Holtek BS82B12A-3, Руководство пользователя

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BS82B12A-3/BS82C16A-3/BS82D20A-3
Touch Key 8-Bit Flash MCU with LED/LCD Driver
BS82B12A-3/BS82C16A-3/BS82D20A-3
Touch Key 8-Bit Flash MCU with LED/LCD Driver
Transmit Break
If the TXBRK bit is set then break characters will be sent on the next transmission. Break character
transmission consists of a start bit, followed by 13×N ‘0’ bits and stop bits, where N=1, 2, etc. If a
break character is to be transmitted then the TXBRK bit must be first set by the application program
and then cleared to generate the stop bits. Transmitting a break character will not generate a transmit
interrupt. Note that a break condition length is at least 13 bits long. If the TXBRK bit is continually
kept at a logic high level then the transmitter circuitry will transmit continuous break characters.
After the application program has cleared the TXBRK bit, the transmitter will finish transmitting the
last break character and subsequently send out one or two stop bits. The automatic logic highs at the
end of the last break character will ensure that the start bit of the next frame is recognized.
UART Receiver
The UART is capable of receiving word lengths of either 8 or 9 bits can be selected by programming
the BNO bit in the UCR register. If the BNO bit is set, the word length will be set to 9 bits with the
MSB being stored in the RX8 bit of the UCR1 register. At the receiver core lies the Receive Serial
Shift Register, commonly known as the RSR. The data which is received on the RX external input
pin is sent to the data recovery block. The data recovery block operating speed is 16 times that of the
baud rate, while the main receive serial shifter operates at the baud rate. After the RX pin is sampled
for the stop bit, the received data in RSR is transferred to the receive data register, if the register is
empty. The data which is received on the external RX input pin is sampled three times by a majority
detect circuit to determine the logic level that has been placed onto the RX pin. It should be noted
that the RSR register, unlike many other registers, is not directly mapped into the Data Memory area
and as such is not available to the application program for direct read/write operations.
Receiving Data
When the UART receiver is receiving data, the data is serially shifted in on the external RX input pin
to the shift register, with the least significant bit LSB first. The RXR register is a two byte deep FIFO
data buffer, where two bytes can be held in the FIFO while a third byte can continue to be received.
Note that the application program must ensure that the data is read from RXR before the third byte
has been completely shifted in, otherwise this third byte will be discarded and an overrun error OERR
will be subsequently indicated. The steps to initiate a data transfer can be summarized as follows:
• Make the correct selection of BNO, PRT and PREN bits to define the word length and parity type.
• Setup the BRG register to select the desired baud rate.
• Set the RXEN bit to ensure that the UART receiver is enabled and the RX pin is used as a UART
receiver pin.
At this point the receiver will be enabled which will begin to look for a start bit.
When a character is received, the following sequence of events will occur:
• The RXIF bit in the USR register will be set when RXR register has data available, at least one
character can be read.
• When the contents of the shift register have been transferred to the RXR register and if the RIE
bit is set, then an interrupt will be generated.
• If during reception, a frame error, noise error, parity error, or an overrun error has been detected,
then the error flags can be set.
• The RXIF bit can be cleared using the following software sequence:
• A USR register access
• An RXR register read execution