TCA9555: FAST MODE

[FAE] Andrew Wang

Genius 10685 points

Part Number: TCA9555

Hi Team,

One quick question, when TCA9555 operating in FAST MODE, will its output DIO runs at same speed (400KHz)?

Thanks!

Andrew

over 4 years ago

0 Max Robertson over 4 years ago

TI__Guru 62910 points

Andrew,

400 kHz would be the bit rate used on the I2C interface. The output ports would toggle at a slower rate, since they would require a full byte transfer on the I2C bus to change their value. Figure 28 in the datasheet may be useful to reference to see the I2C transfers required to toggle both output port values.

Regards,
Max

0 [FAE] Andrew Wang over 4 years ago in reply to Max Robertson

TI__Genius 10685 points

Hi Max,

Thanks for the feedback.

So can I roughly calculate the fastest speed of output ports toggle at 400KHz * (16 ports / 36 SCK) ~ 177KHz ?

Thanks.

Andrew

0 Max Robertson over 4 years ago in reply to [FAE] Andrew Wang

TI__Guru 62910 points

After the byte transfer for the device address and an additional one specifying the output port register, each 8-bit port's set of output values could be toggled via an additional byte transfer. Each transfer is 9 SCK cycles (8 bits + ACK). Since the ports alternate (0/1), you would update port 0, then 9 SCK cycles later would update port 1, then 9 SCK cycles after that you could update port 0 again, etc. So, each one updates every 18 SCK cycles at the fastest. This would be 400 kHz / 18 = 22 kHz.

Max

0 [FAE] Andrew Wang over 4 years ago in reply to Max Robertson

TI__Genius 10685 points

Max

Very useful feedback, thanks a lot!

Andrew

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TCA9555: FAST MODE