# Decisions — guomi-smart-card

## Interface Selection
- SPI1 (SCK1/SS1/MISO1/MOSI1) chosen — Slave mode, CPOL=0 CPHA=0
- spiid=0, base address 0x70000000
- DMA channel 1 (DMACCH1) for SPI RX via LLI

## Protocol Design
- Frame: SYNC(0xAA55,2B) + LEN(2B,big-endian) + CMD(1B) + PAYLOAD(N) + CRC16(2B)
- Response: SYNC + LEN + STATUS(1B) + RESPONSE(N) + CRC16
- 17 commands: 0x01-0x06 (SM2), 0x10 (SM3), 0x20-0x26 (SM4+stream), 0x30-0x31 (key/cert), 0xF0 (info)

## Key Management
- SM2: on-chip generation, private key in eFlash (never exported), 1 slot
- SM4: dynamically injected by host into RAM (not persisted)
- eFlash storage: 4 pages x 512B at 0x00470000, log-structured, magic=0xA5C3

## Testing Strategy
- TDD with national crypto standard test vectors
- On-chip self-test → UART output PASS/FAIL
- No external test framework (bare metal)