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L138: 2.25V IO put device “In Reset”

Paul Singh
Expert 2750 points
Other Parts Discussed in Thread: TPS65070

Hi,

We use TPS65070 to power our L138 board. The L138 has power group A, B and C and we tie all them together so there is actually one single power group for the dual-voltage IO supply voltage.

For some purposes we wanted to adjust the IO voltage. By writing TPS65070’s registers, we could change its DCDC2 output between [0.725V, 3.3V] (see the table on page 57 of TPS65070 manual SLVS950C). Of course, we need to ensure the voltage is not too low for L138’s IO supply, so we are actually changing it somewhere equal and above 1.8V.

We found that 2.25V (setting TPS65070 register DEFDCDCx to 0x0E) seems to be a “redline” — reaching it will put the CPU “In Reset” in CCS’s debug view, but below that everything is fine:

  1. We found 1.85V, 1.90V, 1.95V, 2.0V, 2.05V, 2.10V, 2.15V, 2.20V are all fine and the CPU would not be put into reset.
  2. If we change from 1.80V directly to 2.25V, we immediately see CPU “In Reset”.
  3. Even if we change first to one of the “safe” values, say, 2.20V, wait a while and then switch to 2.25V, we also see CPU immediately becomes “In Reset” in CCS debug view.

I have not tested if this “In Reset” relates to Spectrum emulator’s being connected to the CPU or whether it does not happen when the emulator is removed and the device is booted purely from FLASH, because we are going to need the emulator for the debugging related to the voltage adjustment.

In 4.2 Recommended Operating Conditions of L138 datasheet SPRS586C, we see the supply voltage for dual-voltage IO groups takes two intervals of values:

  1. 1.8V operating point: [1.71V. 1.89V]
  2. 3.3V operating point: [3.15V, 3.45V]

Although 2.25V (what causes problem) is beyond the maximum of [1.71V. 1.89V], values of 1.90V, 1.95V, 2.0V, 2.05V, 2.10V, 2.15V, 2.20V are also beyond that. So I found it difficult to see the real rule clearly: does the device allows only voltages in the two intervals for either nominal 1.8V or 3.3V voltage, or it actually allows for any values in the continuum of [1.71V, 3.15V]? However, none of the assumptions seems to explain why 2.25V causes device “In Reset” plausibly.

 

Paul