Datasheet

PIC16F627A/628A/648A
DS40044A-page 76 Preliminary  2002 Microchip Technology Inc.
FIGURE 12-4: RX PIN SAMPLING SCHEME, BRGH = 0 OR BRGH = 1
12.2 USART Asynchronous Mode
In this mode, the USART uses standard non-return-to-
zero (NRZ) format (one START bit, eight or nine data
bits and one STOP bit). The most common data format
is 8-bit. A dedicated 8-bit baud rate generator is used
to derive baud rate frequencies from the oscillator. The
USART transmits and receives the LSb first. The
USART’s transmitter and receiver are functionally inde-
pendent but use the same data format and baud rate.
The baud rate generator produces a clock either x16 or
x64 of the bit shift rate, depending on bit BRGH
(TXSTA<2>). Parity is not supported by the hardware,
but can be implemented in software (and stored as the
ninth data bit). Asynchronous mode is stopped during
SLEEP.
Asynchronous mode is selected by clearing bit SYNC
(TXSTA<4>).
The USART Asynchronous module consists of the fol-
lowing important elements:
• Baud Rate Generator
• Sampling Circuit
• Asynchronous Transmitter
• Asynchronous Receiver
12.2.1 USART ASYNCHRONOUS
TRANSMITTER
The USART transmitter block diagram is shown in
Figure 12-5. The heart of the transmitter is the transmit
(serial) shift register (TSR). The shift register obtains its
data from the read/write transmit buffer, TXREG. The
TXREG register is loaded with data in software. The
TSR register is not loaded until the STOP bit has been
transmitted from the previous load. As soon as the
STOP bit is transmitted, the TSR is loaded with new
data from the TXREG register (if available). Once the
TXREG register transfers the data to the TSR register
(occurs in one T
CY), the TXREG register is empty and
flag bit TXIF (PIR1<4>) is set. This interrupt can be
enabled/disabled by setting/clearing enable bit TXIE
( PIE1<4>). Flag bit TXIF will be set regardless of the
state of enable bit TXIE and cannot be cleared in soft-
ware. It will RESET only when new data is loaded into
the TXREG register. While flag bit TXIF indicated the
status of the TXREG register, another bit TRMT
(TXSTA<1>) shows the status of the TSR register.
STATUS bit TRMT is a read only bit which is set when
the TSR register is empty. No interrupt logic is tied to
this bit, so the user has to poll this bit in order to deter-
mine if the TSR register is empty.
Transmission is enabled by setting enable bit TXEN
(TXSTA<5>). The actual transmission will not occur
until the TXREG register has been loaded with data
and the baud rate generator (BRG) has produced a
shift clock (Figure 12-5). The transmission can also be
started by first loading the TXREG register and then
setting enable bit TXEN. Normally when transmission
is first started, the TSR register is empty, so a transfer
to the TXREG register will result in an immediate trans-
fer to TSR resulting in an empty TXREG. A back-to-
back transfer is thus possible (Figure 12-7). Clearing
enable bit TXEN during a transmission will cause the
transmission to be aborted and will RESET the trans-
mitter. As a result the RB2/TX/CK pin will revert to hi-
impedance.
In order to select 9-bit transmission, transmit bit TX9
(TXSTA<6>) should be set and the ninth bit should be
written to TX9D (TXSTA<0>). The ninth bit must be
written before writing the 8-bit data to the TXREG reg-
ister. This is because a data write to the TXREG regis-
ter can result in an immediate transfer of the data to the
TSR register (if the TSR is empty). In such a case, an
incorrect ninth data bit maybe loaded in the TSR regis-
ter.
RX
Baud CLK
x16 CLK
START bit
bit0
Samples
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2 3
Baud CLK for all but START bit
(RB1/RX/DT pin)
Note 1: The TSR register is not mapped in data
memory so it is not available to the user.
2: Flag bit TXIF is set when enable bit TXEN
is set.