PGA460-Q1: UART RX is inverted

Stephen Embree14

Part Number: PGA460-Q1
Other Parts Discussed in Thread: PGA460

The UART RX line coming from the PGA460-Q1 is inverted. By this, I mean that the UART RX line voltage is low in-between transmissions of data and then it goes high to transmit data. Why is it inverted and how can I change it to the non-inverted (high in-between transmissions)? See the attached logic trace for an example of what I am talking about. Notice that channel 1, the UART RX from the PGA460-Q1, is low in-between transmissions.

over 4 years ago

0 Akeem Whitehead over 4 years ago

TI__Mastermind 27780 points

Hi Stephen,

The PGA460's USART pins share the tri-state behavior of a UART and SPI combination. This is why the PGA460_RX pin in UART mode can be idle low, which is not UART standard. Thus, I recommend that you add a weak pull-up resistor from the PGA460_RX pin to PGA460_IOREG pin to force an idle high state.

0 Stephen Embree14 over 4 years ago in reply to Akeem Whitehead

Prodigy 120 points

Edit to my question above: it is the UART TX on the PGA460-Q1 that is inverted, not the UART RX. In the trace above the "Async Serial Rx" is the RX pin on the microcontroller, and it is the TX on the PGA460-Q1. Does the UART TX on the PGA460-Q1 also need a pull-up resistor?

0 Akeem Whitehead over 4 years ago in reply to Stephen Embree14

TI__Mastermind 27780 points

Hi Stephen,

Your clarification is correct. It is the PGA460's TXD pin that routes to the master RXD that requires the pull-up. The PGA460's RXD pin already includes an internal pull-up resistor, so only the PGA460's TXD pin requires the external pull-up.

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PGA460-Q1: UART RX is inverted