ST20-GP6 datasheet(13/123 Pages) STMICROELECTRONICS

13 / 123 page

ST20-GP6

13/123

byte header contains: a ‘sync’ byte with the value #1B, and a ‘sample rate’ byte which contains the

two

SampleRate bits from the DSPControl register, see Table 3.1.

Packets are delivered at the rate selected by the

DSPControl register, even if new data is not avail-

able. In this case, the data value for the ﬁeld is set to #8000. This guarantees that synchronism is

maintained between the satellite one-millisecond epochs and the receiver, despite time-of-recep-

tion variations due to the varying path length from the satellite.

Figure 3.2 DSP packet format

3.1

DSP module registers

The GPS hardware channels of the ST20-GP6 are controlled by three sets of registers:

DSPControl register

PRNcode0-11 and PRNphase0-11 registers

NCOfrequency0-11 and NCOphase0-11 registers

The base addresses for the DSP registers are given in the Memory Map chapter.

DSPControl register

The

DSPControl register determines whether the PRN generators are on (normal use) or disabled

(for built-in-self-test of a system), whether the system is in tracking mode (840/970

µs output rate)

or initial acquisition mode (31/62

µs), and selects which of the two rates for each mode. It also

62 byte packet every 840/970/31/62

µs

Absent 16-bit values padded with #8000

12 x 8-bit

time values

12 x 16-bit

Q values

12 x 16-bit

I values

16-bit

header

sync

sample

rate

Acquisition mode

First packet (in SV ms)

T[7:6] = 10

T[5:0] = time[5:0]

Remaining packets

T[7:6] = 00

T[5:0] = sequence number

(sequence numbers are 2 to

16 or 32)

Tracking mode

T[7:6] = 10

T[5:0] = time[5:0]

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