# EET 2261 Unit 8 Seven-Segment Displays; S19 Records; System Clocks

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EET 2261 Unit 8 Seven-Segment Displays; S19 Records; System Clocks
Read Almy, Appendix B and Chapter 16. Homework #8 and Lab #8 due next week. Quiz next week. Handouts: Quiz 7, Checksum practice sheet and Clock Frequencies practice sheet, and slides 8 and 11 on using two seven-segment displays.

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. This topic is discussed in the textbook’s Appendix B.

Common-Cathode or Common-Anode?
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. So to show a 3, what logic level do we need to send to each of the pins shown above?

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 current-limiting 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 seven-segment 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
Brute force approach Simple, but eats up a lot of resources (such as I/O ports on the HCS12). 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 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
Connect left a anode pin through a current-limiting resistor to the HCS12’s PA0. Connect left b anode pin through a resistor to the HCS12’s PA1. And so on for the other anode pins. Similarly for the right display, using PORTB.

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: Send out the code for the first display’s digit. Turn the first display on by setting its cathode LOW and setting all of the other cathodes HIGH. Brief delay. Send out the code for the second display’s digit. Turn the second display on by setting its cathode LOW and setting all of the other cathodes HIGH. And so on for each of the other displays. Repeat the entire sequence from Step 1.

Implementing Approach #2 (Multiplexing) for Two Displays
Connect All a anode pins through a current-limiting resistor to the HCS12’s PB0. All b anode pins through a current-limiting resistor to the HCS12’s PB1. And so on for the other anode pins. The first display’s common-cathode pin to the HCS12’s PTP0. The second display’s common-cathode pin to the HCS12’s PTP1. And so on for any other 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 of Dragon12 manual. On the schematic diagram, note signal names “DIG0”, “DIG1”, etc. which are connected to outputs of the buffer/driver chip below it.

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.

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: S B0520FE9F See next slide for analysis of this record.

Analyzing a Record from an S19 File
We can break the record into five pieces: S B0520FE 9F Address: The starting address in memory where the following data bytes are to be located. Record type: Identifies this record as a data sequence record containing a two-byte address. Data: The data bytes being downloaded. In our case, this is the machine code for a simple program. Checksum: For error-checking. It’s similar to the parity bits that are sometimes attached to data for error-checking. Byte Count: Tells how many bytes follow in this record.

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: Add the byte count, the address bytes, and the data bytes, discarding any carries. Take the one’s-complement of the sum. The result of Step 2 is the checksum byte for this record. Do it for the record shown previously.

Checking the Checksum in an S19 Record
The receiver uses the following method to check for errors: Add the byte count, the address bytes, the data bytes, and the checksum byte, discarding any carries. The result should equal \$FF. If it does not equal \$FF, an error has occurred during transmission. Do checksum practice sheet.

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 special- function 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 special- function register.

Review: List of Special-Function Registers
Pages in the Device User Guide list all of the special-function registers and their addresses.

Finding Details on the Special-Function 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 Special-Function 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: Generating the HCS12’s clock signals 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:
Applying power to the chip’s power pin. Pressing RESET button connected to pin 42. Clock Monitor failure. Computer Operating Properly (COP) timeout.

Clock Generator Three clock signals are used by different parts of the system: Core Clock. Bus Clock, whose frequency is ½ of the Core Clock. 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 -Why might we want to increase or decrease the clock freqs? -Recall that our delay loop code was based on assumption of 41.7 ns cycle time (=1/24 MHz). If we change the frequency of the bus clock, our delay code will give us a longer or shorter delay than we designed it for.

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 of CRG Block User Guide.

Example: Programming the PLL
LDAA # STAA CLKSEL ;Select oscillator, not PLL LDAA # STAA SYNR LDAA # STAA REFDV JSR Delay ;Need about 1 ms delay LDAA #\$ STAA CLKSEL ;Select the PLL LDAA #\$ STAA PLLCTL ;Turn on the PLL. Do clockFreqs practice sheet.

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.

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