Electronic – Why does the AVR act weird (resets, data corruption) when receiving a few bytes on the UART

avrfuse-bitsresetuart

I have an ATmega644 connected to another device using the UART. After receiving a few characters on the UART the device resets and/or acts weird. Some examples of this behavior:

Code before the main loop is executed again but MCUSR is 0 (i.e. no "real" reset)
Code inside the main loop is executed even though the switches connected to the inputs were not pressed (pullups are configured properly).
Sometimes the MCU just hangs
Sometimes ports that were not configured as outputs act as outputs (e.g. LEDs connected to them light up even though the DDR for that port is 0).

The device is using an external 18.432 MHz crystal and the UART is set to 19200 baud. It is powered with 5V; the RX pin is connected to a 3V3 RPi.

Best Answer

Even if it the problem with this particular setup was something else (as pointed out by another answer), another common reason is that the controller is being operated outside the "safe operating area" with respect to Vcc VS clock speed.

atmega644 Vcc VS clock speed

With 18.432 MHz clock speed the device would require at least about 4.2V to function properly.

Related Solutions

Electronic – Why does the delay between bytes increase when I increase UART baud rate

The higher baudrate makes the UART module transmit faster, so you have more "spacing" between the bytes, but it seems to me that the rate at which you're transmitting the bytes is still the same. Your timing diagrams are missing total time so its hard to see if this is the case. Don't compare last edge to first edge. Compare the first edges of the the two bytes. Are these still the same?

Electronic – need to wait a few moments before sending serial data on the AVR

Basically what you do in

// enable transmitter
UCSR0B |= (1 << TXEN0);

_delay_ms(10);  // XXX

is inserting 10ms of stop bits (1). This helps your receiver to synchronize with the data stream (which conveniently starts with a start bit 0). Until then the AVR pin is high impedance and filling your input buffer with garbage to which your receiver is trying to synchronize.

Every time the receiver reads 10 bits that fit the frame (start-bit 0, 8 data bits, stop-bit 1) the data bits are pushed in the input buffer. Only when the frame doesn't match, bits get discarded until it does find a matching 10 bit string.

This also explains the importance for using start and stop bits in asynchronous transmission. This also explains why using 1.5 or 2 stop bits improves speed of synchronization; the probability of received data accidentally fitting the frame for a single byte decreased.

Best Answer

Related Solutions

Electronic – Why does the delay between bytes increase when I increase UART baud rate

Electronic – need to wait a few moments before sending serial data on the AVR

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