AM3352: UART timings

HS FAE Germany

Part Number: AM3352

Hello,
customer has to receive a 38baud UART stream. The bit timings is bad with a tolerance of +/- 7% (due to an cheap optocoupler). Our datasheet gurantees 4/5% (page 717). Customer said that the AM3352 received the bit stream without problems.

- Do you have any experience with tolerances higher than 5%?
- How does the internal AM3352 bit sampling look like. How often is a UART bit sampled?

Regards, Holger

over 7 years ago

0 Biser Gatchev-XID over 7 years ago

TI__Guru**** 393215 points

Hi,

The AM335x UART works with 13x or 16x oversampling, depending on how it's configured. See section 4.4.4.2.1 in the AM335x TRM Rev. P for details.

0 HS FAE Germany over 7 years ago in reply to Biser Gatchev-XID

TI__Mastermind 31440 points

Hello Biser,
the TRM writes that "When receiving, the UART samples the bit in the 8th cycle.". This means in the middle of one UART bit. Do you know why the d/s allows 5%?

Regards, Holger

0 Biser Gatchev-XID over 7 years ago in reply to HS FAE Germany

TI__Guru**** 393215 points

This tolerance becomes critical at higher baud rates, where the bit duration is very small. The UART is capable to receive at up to 3.6864Mbps.

0 HS FAE Germany over 7 years ago in reply to Biser Gatchev-XID

TI__Mastermind 31440 points

does this mean I can have a higher tolerance than 5% with a baudrate of 38400baud?
Do know if there is voting of the receive samples?

0 Biser Gatchev-XID over 7 years ago in reply to HS FAE Germany

TI__Guru**** 393215 points

Basically the UART works like this: when the first falling edge is detected, the sampling coulter is started. Then it samples the input after 8 clocks. If it's 0, this is deemed a start bit, the counter is reconfigured to count to 16, and next bit is sampled after 16 clocks, and so on, until the entire frame is sampled.

In your case it seems that the bit tolerances are more or less evenly spread, so that bits are sampled at the correct levels. This does not mean that the specs can be stretched for low bit rates. You should think of changing the optocoupler, because I cannot predict what will happen at different temperatures, or when the optocoupler starts aging.

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AM3352: UART timings