Hi, I’m trying to better understand how the USB PMA and BTABLE work on the STM32F103. After reading the reference manual, I’m still unsure how the descriptor table size relates to the number of endpoints.
I have two possible interpretations of how the USB Descriptor Table and PMA layout work on the STM32F103, and I need clarification from ST to confirm which one is correct.
I will provide two diagrams (Figure A and Figure B) to illustrate the scenario, assuming BTABLE offset = 0.
Scenario: BTABLE = 0x0000
Figure A (Fixed 8×8 bytes BTABLE)
In this interpretation:
The USB descriptor table is always 8 × 8 bytes = 64 bytes (0x0000–0x003F), assuming the device supports 8 endpoints.
The usable PMA buffer area always starts at 0x0040, regardless of how many endpoints are actually used (for example, even if only 3 endpoints are used).
Therefore the usable PMA region would always be 0x0040–0x0200.
Question A:
Is this interpretation correct? Is the descriptor table size always fixed at 64 bytes in the STM32F103, independent of the number of endpoints actually used?
Figure B (Descriptor Table Depends on Number of Endpoints Used)
In this interpretation:
The BTABLE size depends on the number of endpoints used.
For example, if I use 4 endpoints:
4 × 8 bytes = 32 bytes → 0x0000–0x001F.The PMA usable buffer area starts immediately after the descriptor table.
Example: 0x0020–0x0200.
Question B:
Is this interpretation instead the correct one? Does the size of the BTABLE section shrink or grow depending on how many endpoints are enabled in the firmware?
Summary of My Main Question:
When BTABLE = 0x0000, does the usable PMA buffer area always begin at 0x0040,
or does it shift depending on how many endpoints are configured?
Thank you very much—I want to ensure I place my PMA buffer addresses correctly.
