i2c: values for tlow lower than the minimum values specified in the I2C specification

[fivdi]

Per table 10 on page 48 of the I²C-bus specification and user manual the minimum values for t_LOW and t_HIGH are as follows:

Symbol	Parameter	Standard-mode	Fast-mode	Fast-mode Plus	Unit
tLOW	LOW period of the SCL clock	4.7	1.3	0.5	μs
tHIGH	HIGH period of the SCL clock	4.0	0.6	0.26	μs

With pico-sdk v1.1.0, the actual values used for t_LOW are lower than the minimum values specified in the specification (¹):

Symbol	Parameter	Standard-mode	Fast-mode	Fast-mode Plus	Unit
tLOW	LOW period of the SCL clock	4.01	1.0081	0.41	μs
tHIGH	HIGH period of the SCL clock	6.0	1.496	0.6	μs

The corresponding values written to the IC_FS_SCL_LCNT and IC_FS_SCL_HCNT registers are:

Symbol	Register	Standard-mode	Fast-mode	Fast-mode Plus
lcnt	IC_FS_SCL_LCNT	500	126	50
hcnt	IC_FS_SCL_HCNT	750	187	75

This isn't resulting in any issues with the I²C devices that I'm using with pico-sdk but it would probably be worth changing. Perhaps the values written to lcnt should have been written to hcnt and vice versa? See here.

[fivdi]

I just noticed that @kevinjwalters also spotted this issue on a logic analyzer here.

[kevinjwalters]

@fivdi Yes, I was hoping for someone else to confirm my analysis before I put a ticket in for it :)

[Wren6991]

#314 is now merged, thank you @fivdi