspi

I built a small PCB board that runs my HMC833 phase lock loop using the reference design provided by analog devices, and it works.

The pll is controlled using spi to set the frequency, so I placed a 12 pin header on the board to directly program the pll from my computer, a USB to SPI converter is used along with the provided pll programming software by analog device.

Below is the schematic of the HMC833 Eval board, and my design is almost identical to the eval board except that I customized the reference frequency input part for my own purposes:
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WHAT IS CONFUSING me is the purpose of the V_IO voltage. V_IO is a 3.3V supplied by the voltage regulator U4 to the SPI header J7, and that is it. V_IO is not supplying voltage to anything else on the board.

This concerns me because when I tested my PCB board, I connected my usb-spi interface to the spi header on the board. I turned the power on, program the registers and it indeed produced my desired frequency. BUT when I unplug pin 4 at the header (U7), it looks like everything is then turned off on the board. See photos below:

  1. When all spi pins and V_IO pins are connected, power source has current output and spectrum analyzer has the desire frequency (power amp wasn't power in the pic but it did produced frequency when I turn the amp on)
    enter image description here

  2. when pin 4 is unplug, no current output from the source
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  3. and this is the usb-spi converter I am using, it is provided when I purchased the eval board from Hitties(analog device). There isn't much information I can find regarding what exactly does this programmer do…
    enter image description here

Right now I need to draw a second version of my own board but this time I am putting in an MCU to drive the PLL instead of using a PC. I am stuck at what to do with the V_IO pin because I don't know what does V_IO actually do. From the schematic it looks useless because it is not connected to anything else on the board but everything seems to turned off if I don't connect them.

Anybody has any idea?
Regards,

Best Answer

Vio is the supply for external interfaces of a chip.

It may be separate from the core supplymerely for electrical integrity purposes related to having enough supply pins near the load.

But often it is separate because the I/O bank will or could run at a signalling different voltage from the core.

If you intend to communicate via 3v3 signalling, something needs to supply this pin with 3v3.

And like any supply, it needs good bypassing right where it enters the chip.

In your specific case the chip consuming Vio would probably the USB bridge, and it looks like your target board is perhaps taking a supply from that board on one pin and sending it back on another, to basically say "this is the signal voltage I want"

But then you don't seem to have shown the power supply section of the target board, so it's possible there is a source there you are overlooking.

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