EET 2261 Unit 8
Seven-Segment Displays; S19
Records; System Clocks
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Read Almy, Appendix B and Chapter
16.
Homework #8 and Lab #8 due next
week.
Quiz next week.
Seven-Segment Displays
•
Recall that a seven-segment display has
LEDs named a through g (and possibly one
or two LEDs for decimal points) that can be
lit up to display characters.
Common-Cathode or CommonAnode?
•
•
Every seven-segment display is either:
•
Common-cathode: each LED has its own
anode pin, and all LEDs share a single cathode
pin.
•
Or common-anode: each LED has its own
cathode pin, and they all share a single anode
pin.
The Dragon12’s are common-cathode.
Current-Limiting Resistors
•
Recall that LEDs will burn out if you pass
too much current through them.
•
Each LED should always be connected in
series with its own current-limiting resistor,
typically between 220 Ω and 470 Ω.
•
So if you wire a seven-segment display,
you’ll need about eight of these currentlimiting resistors: one for each segment and
decimal point. (This is already done for us
on the Dragon12 board.)
Banks of Seven-Segment Displays
•
Often we use banks of several sevensegment displays to show several digits.
•
Example: For a clock that displays hours,
minutes, and seconds, we’d need six
seven-segment displays.
•
The result: a lot of connections and a lot of
current-limiting resistors (if you take the
“brute-force” approach described below).
Two Approaches to Controlling a
Bank of Seven-Segment Displays
1. Brute force approach
•
Simple, but eats up a lot of resources
(such as I/O ports on the HCS12).
2. Multiplexing approach
•
•
Harder to understand, but much more
efficient in its use of limited resources
(such as I/O ports).
This approach is described on pages 2426 of Dragon12 manual and in the
textbook’s Appendix B.
Approach #1: Brute Force
•
Connect a seven-segment display’s eight
anode pins through current-limiting
resistors to one of the HCS12 chip’s ports.
•
Also connect the display’s common cathode
pin to ground.
•
Driving several displays in this way will use
several of the I/O ports on our HCS12 chip.
Implementing Approach #1 (Brute
Force) for Two Displays
Approach #2: Multiplexing
•
In the second approach, we use a single
HCS12 port to drive the anode pins on all of
the displays.
•
We also connect the common-cathode pins
to another HCS12 port. This lets us turn a
particular display on (by making the
common-cathode pin LOW) or off (by
setting the common cathode pin HIGH).
Approach #2: Multiplexing (Cont.)
•
We sequence quickly through the displays,
turning each one on for an instant:
1. Send out the code for the first display’s digit.
2. Turn the first display on by setting its cathode
LOW and setting all of the other cathodes HIGH.
3. Brief delay.
4. Send out the code for the second display’s digit.
5. Turn the second display on by setting its
cathode LOW and setting all of the other
cathodes HIGH.
6. Brief delay.
7. And so on for each of the other displays.
8. Repeat the entire sequence from Step 1.
Implementing Approach #2
(Multiplexing) for Two Displays
Seven-Segment Displays on the
Dragon12 Trainer
•
Our trainer board uses the multiplexing
approach for its four seven-segment
displays.
•
See Schematic Diagram 4 and pages 24-26
of Dragon12 manual.
Reminder: You Must Configure
Ports as Inputs or Outputs
•
We’ve seen that the seven-segment
displays’ anode pins are wired to Port B and
their common cathode pins are wired to Port
P.
•
So to use the seven-segment displays, we
must first configure Ports B and P as
outputs.
Downloading a Program from
CodeWarrior to the HCS12
•
When CodeWarrior downloads a program to
the HCS12, it does so using a widely used
standard called the Motorola S19 file format.
(http://en.wikipedia.org/wiki/S19_(file_format))
•
In Lab #3 you briefly examined an S19 file
for a program that you were downloading.
One line (or “record”) of that file looked like
this:
S109200086038B0520FE9F
•
See next slide for analysis of this record.
Analyzing a Record from an S19
File
•
We can break the record into five pieces:
S1 09 2000 86038B0520FE 9F
The
starting
Data:
Checksum:
The data bytes
For errorRecordAddress:
type: Identifies
Byte
Count:
Tellschecking.
how
address
in
memory
where It’s similar
being
downloaded.
In
to
this record as a data
many
bytes follow
in
this
therecord
following
data
bytes
our
case,
the
parity
this isbits
thethat are
sequence
record.
are to
be located.
machine
sometimes
code for
attached
a
to
containing
a two-byte
simple
data
program.
for error-checking.
address.
Checksums
•
A checksum byte is an extra byte
appended to data that is being transmitted.
Its purpose is to allow error checking by the
receiver. (Similar to a parity bit.)
•
There are different ways to compute
checksum bytes. They’re all effective, as
long as the sender and the receiver are
using the same method. See next two slides
for the checksum method used in S19 files.
Generating the Checksum in an
S19 Record
• The sender uses the following method
to calculate the checksum byte:
1. Add the byte count, the address bytes,
and the data bytes, discarding any
carries.
2. Take the one’s-complement of the
sum.
• The result of Step 2 is the checksum
byte for this record.
Checking the Checksum in an S19
Record
• The receiver uses the following
method to check for errors:
1. Add the byte count, the address bytes,
the data bytes, and the checksum
byte, discarding any carries.
2. The result should equal $FF. If it does
not equal $FF, an error has occurred
during transmission.
Review: HCS12 Block Diagram
•Up to now we’ve focused
on the HCS12’s central
processing unit, CPU12 in
the block diagram (on page
6 of textbook or page 23 of
Device User Guide).
• We’ve also looked at the
memory blocks (Flash,
RAM, EEPROM).
•And we’ve looked at the
general-purpose I/O ports
(PTA, PTB, etc. in the
diagram).
Review: HCS12 Block Diagram
•In coming weeks we’ll study other hardware
“blocks” (or subsystems) in the HCS12, such
as:
•Clock and Reset Generator Block
•Enhanced Capture Timer Block
•Interrupt Block
•And others
•Up to now our primary reference guide has
been the CPU reference manual, but now
we’ll need the reference guides for the other
blocks, listed here.
Review: Special-Function
Registers
•
Recall that the HCS12 has hundreds of
special-function registers.
•
Most of these registers are either:
•
Data registers, which transfer data from place
to place. (Example: PORTA)
•
Control registers, which control various
aspects of the chip’s operation. (Example:
DDRA)
•
Status registers, which hold status information
about events that have occurred.
Review: How to Access the
Special-Function Registers
•
In the HCS12’s memory map (page 26 of
Device User Guide), addresses from $0000
to $03FF are assigned to the specialfunction registers.
•
When you execute an LDAA or STAA
instruction to one of these addresses, you’re
not reading or writing to memory; instead,
you’re reading from or writing to a specialfunction register.
Review: List of Special-Function
Registers
•
Pages 27-49 in the Device User Guide list
all of the special-function registers and their
addresses.
Finding Details on the SpecialFunction Registers
•
Up to now the only special-function registers
we’ve used are the ones associated with I/O
ports, such as PORTA, DDRA, etc.
•
In coming weeks we’ll discuss many more
special-function registers, which are
associated with individual hardware blocks.
•
For details on how these registers are used,
refer to the block user guides.
Finding Details on the SpecialFunction Registers: Example
•
Example: The register at address $0034,
named SYNR, is one of many associated
with the Clock and Reset Generator.
•
For details on this register, refer to p. 15 of
the Clock and Reset Generator Block User
Guide.
Clock and Reset Generator (CRG)
Block
•Next we’ll look at the Clock
and Reset Generator block,
which includes an
important circuit called a
Phase-Locked Loop (PLL).
•Figure from p. 6 of
textbook or p. 23 of Device
User Guide).
Overview: Clock and Reset
Generator (CRG) Block
•
As its name suggests, this hardware block
has two major responsibilities:
1. Generating the HCS12’s clock signals
2. Resetting the HCS12 under certain
conditions.
• See block
diagram on
p. 11 of the
CRG Block
User Guide.
Reset Generator
•
Four conditions can cause the system to
reset itself:
1. Applying power to the chip’s power pin.
2. Pressing RESET button connected to pin
42.
3. Clock Monitor
failure.
4. Computer
Operating
Properly
(COP)
timeout.
Clock Generator
•
Three clock signals are used by different
parts of the system:
1. Core Clock.
2. Bus Clock, whose frequency is ½ of the
Core Clock.
3. Oscillator
Clock.
•
The one we care
most about is the
Bus Clock, which
tells us the
instruction cycle
time.
Oscillator and External Crystal
•
All three clock signals are derived ultimately
from the external crystal connected to the
chip’s oscillator.
•
See diagram
on p. 32 of
the CRG
Block User
Guide.
Bypassing the Phase Locked Loop
(PLL)
•
In the simplest case, we bypass the internal
phase locked loop (PLL) circuit and just use
the oscillator’s output.
•
In this case, the Core Clock and Oscillator
Clock will run at the crystal frequency, and
the Bus Clock will run at ½ this frequency.
•
Example: The Dragon12 has an 8-MHz crystal, so if
we bypass the PLL, our clock frequencies will be:
• Oscillator Clock = 8 MHz
• Core Clock = 8 MHz
• Bus Clock = 4 MHz
Using the PLL
•
The PLL circuit lets us increase or decrease
the Core Clock and Bus Clock frequencies
above or below what they would be if we just
used the crystal and oscillator.
•
Example: By default, CodeWarrior uses the
Dragon12’s PLL to multiply the oscillator frequency
by 6. Since we have an 8-MHz crystal, our clock
frequencies will be:
• Oscillator Clock = 8 MHz
• Core Clock = 48 MHz
• Bus Clock = 24 MHz
Special-Function Registers
Associated with the CRG Block
•
The twelve special-function registers located
at addresses $0034 to $003F let us control
the operation of the CRG block.
•
Figure from p. 30 of the Device User Guide.
Special-Function Registers That
Control the PLL
•
Of the registers associated with the CRG,
we care most about these four:
•
•
•
•
•
CRG PLL Control Register (PLLCTL)
CRG Clock Select Register (CLKSEL)
CRG Synthesizer Register (SYNR)
CRG Reference Divider Register (REFDV)
These let us tell the system whether we
want to use the PLL or bypass it, and also let
us specify how much we want to increase or
decrease the clock frequency.
CRG PLL Control Register
(PLLCTL)
•
The main bit we care about in this register is
bit 6 (PLLON), which turns the PLL circuit on
or off.
•
Figure from p. 21 of CRG Block User Guide, which
also provides detailed explanation.
CRG Clock Select Register
(CLKSEL)
•
The main bit we care about in this register is
bit 7 (PLLSEL), which says whether we want
to bypass the PLL or use it.
•
Figure from p. 19 of CRG Block User Guide, which
also provides detailed explanation.
CRG Synthesizer Register (SYNR) &
Reference Divider Register (REFDV)
•
These two registers determine how much we
increase or decrease the clock frequency,
according to this formula:
•
See pp. 15-16 of CRG Block User Guide.
Example: Programming the PLL
LDAA #0
STAA CLKSEL
LDAA #2
STAA SYNR
LDAA #1
STAA REFDV
JSR Delay
LDAA #$80
STAA CLKSEL
LDAA #$60
STAA PLLCTL
;Select oscillator, not PLL.
;Need about 1 ms delay.
;Select the PLL.
;Turn on the PLL.
Measuring the Bus Clock Frequency
•
None of the HCS12’s clock signals are
brought out to pins on the chip. So we can’t
directly measure the clock frequencies.
•
But we can indirectly measure the bus
clock frequency. (We’ll do this in Lab 9.)
•
Suppose we have a program that uses a delay
loop to toggle a pin on a port. The time delay
from the delay loop depends on the bus clock
frequency. (A faster clock frequency results in a
shorter delay.)
•
So by measuring the toggle rate, we can figure
out what the bus clock frequency is.