Thanks for the idea - I purchased the X-nucleo-IHM03A1 for testing. I am supplying the 3.3V for VDD and VREG externally and connected the SPI port to an external MCU. I first drive Reset high before trying to initialize the ST registers. When the VS power is off, the SPI queries all return 0, and the FLAG/BUSY LED's remain off. Once I supply 24V to the VS for the motor supply, I can read and write the ST registers just fine.
I double checked the schematic, the 3.3V supply is driving both the VDDIO and the VREG while VCC & VS would be off & protected by external diodes in this mode.
Suspecting the oscillator wasn't running, I checked the pads at R22, (OSCOUT) pin to see if the internal oscillator had booted up... sure enough no clock until VS has power.
Reading the datasheet pg 22, the only power up conditions for the logic are as follows, so something is either wrong with the datasheet or the IC design:
7.1 Device power-up
During power-up, the device is under reset (all logic IOs disabled and power bridges in high impedance state) until the following conditions are satisfied:
- VREG is greater than VREGthOn
- Internal oscillator is operative
- STBY/RESET input is forced high.
After power-up, the device state is the following:
- Parameters are set to default
- Internal logic is driven by internal oscillator and a 2-MHz clock is provided by the OSCOUT pin
- Bridges are disabled (high impedance).
- FLAG output is forced low (UVLO failure indication).
After power-up, a period of tlogicwu must pass before applying a command to allow proper oscillator and logic startup. Any movement command makes the device exit from High Z state (HardStop and SoftStop included).
Further looking into the clock - there isn't anything mentioned about which power supply powers the internal clock (pg 29):
7.8 Internal oscillator and oscillator driver
The control logic clock can be supplied by the internal 16-MHz oscillator, an external oscillator (crystal or ceramic resonator) or a direct clock signal.
These working modes can be selected by EXT_CLK and OSC_SEL parameters in the CONFIG register (see Table 41 on page 66).
At power-up the device starts using the internal oscillator and provides a 2-MHz clock signal on the OSCOUT pin
Unfortunately I think I am at a dead end for this quest. It would have been nice to use the UVLO features and see the device from the MCU well before the VS motor power is enabled.
