GPS Free Stack/Notes About Namuru
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Acknowledgments
Sampling
Accumulator interrupt signal need to be read (polling) or managed by CPU in order to read accumulators before they get overwrite.
Accumulators provides correlation results that can lately be interpreted/tracked to extract chips/bits once signal peak is confirmed trough acquisition.
Timing
Accumulator interrupt is asserted high according to ACCUM_COUNT value:
The Accumulator interrupt signal and flag needs to have between 0.5 ms and about 1 ms period. This is to ensure that accumulation data can be read before it is written over by new data. The accumulators are asynchronous to each other and have a dump period of nominally 1ms.
ACCUM_INT period = (accum_divide + 1) / 16.384MHz For 0.5 ms accumulator interrupt accum_divide = 16.384x10^6 * 0.0005 - 1 accum_divide = 0x1FFF
Accumulator interrupt MUST be asserted low after status read bit from CLEAR_STATUS register is set high.
TIC allows to sync measurements across all channels, PROG_TIC register control measurement period.
tic period = (tic_divide + 1) * Clk period If clocked by SiGE 4162 Realmode 16.384Mhz : tic period = (tic_divide + 1) / 16.384 Mhz For default tic period (0.1s) tic_divide = 0x18ffff
Channel tracking =
To initialize a channel a PRN Key must be set (this should be decided upon satellite on sky). Set carrier NCO value (used adjust doppler) Set the C/A code generator at the required frequency (2*1023)
Registers
(note some register are nor part of namuru upstream)
Channel related registers
| Register | Address | Description |
|---|---|---|
| CH0_PRN_KEY | (0x00) | Gold code sequence |
| CH0_CARRIER_NCO | (0x04) | Local carrier oscilator frequency control |
| CH0_CODE_NCO | (0x08) | Local C/A code oscilator frequency control |
| CH0_CODE_SLEW | (0x0c) | C/A code delay control |
| CH0_I_EARLY | (0x10) | I Early Accumulator |
| CH0_Q_EARLY | (0x14) | Q Early Accumulator |
| CH0_I_PROMPT | (0x18) | I Prompt Accumulator |
| CH0_Q_PROMPT | (0x1c) | Q Prompt Accumulator |
| CH0_I_LATE | (0x20) | I Late Accumulator |
| CH0_Q_LATE | (0x24) | Q Late Accumulator |
| CH0_CARRIER_MEASUREMENT | (0x28) | Carrier cycle count and phase since last TIC |
| CH0_CODE_MEASUREMENT | (0x2c) | Code measurement (code half-chip, code NCO phase) |
| CH0_EPOCH | (0x30) | C/A code count latched on TIC |
| CH0_EPOCH_CHECK | (0x34) | same as EPOCH but no latched to TIC |
| CH0_EPOCH_LOAD | (0x38) | Modify epoch counter |
Status
| STATUS | (0x380) | bit 0 TIC occurred, bit 1 accumulator int, need to be clean manually after read |
| NEW_DATA | (0x384) | data dump one bit per channel |
| TIC_COUNT | (0x388) | Provides TIC down counter value |
| ACCUM_COUNT | (0x38c) | Provides ACCUM INT down counter value |
| CLEAR_STATUS | (0x390) | Clear STATUS flags (need to be cleared) |
| HW_ID | (0x3bc) |
Control (Mostly time-base related)
| RESET | (0x3c0) | Software reset (need to be cleared) |
| PROG_TIC | (0x3c4) | Control TIC period value for down counter |
| PROG_ACCUM_INT | (0x3c8) | Control ACCUM INT period (must be less 1 ms) |
