Datasheet

dsPIC30F6011A/6012A/6013A/6014A
DS70143E-page 106 © 2011 Microchip Technology Inc.
16.3.4 TRANSMIT INTERRUPT
The transmit interrupt flag (U1TXIF or U2TXIF) is
located in the corresponding interrupt flag register.
The transmitter generates an edge to set the UxTXIF
bit. The condition for generating the interrupt depends
on the UTXISEL control bit:
1. If UTXISEL = 0, an interrupt is generated when
a word is transferred from the transmit buffer to
the Transmit Shift register (UxTSR). This implies
that the transmit buffer has at least one empty
word.
2. If UTXISEL = 1, an interrupt is generated when
a word is transferred from the transmit buffer to
the Transmit Shift register (UxTSR) and the
transmit buffer is empty.
Switching between the two Interrupt modes during
operation is possible and sometimes offers more
flexibility.
16.3.5 TRANSMIT BREAK
Setting the UTXBRK bit (UxSTA<11>) will cause the
UxTX line to be driven to logic ‘0’. The UTXBRK bit
overrides all transmission activity. Therefore, the user
should generally wait for the transmitter to be Idle
before setting UTXBRK.
To send a break character, the UTXBRK bit must be set
by software and must remain set for a minimum of 13
baud clock cycles. The UTXBRK bit is then cleared by
software to generate Stop bits. The user must wait for
a duration of at least one or two baud clock cycles in
order to ensure a valid Stop bit(s) before reloading the
UxTXB, or starting other transmitter activity. Transmis-
sion of a break character does not generate a transmit
interrupt.
16.4 Receiving Data
16.4.1 RECEIVING IN 8-BIT OR 9-BIT
DATA MODE
The following steps must be performed while receiving
8-bit or 9-bit data:
1. Set up the UART (see Section 16.3.1
“Transmitting in 8-bit data mode”).
2. Enable the UART (see Section 16.3.1
“Transmitting in 8-bit data mode”).
3. A receive interrupt will be generated when one
or more data words have been received,
depending on the receive interrupt settings
specified by the URXISEL bits (UxSTA<7:6>).
4. Read the OERR bit to determine if an overrun
error has occurred. The OERR bit must be reset
in software.
5. Read the received data from UxRXREG. The
act of reading UxRXREG will move the next
word to the top of the receive FIFO, and the
PERR and FERR values will be updated.
16.4.2 RECEIVE BUFFER (UXRXB)
The receive buffer is 4 words deep. Including the
Receive Shift register (UxRSR), the user effectively
has a 5-word deep FIFO buffer.
URXDA (UxSTA<0>) = 1 indicates that the receive buf-
fer has data available. URXDA = 0 implies that the buf-
fer is empty. If a user attempts to read an empty buffer,
the old values in the buffer will be read and no data shift
will occur within the FIFO.
The FIFO is reset during any device Reset. It is not
affected when the device enters or wakes up from a
Power-Saving mode.
16.4.3 RECEIVE INTERRUPT
The receive interrupt flag (U1RXIF or U2RXIF) can be
read from the corresponding interrupt flag register. The
interrupt flag is set by an edge generated by the
receiver. The condition for setting the receive interrupt
flag depends on the settings specified by the
URXISEL<1:0> (UxSTA<7:6>) control bits.
a) If URXISEL<1:0> = 00 or 01, an interrupt is gen-
erated every time a data word is transferred
from the Receive Shift register (UxRSR) to the
receive buffer. There may be one or more
characters in the receive buffer.
b) If URXISEL<1:0> = 10, an interrupt is generated
when a word is transferred from the Receive Shift
register (UxRSR) to the receive buffer, which as a
result of the transfer, contains 3 characters.
c) If URXISEL<1:0> = 11, an interrupt is set when
a word is transferred from the Receive Shift reg-
ister (UxRSR) to the receive buffer, which as a
result of the transfer, contains 4 characters (i.e.,
becomes full).
Switching between the Interrupt modes during opera-
tion is possible, though generally not advisable during
normal operation.
16.5 Reception Error Handling
16.5.1 RECEIVE BUFFER OVERRUN
ERROR (OERR BIT)
The OERR bit (UxSTA<1>) is set if all of the following
conditions occur:
a) The receive buffer is full.
b) The Receive Shift register is full, but unable to
transfer the character to the receive buffer.
c) The Stop bit of the character in the UxRSR is
detected, indicating that the UxRSR needs to
transfer the character to the buffer.
Once OERR is set, no further data is shifted in UxRSR
(until the OERR bit is cleared in software or a Reset
occurs). The data held in UxRSR and UxRXREG
remains valid.