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15: 1PPS Timemark Generator
GP4020 GPS Baseband Processor Design Manual 151
TIC
ARM_TIMEMARK
RAW TIMEMARK
0.0999999s
10 TIC Periods
= 0.999999s
Figure 15.2 Timemark output using ARM_TIMEMARK signal, triggered from software.
The TIC period can only be adjusted in steps of 175ns ( 7 / 40MHz = 175ns). Therefore, the closest that TIC can be
set to 0.1000000s is either 0.0999999s or 0.100000075s. The GP4020 employs some additional fine-resolution
adjustment techniques to effectively reduce the TIC period adjustment resolution from 175ns to 25ns; these
techniques are indicated later in this section. The TIC period is set to 0.0999999s by default. The period of TIC can
be set using the PROG_TIC_HI (5-bits) and PROG_TIC_LO (16-bits) registers, using the following programming
routine:
TIC_PERIOD = ((PROG_TIC_HIGH * 65536) + PROG_TIC_LO + 1) * 7 / 40000000
To set TIC from software to be 0.0999999 seconds:
PROG_TIC_HIGH = 0x08,
PROG_TIC_LOW = 0xB823
To set TIC from software to be 0.100000075 seconds:
PROG_TIC_HIGH = 0x08,
PROG_TIC_LOW = 0xB824
If TIMEMARK is set-up to be generated once in every 10 TIC events (where TIC is 0.0999999s), the Timemark
period would be 1µs less than 1PPS, and so it would gradually drift away from UTC. It is therefore necessary to
continually monitor the relationship between TIC and UTC to effectively keep TIC in phase with UTC. This can be
achieved by either slewing the TIC period, or adding a delay after a TIC has occurred.
Figure 15.3 below shows a timing diagram of how Raw Timemark and Timemark are related to TIC, and how small
delays are added to align Timemark to UTC. Note the diagram is not to scale, in order to emphasise the delay.