EET 2261 Unit 11
Serial Communications Interface
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
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Read Almy, Chapter 24.
Homework #11 and Lab #11 due next
week.
Quiz next week.
Communicating with External
Devices
•We’ve used the HCS12’s general-purpose I/O
ports (Ports A, B, E,…) to communicate with
simple devices, such as switches and LEDs,
that use standard TTL-level signals (0 V for
LOW and 5 V for HIGH).
•In addition to this low-tech form of
communication, our HCS12 contains several
blocks that implement more complicated
communication bus standards.
Serial Communications Interface
(SCI) Blocks on the HCS12
•The HCS12 has two
asynchronous serial
communications interface
blocks named SCI0 & SCI1.
•Each SCI block has two I/O
pins: RXD for receiving data,
TXD for transmitting data.
These pins are shared with
the general-purpose I/O port
S.
•Figure from p. 6 of textbook
or page 23 of Device User
Guide).
SCI0 and SCI1 on the Dragon12
•On the Dragon12 board:
•The HCS12’s SCI0 is normally
connected to the board’s USB port,
through which the chip
communicates with CodeWarrior on
the personal computer.
•The HCS12’s SCI1 is normally
connected to the RS232 interface.
•The board has movable jumpers that let us
reconfigure the board’s communication resources for
different needs. But we’ll leave these jumpers alone.
Block Diagram of Serial
Communications Interface (SCI) Block
Diagram from
page 12 of the
SCI Block User
Guide.
•
Remember: there are two separate copies of
this circuit on our HCS12 chip. One is called
SCI0, and the other is SCI1. We’ll use SCI1.
Special-Function Registers
Associated with the SCI Block
•
The 8 special-function registers located at
addresses $00C8 to $00CF let us control the
operation of the SCI0 block.
•
The 8 registers at addresses $00D0 to
$00D7 are for the SCI1 block.
•
See p. 37 of Device User Guide.
SCI Baud Rate Registers
(SCInBDH and SCnIBDL)
•
•
SCInBDH and SCnIBDL combined hold 16 bits,
of which 13 form a number called SBR.
•
The SCI’s baud rate depends on SBR and the
system’s bus clock frequency, as follows:
Baud rate = Bus clock freq / (16 × SBR)
Figure from
p. 5 of SCI
Block User
Guide.
Possible Baud Rates
•
As stated on the previous slide, the SCI’s baud
rate is given by
Baud rate = Bus clock freq / (16 × SBR)
where SBR is a 13-bit number.
•
So it’s usually not possible to set the baud rate
exactly to one of the standard values used by
other communications devices (such as 300,
600, 1200, 2400, 4800, 9600, 19200).
•
But usually we can get close enough for
communication to take place successfully.
SCI Data Registers (SCInDRH and
SCInDRL)
•
•
The SCInDRH and SCInDRL registers hold the
data that is being either transmitted or received.
•
For 8-bit data (the most common situation), only
SCInDRL is used.
Figure from
p. 11 of SCI
Block User
Guide.
SCIn Control Register 1
(SCInCR1)
•
The bits we care most about in this control
register are:
• M, which we use to specify whether we’re
working with 8-bit or 9-bit data.
• PE, which we use to enable or disable parity
generation/checking.
• PT, which (if parity is enabled) we use to
specify whether even or odd parity is used.
•
Figure from p. 6 of SCI Block User Guide.
SCIn Control Register 2
(SCInCR2)
•
The bits we care most about in this control
register are:
• TE, which we use to enable or disable data
transmission.
• RE, which we use to enable or disable data
reception.
•
Figure from p. 7 of SCI Block User Guide.
SCIn Status Register 1 (SCInSR1)
•
The bits we care most about in this status
register are:
• TDRE, which tells us whether the
transmission data register is empty.
• RDRF, which tells us whether the receive
data register is full.
•
Figure from p. 8 of SCI Block User Guide.
Steps for Transmitting Data
• Programming the SCI to transmit data
(without using interrupts):
1. Set baud rate using SCInBDH:SCInBDL registers.
2. Write $00 to SCInCR1 register, indicating 8-bit
data frame, no parity bit.
3. Write $08 to SCInCR2 register to enable
transmission, also disabling interrupts.
4. Monitor the TDRE bit of the SCInSR1 register to
make sure data register is empty before sending
a byte to SCInDRL. If TDRE = 1, then go to the
next step.
5. Write the byte to be transmitted to SCInDRL.
6. To transfer another byte, go to Step 4.
Steps for Receiving Data
• Programming the SCI to receive data
(without using interrupts):
1. Set baud rate using SCInBDH:SCInBDL registers.
2. Write $00 to SCInCR1 register, indicating 8-bit
data frame, no parity bit.
3. Write $04 to SCInCR2 register to enable
reception, also disabling interrupts.
4. Monitor the RDRF bit of the SCInSR1 register to
see if an entire byte has been received. If RDRF
= 1, then go to the next step.
5. Read the received byte from SCInDRL.
6. To receive another byte, go to Step 4.
Polling Versus Interrupts for Serial
Communications
•The two previous slides assumed that we’re not
using interrupts. Without interrupts, our program
will sit in a loop, polling the TDRE bit (or the
RDRF bit) repeatedly until it is set, at which point
the program proceeds to take some other action:
Over: BRCLR SCI1SR1, %10000000, Over
•Another way is to use interrupts instead of
repeatedly polling the TDRE bit (or the RDRF
bit).
Serial Communications Interface
(SCI) Interrupt
•Each SCI module has its own interrupt, which can be
caused by either the receive data register being full or
the transmit data register being empty:
•Enabled or disabled by TIE and RIE in SCInCR2:
•
We’re familiar with the TDRE and RDRF flag bits in
SCInSR1:
Interrupt Vectors for SCIn
Interrupts
•In the interrupt vector table, the two words
starting at $FFD6 and $FFD4 are reserved for
the starting addresses of the service routines
for SCI0 and SCI1 interrupts, respectively.
From table on page 75
of the Device User Guide.
•Remember: the programmer is responsible for
setting up correct addresses in the vector
table.
Microsoft HyperTerminal
•A Microsoft Windows
accessory program
named HyperTerminal
lets you send or
receive data over your
computer’s RS-232
port.
•Shown here is a
dialog box for
configuring
HyperTerminal.
Review: Communicating with
External Devices
•In previous weeks we used the HCS12’s
general-purpose I/O ports (Ports A, B, E,…) to
communicate with simple devices, such as
switches and LEDs, that use standard TTLlevel signals (0 V for LOW and 5 V for
HIGH).
•In addition to this low-tech form of
communication, our HCS12 contains many
blocks that implement more complicated
communication bus standards.
Communication Blocks in the
HCS12
•These blocks include:
•Two serial communications
interface (SCI) blocks.
•Three serial peripheral
interface (SPI) blocks.
•Two controller area network
(CAN) blocks.
•One inter-integrated circuit
(IIC) block.
• Figure from p. 6 of textbook
or page 23 of Device User
Guide).
Two Meanings of “Bus”
•Sometimes the term bus simply refers to a
group of conductors (wires or circuit board
traces) that carry signals from one device to
another.
•Examples: address bus, data bus
•At other times it refers to a standard set of
specifications (voltage levels, timing specs,
connectors, etc.) used for communication
between devices.
•Examples: RS-232, SPI, USB
Many Bus Standards
•There are dozens of bus standards in common
use. From Wikipedia’s article on the USB bus:
We’ll Focus on the SCI Blocks
•With all of these HCS12 blocks and bus
standards, this is a large and complex topic.
•We’ll restrict our attention the HCS12’s SCI
blocks (Chapter 24).
•If you’re interested in the SPI blocks, see
Chapter 25. For the IIC block, see Chapter
26.
Terminology: Serial vs. Parallel
•Some bus standards apply to serial
communication (1 data bit transferred at a
time).
•Others apply to parallel communication
(several data bits—usually 8—transferred at a
time).
Terminology: Bits per Second and
Baud Rate
•These are two common measures of speed in
communications. Many writers loosely treat
these as being synonyms, but this is not strictly
correct.
•Bits per second (bps) is the easier to
understand. Often expressed as kbps or Mbps.
•In the simplest cases, baud rate equals bps.
In more sophisticated schemes, the two are
related but not equal. Traditional baud rates
are 300, 600, 1200, 2400, 4800, 9600, 19200.
Terminology: Simplex vs. Duplex
•Simplex: Information flows in one direction
only.
•Example: a temperature sensor sending
data to a personal computer.
•Half-duplex: Information can flow in both
directions, but only one at a time.
•Example: walkie-talkie.
•Full-duplex: Information can flow in both
directions at the same time.
•Example: telephone.
Communications Terminology:
Asynchronous vs. Synchronous
•In asynchronous communication, the two
devices do not share clock signals, so they use
“handshaking” signals or some other method to
coordinate their activity.
•Widely used serial asynchronous standards:
•RS-232
•RS-423
•RS-422
•RS-485
•USB (Universal Serial Bus): can operate
synchronously or asynchronously.
Communications Terminology:
Asynchronous vs. Synchronous
•In synchronous communication, the sender
and receiver share a clock signal.
•Widely used serial synchronous standards:
•SPI (“four-wire,” or “three-wire” variant)
•IIC or I2C (also called “two-wire”)
•1-wire
Ports on a Typical Laptop
Computer
USB
(Serial)
IEEE
1394
(Serial)
RS-232
(Serial)
Printer
(Parallel)
PS/2 Mouse
(Serial)
VGA (Analog
video)
Ports on a Fluke 45 DMM
IEEE 488 (“GPIB”) option
not installed (Parallel)
RS-232
(Serial)
Communications Terminology:
Mark and Space
•The terms “mark” and “space” are old terms
from the days of telegraphs. These terms are
still often used when discussing serial
communication.
•Mark simply means a binary 1 level on the
serial line. (When no data is being transmitted,
the line sits high, and we are “marking time.”)
•Space simply means a binary 0 level on the
line.
Communications Terminology:
Start Bits, Stop Bits
•Asynchronous communication standards such as
RS232 generally use start bits and stop bits at the
beginning and end of a transmitted byte.
•The start bit is binary 0.
•The stop bits—there may be one or two—are binary 1.
•These bits are not part of the data being
transmitted. They form a “frame” around the data.
Figure from page 229
of the textbook.
Communications Terminology:
Parity Bits
•Parity is an error-checking system that attaches
an extra bit (called the parity bit) to a byte when
the byte is transmitted.
•When we configure a serial device, we often
have the choice of using odd parity, even parity,
or no parity.
Figure from page 229
of the textbook.
As shown here, we
transmit the data byte’s
LSB first.
RS-232 Standard
•First version created in early 1960’s.
•Obsolete in some respects, but still very widely
used.
•In recent years, has been applied in ways that
its original creators never imagined, sometimes
leading to problems.
•Original spec defined 25 pins (signals), but
often only 9 or fewer are used.
Terminology: DCE vs DTE
•In any RS-232 application, each device is
designated as either Data Terminal Equipment
(DTE) or Data Communications Equipment
(DCE).
•Simple case: When you connect a personal
computer to a modem, the computer is the DTE
and the modem is the DCE.
Connectors
•Original RS-232 standard called for a DB-25
connector. Since many later applications didn’t
use most of the pins, it became common to use
DE-9 connectors (often incorrectly referred to
as DB-9).
RS-232 Signals
•The nine most important signals:
Description
Abbrev.
Direction
DTE - DCE
DB-25
Pin #
DE-9
Pin #
Transmitted data
TxD

2
3
Received data
RxD

3
2
Request to send
RTS

4
7
Clear to send
CTS

5
8
Signal Ground
7
5
Protective Ground
1
Data set Ready
DSR

6
6
Data carrier detect
DCD

8
1
Data terminal ready
DTR

20
4
HCS12 Uses a Scaled-Down RS232 Interface
•As we’ve seen, the original RS-232 standard
defines 25 lines, but most applications make do
with far fewer than 25—usually 9 or even less.
•With the HCS12’s Serial Communications
Interface, we use only three lines: RxD, TxD,
and Signal Ground.
•So we’re not using any of the handshaking
lines (such as CTS, RTS, DSR, DTR). As a
result, data can be lost if the sender transmits
data when the receiver is not ready to receive
it.
RS-232 Voltage Levels
•TTL voltage levels are:
•  0 V for a binary 0.
•  +5 V for a binary 1.
•This scheme is “unipolar” because it doesn’t
use negative voltages.
•For transmission over a cable, it’s undesirable
to have either logic level close to 0 V.
•So RS-232 uses a “bipolar” scheme, with:
• +3 V to +25 V for a binary 0 (“space”)
• -3 V to -25 V for a binary 1 (“mark”)
MAX232 Chip
•Since TTL voltage levels are incompatible with
RS-232 voltage levels, many digital systems
need to translate from one to the other.
•A popular chip for this purpose is Maxim’s
MAX232A.
RS-232 on the Dragon12
•The Dragon12 board contains an RS-232
interface, which consists of a MAX232A chip
and a DE9 connector.
•See the Dragon12 Schematic Diagram 3.