cop8sgr5vej8xxx National Semiconductor Corporation, cop8sgr5vej8xxx Datasheet - Page 33

• Manufacturer Part Number: cop8sgr5vej8xxx
• Description: 8-bit Cmos Rom Based And Otp Microcontrollers With 8k To 32k Memory, Two Comparators And Usart
• Manufacturer: National Semiconductor Corporation

8.0 USART
As an example, considering Asynchronous Mode and a CKI
clock of 4.608 MHz, the prescaler factor selected is:
The 2.5 entry is available in Table 5 . The 1.8432 MHz
prescaler output is then used with proper Baud Rate Divisor
( Table 4 ) to obtain different baud rates. For a baud rate of
19200 e.g., the entry in Table 4 is 5.
The divide by 16 is performed because in the asynchronous
mode, the input frequency to the USART is 16 times the
baud rate. The equation to calculate baud rates is given
below.
The actual Baud Rate may be found from:
Where:
BR is the Baud Rate
Fc is the CKI frequency
N is the Baud Rate Divisor ( Table 4 ).
P is the Prescaler Divide Factor selected by the value in the
Prescaler Select Register ( Table 5 )
Note: In the Synchronous Mode, the divisor 16 is replaced by two.
Example:
Asynchronous Mode:
Using the above equation N x P can be calculated first.
Now 32.552 is divided by each Prescaler Factor ( Table 5 ) to
obtain a value closest to an integer. This factor happens to
be 6.5 (P = 6.5).
The programmed value (from Table 4 ) should be 4 (N − 1).
Using the above values calculated for N and P:
8.8 Effect of HALT/IDLE
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 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 Wakeup scheme
provided on the device.
Before entering the HALT or IDLE modes the user program
must select the Wakeup source to be on the RDX pin. This
selection is done by setting bit 3 of WKEN (Wakeup Enable)
register. The Wakeup trigger condition is then selected to be
high to low transition. This is done via the WKEDG register
(Bit 3 is one.)
If the device is halted and crystal oscillator is used, the
Wakeup signal will not start the chip running immediately
N − 1 = 5 (N − 1 is the value from Table 4 )
Crystal Frequency = 5 MHz
Desired baud rate = 9600
N = 6 (N is the Baud Rate Divisor)
% error = (9615.385 − 9600)/9600 x 100 = 0.16%
Baud Rate = 1.8432 MHz/(16 x 6) = 19200
BR = (5 x 10
N x P = (5 x 10
N = 32.552/6.5 = 5.008 (N = 5)
BR = Fc/(16 x N x P)
4.608/1.8432 = 2.5
(Continued)
6
)/(16 x 5 x 6.5) = 9615.384
6
)/(16 x 9600) = 32.552
33
because of the finite start up time requirement of the crystal
oscillator. The idle timer (T0) generates a fixed (256 t
to ensure that the oscillator has indeed stabilized before
allowing the device to execute code. The user has to con-
sider this delay when data transfer is expected immediately
after exiting the HALT mode.
8.9 Diagnostic
Bits CHARL0 and CHARL1 in the ENU register provide a
loopback feature for diagnostic testing of the USART. When
these bits are set to one, the following occur: 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 re-
ceive data paths of the USART.
Note that the framing format for this mode is the nine bit
format; one Start bit, nine data bits, and 7/8, one or two Stop
bits. Parity is not generated or verified in this mode.
8.10 Attention Mode
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 7/8, 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.
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).
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.
9.0 Comparators
The device contains two differential comparators, each with
a pair of inputs (positive and negative) and an output. Ports
F1–F3 and F4–F6 are used for the comparators. The follow-
ing is the Port F assignment:
F6 Comparator2 output
F5 Comparator2 positive input
F4 Comparator2 negative input
F3 Comparator1 output
F2 Comparator1 positive input
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