The core does not consider the LP to HS delay in the video timing calculations by default. Hence, you see slightly fewer Frames Per Second (FPS) than intended. In situations where you need to consider the D-PHY LP to HS latency, you are required to program the register given below.
The D-PHY LP to HS (more specifically the time from PPI signal txrequesths to txreadyhs) delay varies based on Line Rate (Mbps), LPX Period (ns), internal design logic, and CDC stages.
User Inputs:
- P1 = UI in ns
- P2 = LPX Period in ns
- P3 = INT(PPI Byteclk in ns) is the calculation to convert the specified value into an integer number.
- P4 = PPI Byteclk in ns (with decimal places)
| Linerate | L1 |
|---|---|
| <150 | INT(P1) |
| 150 to <300 | INT(P1*2) |
| 301 to <600 | INT(P1*4) |
| 601 and above | P3 |
L2 = P2 + 25
L3 = 85 + 6*P1
L4 = ((L3-3*L1)/5)-1 L5 = (L4*5 + L2)/P4
L6 = INT((105+6*P1)/P3) + 4
LP-HS delay = L5 + L6
Example LP-HS delay for certain line rates are given below:
| Line rate | P1 | P2 | P3 | L1 | P4 | L2 | L3 | L4 | L5 | L6 |
LP-HS Delay |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 80 | 12.5 | 50 | 100 | 12 | 100 | 75 | 160 | 23 | 1 | 5 | 6 |
| 150 | 6.666667 | 50 | 53 | 13 | 53.33333333 | 75 | 125 | 16 | 2 | 6 | 8 |
| 550 | 1.818182 | 50 | 14 | 7 | 14.54545455 | 75 | 95.90909091 | 13 | 9 | 12 | 21 |
| 1000 | 1 | 50 | 8 | 8 | 8 | 75 | 91 | 12 | 16 | 17 | 33 |
| 1188 | 0.841751 | 50 | 6 | 6 | 6.734006734 | 75 | 90.05050505 | 13 | 20 | 22 | 42 |
| 1440 | 0.694444 | 50 | 5 | 5 | 5.555555556 | 75 | 89.16666667 | 13 | 25 | 25 | 50 |
Due to CDC stages and decimal numbers of byteclk period, you might need to program a slightly higher or lower value obtained in the preceding table to get the closer FPS on the hardware.
The final value to program is “LP-HS Delay” * DSI Lanes.
| Bits | Name | Reset Value | Access | Description |
|---|---|---|---|---|
| 31:16 | Reserved | NA | NA | Reserved |
| 15:0 | D-PHY LP HS offset | 0x0 | R/W | To offset D-PHY LP to HS transition delay in the video timing calculations |