how to set HS TX pre-emphasis of SN75dphy440ss to 1.5dB?

how to set HS TX pre-emphasis of SN75dphy440ss to 1.5dB?

a customer is using SN75dphy440ss for MIPI signal extending. From the datasheet, we may use hardware method to set the pre-emphasis to 0dB or 2.5dB by pull PRE_CFG1 pin to VIL/ViM or VIH, but the test result shows that if the pre-emphasis is set to 0dB, the HS CLOCK TX differential voltage is lower than the bottom limit;  if pre-emphasis is set to 2.5dB, the HS CLOCK TX static common mode voltage will be higher than the top limit. so we are planning to try to set pre-emphasis to 1.5dB. but as the description in the datasheet about HSTX_PRE (pre-emphasis):

This field controls the HS TX pre-emphasis level. The value of
this field will match the sampled state of CFG1 pin. Software can
change the value of this field at a later time.
00 – 1.5 dB
01 – 0 dB (CFG1 = VIM or VIL)
1X – 2.5 dB (CFG1 = VIH)

 

Seems the only way to set HSTX_PRE to 1.5dB is to set via writing to Standard CSR Register (0x0E), and could not set it via PRE_CFG1 pin. but the customer does not have I2C function to communicate with SN75dphy440ss. would you please help to check if we may set  HSTX_PRE to 1.5dB  via any hardware method?

thanks!