National Instruments HPC167064 - Manual (Page 23)

HPC167064 UART
The HPC167064 contains a software programmable UART
The UART (see
Figure 26 ) consists of a transmit shift regis-
ter a receiver shift register and five addressable registers
as follows a transmit buffer register (TBUF) a receiver buff-
er register (RBUF) a UART control and status register
(ENU) a UART receive control and status register (ENUR)
and a UART interrupt and clock source register (ENUI) The
ENU register contains flags for transmit and receive func-
tions this register also determines the length of the data
frame (8 or 9 bits) and the value of the ninth bit in transmis-
sion The ENUR register flags framing and data overrun er-
rors while the UART is receiving Other functions of the
ENUR register include saving the ninth bit received in the
data frame and enabling or disabling the UART’s Attention
Mode of operation The determination of an internal or ex-
ternal clock source is done by the ENUI register as well as
selecting the number of stop bits and enabling or disabling
transmit and receive interrupts
The baud rate clock for the Receiver and Transmitter can
be selected for either an internal or external source using
two bits in the ENUI register The internal baud rate is pro-
grammed by the DIVBY register The baud rate may be se-
lected from a range of 8 Hz to 128 kHz in binary steps or T3
underflow By selecting a 9 83 MHz crystal all standard
baud rates from 75 baud to 38 4 kBaud can be generated
The external baud clock source comes from the CKX pin
The Transmitter and Receiver can be run at different rates
by selecting one to operate from the internal clock and the
other from an external source
The HPC167064 UART supports two data formats The first
format for data transmission consists of one start bit eight
data bits and one or two stop bits The second data format
for transmission consists of one start bit nine data bits and
one or two stop bits Receiving formats differ from transmis-
sion only in that the Receiver always requires only one stop
bit in a data frame
UART Wake-Up Mode
The HPC167064 UART features a Wake-Up Mode of opera-
tion This mode of operation enables the HPC167064 to be
networked with other processors Typically in such environ-
ments the messages consist of addresses and actual data
Addresses are specified by having the ninth bit in the data
frame set to 1 Data in the message is specified by having
the ninth bit in the data frame reset to 0
The UART monitors the communication stream looking for
addresses When the data word with the ninth bit set is
received the UART signals the HPC167064 with an inter-
rupt The processor then examines the content of the re-
ceiver buffer to decide whether it has been addressed and
whether to accept subsequent data
TL DD 11046 – 34
FIGURE 26 UART Block Diagram
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