Rev. 1.50
164
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Rev. 1.50
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HT66F0175/HT66F0185
A/D Flash MCU with EEPROM
HT66F0175/HT66F0185
A/D Flash MCU with EEPROM
This sequence of events can now be repeated to send additional data. It should be noted that when
TXIF=0, data will be inhibited from being written to the TXR register. Clearing the TXIF flag is
always achieved using the following software sequence:
1. A USR register access
2. A TXR register write execution
The read-only TXIF flag is set by the UART hardware and if set indicates that the TXR register is
empty and that other data can now be written into the TXR register without overwriting the previous
data. If the TEIE bit is set, then the TXIF flag will generate an interrupt. During a data transmission,
a write instruction to the TXR register will place the data into the TXR register, which will be
copied to the shift register at the end of the present transmission. When there is no data transmission
in progress, a write instruction to the TXR register will place the data directly into the shift register,
resulting in the commencement of data transmission, and the TXIF bit being immediately set. When
a frame transmission is complete, which happens after stop bits are sent or after the break frame, the
TIDLE bit will be set. To clear the TIDLE bit the following software sequence is used:
1. A USR register access
2. A TXR register write execution
Note that both the TXIF and TIDLE bits are cleared by the same software sequence.
Transmitting Break
If the TXBRK bit is set, then the break characters will be sent on the next transmission. Break
character transmission consists of a start bit, followed by 13xN “0” 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 high 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 UCR1 register. When BNO bit is set, the word length will be set to 9 bits. In
this case the 9
th
bit, which is the MSB, will be stored in the RX8 bit in the UCR1 register. At the
receiver core lies the Receiver Shift Register more 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 the 3
rd
byte can continue to be
received. Note that the application program must ensure that the data is read from RXR before the
