N = 32.552/6.5 = 5.008 (N = 5)
14.0 USART (Continued)
Using the above equation N x P can be calculated first.
N x P = (5 x 106 x 2)/(16 x 19200) = 32.552
The programmed value (from Table 19) should be 4 (N - 1).
Using the above values calculated for N and P:
BR = (5 x 106 x 2)/(16 x 5 x 6.5) = 19230.769
error = (19230.769 - 19200) x 100/19200 = 0.16%
Now 32.552 is divided by each Prescaler Factor (Table 20) to
obtain a value closest to an integer. This factor happens to
be 6.5 (P = 6.5).
20006328
FIGURE 24. USART BAUD Clock Divisor Registers
14.8 EFFECT OF HALT/IDLE
14.10 ATTENTION MODE
The USART logic is reinitialized when either the HALT or
IDLE modes are entered. This reinitialization sets the TBMT
flag and resets all read only bits in the USART control and
status registers. Read/Write bits remain unchanged. The
Transmit Buffer (TBUF) is not affected, but the Transmit Shift
register (TSFT) bits are set to one. The receiver registers
RBUF and RSFT are not affected.
The USART Receiver section supports an alternate mode of
operation, referred to as ATTENTION Mode. This mode of
operation is selected by the ATTN bit in the ENUR register.
The data format for transmission must also be selected as
having nine Data bits and either one or two Stop bits.
The ATTENTION mode of operation is intended for use in
networking the device with other processors. Typically in
such environments the messages consists of device ad-
dresses, indicating which of several destinations should re-
ceive them, and the actual data. This Mode supports a
scheme in which addresses are flagged by having the ninth
bit of the data field set to a 1. If the ninth bit is reset to a zero
the byte is a Data byte.
The device will exit from the HALT/IDLE modes when the
Start bit of a character is detected at the RDX (L3) pin. This
feature is obtained by using the Multi-Input Wake-up scheme
provided on the device.
Before entering the HALT or IDLE modes the user program
must select the Wake-up source to be on the RDX pin. This
selection is done by setting bit 3 of WKEN (Wake-up Enable)
register. The Wake-up trigger condition is then selected to be
high to low transition. This is done via the WKEDG register
(Bit 3 is one).
While in ATTENTION mode, the USART monitors the com-
munication flow, but ignores all characters until an address
character is received. Upon receiving an address character,
the USART signals that the character is ready by setting the
RBFL flag, which in turn interrupts the processor if USART
Receiver interrupts are enabled. The ATTN bit is also cleared
automatically at this point, so that data characters as well as
address characters are recognized. Software examines the
contents of the RBUF and responds by deciding either to
accept the subsequent data stream (by leaving the ATTN bit
reset) or to wait until the next address character is seen (by
setting the ATTN bit again).
If the device is halted and crystal oscillator is used, the
Wake-up signal will not start the chip running immediately
because of the finite start up time requirement of the crystal
oscillator. The IDLE timer (T0) generates a fixed (256 tC)
delay to ensure that the oscillator has indeed stabilized
before allowing the device to execute code. The user has to
consider this delay when data transfer is expected immedi-
ately after exiting the HALT mode.
Operation of the USART Transmitter is not affected by se-
lection of this Mode. The value of the ninth bit to be trans-
mitted is programmed by setting XBIT9 appropriately. The
value of the ninth bit received is obtained by reading RBIT9.
Since this bit is located in ENUR register where the error
flags reside, a bit operation on it will reset the error flags.
14.9 DIAGNOSTIC
Bits CHL0 and CHL1 in the ENU register provide a loopback
feature for diagnostic testing of the USART. When both bits
are set to one, the following occurs: The receiver input pin
(RDX) is internally connected to the transmitter output pin
(TDX); the output of the Transmitter Shift Register is “looped
back” into the Receive Shift Register input. In this mode,
data that is transmitted is immediately received. This feature
allows the processor to verify the transmit and receive data
paths of the USART.
14.11 BREAK GENERATION
To generate a line break, the user software should set the
BRK bit in the ENUI register. This will force the TDX pin to 0
and hold it there until the BRK bit is reset.
Note that the framing format for this mode is the nine bit
format; one Start bit, nine data bits, and one or two Stop bits.
Parity is not generated or verified in this mode.
www.national.com
48
NSC [ National Semiconductor ]
