AM263P4: UART RTS timing at high baud rate

Hi Kien,

Good question — this is a real timing concern at 6 Mbps. Let me walk through what the documentation tells us and flag a critical transceiver issue that will likely dominate your design.

Key Clarification Needed First

How are you wiring DE and RE on the THVD1400V? This matters enormously because the transceiver's driver enable time changes by two orders of magnitude depending on the RE pin state:

At 6 Mbps, your bit time is ~167 ns. If you're toggling both DE and RE together via a single RTS signal (common RS-485 wiring), the 13 µs worst-case enable time will destroy your first ~78 bits — not just the first bit. This is the dominant risk in your design.

AM263P4 UART RTS Timing

The AM263P4 UART does support RS-485 direction control via RTS [2][3]. Here's what the documentation confirms:

• RTS asserts at the Start Bit and de-asserts after the Stop Bit plus a programmable TIMEGUARD period [4]
• The TIMEGUARD is configurable (example shows 3 baud clock cycles) and extends RTS hold after the last stop bit to prevent premature direction change [5]
• A built-in guard band delay counter ensures the transceiver switches to RX mode only after both the TX FIFO and shift register are empty and the guard delay has elapsed [6]

However, explicit AC timing specs (setup/hold times in nanoseconds) for RTS relative to TX data edges are not documented in either the datasheet or TRM [7][8]. The datasheet does specify TX pulse width jitter of ±2.2 ns relative to the baud time [9], and the UART supports up to 12 Mbps with a 192 MHz functional clock [10], so 6 Mbps is within the UART's capability — but you'll need to ensure your functional clock is set to at least 96 MHz (not the default 48 MHz, which caps at ~3.7 Mbps) [11].

Recommendations

1. Do NOT tie DE and RE̅ together on a single RTS line. Instead, keep RE̅ tied low (receiver always enabled) and drive only DE from RTS. This reduces driver enable time from 13 µs to 70 ns max [1] — well under one bit time at 6 Mbps. You'll see your own transmissions on RX, but that's easily handled in software.

2. Configure TIMEGUARD appropriately. The driver disable time is 50 ns max [1], and the driver propagation delay is 40 ns max [1]. Set TIMEGUARD to at least 1–2 baud clock cycles to ensure the stop bit fully propagates through the transceiver before RTS de-asserts.

3. Verify your UART functional clock. At the default 48 MHz, the max baud rate is only ~3.7 Mbps [11]. You need to configure the clock to ≥96 MHz (192 MHz recommended for 16x oversampling at 6 Mbps) to reach 6 Mbps [10].

4. Validate with a scope. Since RTS setup time relative to the first TX bit isn't specified in the datasheet, measure the actual RTS-to-Start-Bit timing on hardware at 6 Mbps to confirm the transceiver is fully enabled before the first bit arrives on the bus.

Bottom Line

The RTS mechanism will work for direction control at 6 Mbps, but only if RE̅ is held low (not toggled with DE). If you toggle both pins together, the THVD1400V's 6–13 µs charge pump startup will cause massive bit loss. With RE̅ tied low, the 70 ns max enable time gives you adequate margin within a 167 ns bit period, and the programmable TIMEGUARD handles the tail end.

1. THVD1400V Datasheet - Switching Characteristics
2. AM263P TRM - UART RS-485 Direction Control
3. AM263P TRM - UART RS-485 Auto Flow Control
4. AM263P TRM - RS-485 RTS Timing and TIMEGUARD
5. AM263P TRM - Guard Band Delay Counter
6. AM263P TRM - TX FIFO/Shift Register Empty Condition
7. AM263Px Datasheet - UART Timing (no RTS AC specs)
8. AM263P TRM - RTSn Pin Description (no timing specs)
9. AM263Px Datasheet - UART Switching Characteristics
10. AM263Px Datasheet - UART Max Baud Rate 12 Mbps
11. AM263P TRM - 48 MHz Clock Max 3.6 Mbps

Best Regards,

Zackary Fleenor