I am using a SPIRIT1 as a transmitterI switch the (2.8v) rail onI wait for the part to become ready (as indicated on GPIO0)I then program it.On MOST samples this is OK, but about one in ten units intermittently fail to program correctly (the SMPS doe...

Very standard "app note" circuitGood decoupling. 2.8v switched supply railXO is up within 350uS of startupDevice is programmed about 100uS after detection of the "reset OK" state