ZHCSTD7A December 2021 – November 2023 BQ79631-Q1
PRODUCTION DATA
The UART interface follows the standard serial protocol of 8-N-1, where it sends information as a START bit, followed by eight data bits, and then one STOP bit. The STOP bit indicates the end of the byte. If a byte is received that does not have the STOP bit set, the FAULT_COMM1[STOP_DET] bit is set, indicating there may be a baud rate issue between the host and the device. The device supports 1-Mbps baud rate. Additionally, during development, a slower baud rate is needed to debug the communication, an optional 250-kbps baud rate can be enabled under communication debug mode.
The UART sends data on the TX pin and receives data on the RX pin. When idle, the TX and RX pins are high. The UART interface requires that RX is pulled up to CVDD through a resistor on the base device. The RX is pulled up on the device side. Do not leave RX unconnected. Ensure RX is connected directly to CVDD for stack devices.
The TX pin is disabled in stack devices, but must be pulled high through a resistor on the host side on base device to prevent triggering an invalid communications frame when the communication cable is not attached, or during power-off or SHUTDOWN state when TX is high impedance. TX is always pulled to CVDD internally while in ACTIVE or SLEEP mode, whether enabled or disabled. Leave TX unconnected if not used in stack devices.
The UART interface is strictly a half-duplex interface. While transmitting, any attempted communication on RX is ignored. The only exception is COMM CLEAR signal on RX pin, which immediately terminates the communication. See Section 8.3.5.1.1.1.3 for details.
Using two STOP bits in UART:
The device can be set up with two stop bits (DEV_CONF[TWO_STOP_EN] = 1), the UART response frame transmits from device to host will always return with two STOP bits as shown below. Host is not required to send the command frame to the device with two STOP bits. The device is able to receive one or more stop bits with or without this function enabled.
Potential use of the two stop bits may be to:
Although UART is only used by the base device, if the [TWO_STOP_EN] = 1, the stack devices also set the [TWO_STOP_EN] = 1 even though UART is not used in stacks. It is because the stack devices will use the bit setting to determine the proper gap applying between two communication frames.