# How to calculate bit time of any signal?

Given a data rate or clock rate, how to calculate bit time of any signal/interface?

As per what I know, bit time = 1/data rate. Is it correct?

I got a little confused when I could not calculate the TBIT values (and eventually skew) in the following:

This time difference, also known as intra-pair skew, is specified by HDMI for a receiver with 0.4 · TBIT for a TMDS clock rate of 225 MHz, which translates to 178 ps maximum. For an HDMI transmitter the specification calls for 0.15 · TBIT for a TMDS clock rate of 225 MHz, which translates to 66 ps maximum.

Am I missing something here?

Any help is appreciated.

• I don't see anything wrong with the numbers. Perhaps you are getting confused with the overall link calculations? – Peter Smith Aug 6 '17 at 8:25
• Yes. I am not doubting the calculations given there, but I am not able to calculate them myself. I thought I'd better get clarification about bit time in general, so that I can do the calculations for any interface. – LoveEnigma Aug 6 '17 at 9:14
• clock rate 225MHz, bit time 1/225 = 4.44nS, 0.4 bit time = 1.78nS. Looks like someone's slipped a decimal place somewhere. Ah! Does HDMI transmit bytes with 10/8 encoding, and bytes are at 225MHz at the parallel to serial encoder (is that what TMDS means?), and bits are at 2.25GHz on the serial interface? 225MHz is far too slow for an interface like HDMI, SATA runs at 3GHz, DVI similar. How to get bit time of a signal? Understand what all terms mean. – Neil_UK Aug 6 '17 at 9:25
• HDMI (and others) transmit 10 bits per pixel clock, which is not always clearly stated. Tbit is therefore Tclk / 10 – Peter Smith Aug 6 '17 at 9:29