You can enter into DEBUG mode for software debug.
In datasheet page14 REV0.8
14.5 Debug mode
The FS85/FS84 enters in Debug mode with the sequence described in Figure 8:
1. DBG pin = VDBG and VSUP > VSUP_UVH
2. WAKE1 or WAKE2 > WAKE12VIH
VDBG and VSUP can come up at the same time as long as WAKE1 or WAKE2 comes up