[HMC830] REG_EN(AD4) net

REG_EN turns on the 3.3/5V HMC830 power with the eval board GUI "ChipEN" button.  If you don't want to do this leave pin 11 open at the 12 pin SPI header.  Another idea is connect a LED to pin 11 -- you could toggle the LED with ChipEN  to check if the USB interface is working.  You need 3.3V at pin 2 and 4 on the 12 pin header to power SPI signals from the USB interface.

Just be aware the HMC830 only performs SPI mode selection (HMC or Open mode) during power-on reset.   You must start with SCK and SEN low at power-on or the HMC830 could enter the wrong SPI mode.  The REG_EN signal gives software control over power-on reset.

Thank you for reply.

When you say "You must start with SCK and SEN low at power-on or the HMC830 could enter the wrong SPI mode.  The REG_EN signal gives software control over power-on reset.", you are saying me that the SCK and SEN signal are controlled by interface tool via software? By software I should set the SCK and SEN low at power-on?

Many thanks.

Antonio

The FTDI chip is powered from the USB 5V and is reset after connecting the USB cable.  All SPI outputs from the FT2232C chip are initialized to "1" but do not pass through the isolator until 5.5V power is turned on. The eval board software controls SPI signals via FTDI's MPSSE interface.   During initialization it clears all SPI signals, sets reg_en to power the HMC830 then performs mode selection.

After turning on the isolator there is a potential problem since SCK and SEN inputs are turned on at the same time reg_en turns on HMC830 power.  Depending on the power-on reset timing the HMC830 may detect a false mode selection as SCK and SEN slew high.   The solution is toggle 5.5V (or use ChipEN) and restart the eval software.  This forces a HMC830 mode selection but now the SPI signals are set low as the HMC830 comes out of reset.