AP500/AP520/AP521 ACROPACK
USER
’S MANUAL
Acromag, Inc. Tel: 248-295-0310
- 26 - http://www.acromag.com
- 26 -
www.acromag.com
MCRDIV = 1 If MCR bit-7=0
MCRDIV = 4 If MCR bit-7=1
Table 3.5
shows the correct divisor to use for generation of some standard
baud rates (based on the 14.7456MHz clock). A different external crystal
can replace the 14.7456MHz crystal on the circuit board to obtain unique
clock rates. You may contact Acromag Applications Engineering to explore
options in this area.
With respect to this device, the baud rate may be considered equal to the
number of bits transmitted per second (bps). The bit rate (bps), or baud
rate, defines the bit time. This is the length of time a bit will be held on
before the next bit is transmitted. A receiver and transmitter must be
communicating at the same bit rate, or data will be garbled. A receiver is
alerted to an incoming character by the start bit, which marks the beginning
of the character. It then times the incoming signal, sampling each bit as
near to the center of the bit time as possible.
To better understand the asynchronous timing used by this device, note that
the receive data line (RxD) is monitored for a high-to-low transition (start
bit). When the start bit is detected, a counter is reset and counts the 16x
sampling clock to 7-1/2 (the center of the start bit). The receiver then
counts from 0 to 15 to sample the next bit near its center, and so on, until a
stop bit is detected, signaling the end of the data stream. Use of a sampling
rate 16x the baud rate reduces the synchronization error that builds up in
estimating the center of each successive bit following the start bit. As such,
if the data on RxD is a symmetrical square wave, the center of each
successive data cell will occur /-3.125% of the actual center (this is
50% / 16, providing an error margin of 46.875%). Thus, the start bit can
begin as much as one 16x clock cycle prior to being detected.
Table 3.5 Baud Rate
Divisors(14.7456MHZ)
BAUD RATE
DIVISOR (N)
MCR
Bit-7=1
MCR
Bit-7=0
Decimal
DLM
(HEX)
DLL
(HEX)
50
200
4608
12
00
300
1200
768
03
00
600
2400
384
01
80
1200
4800
192
00
C0
2400
9600
96
00
60
3600
14,400
64
00
40
