Chapter 4 9S12 Architecture

From the text by Valvano: Introduction to Embedded Systems:

Interfacing to the Freescale 9S12

Chapter 4 Objectives

• Present the basic microcomputer architecture

• Study software execution at the bus cycle level

• List three 9S12 microcomputers and their memory and I/O port configurations

• Describe the timer and use it to create fixed time delays

4.1 Introduction

• 4.1.1 Big and Little Endian– Freescale microcomputers implement the big

endian approach for data storage—the most significant byte first.

– Intel microcomputers implement the little endian approach—the least significant byte is stored first.

– Consider storage of 1000 ($03E8) –Figure 4.1, page 112 of the text.

Endians

Consider storage of 1000 ($03E8) –Figure 4.1, page 112 of the text.

Note that the MSB is placed $0850 for the BIG Endian approach and the LSB is placed in $0851.

4.1 Introduction

• 4.1.2 Memory-Mapped I/O– The embedded system architecture defines

how the processor, RAM, ROM, and I/O devices are connected.

– The 9S12 implements Memory-Mapped I/O.– The I/O devices are connected to the

processor in a manner similar to memory (see Figure 4.4, page 113 of text).

4.1 Introduction

• 4.1.3 I/O Mapped I/O —the control bus for the I/O is separate from the control bus for memory. (Figure 4.5, page 113.)

• 4.1.4 Segmented or Partitioned Memory — memory is divided into different groups, according to function—the Intel 8051 has 3 segments.

4.1.5 Memory Bus Cycles

• The bus contains address, data , and control information.– Address– specifies which module will

communicate with the processor.– Data—the information being transferred.– Control—indicates the direction of transfer.

• A bus cycle is a complete data transfer.

Checkpoints

• Checkpoint 4.1: The 9S12C32 has a 16-bit address bus. How many locations can it address? (2^16 or 65,536 locations)

• Checkpoint 4.2: Both the 9S12DP512 and the 9S12E128 can access 1 mebibyte, including internal and external memory. How many address lines are in their busses? (recall Table 3.3, page 59). (1 mebibyte is 2^20, so there are 20 address lines.)

Checkpoints

• Checkpoint 4.3: The 9S12C32 in single-chip mode has a 16-bit address bus and a 16-bit data bus, but can still only address 65,536 bytes of memory. Why?– 9S12 accesses individual bytes, so each

location requires a unique address.

4.1.6 Processor Architecture

• 4 basic parts (BIU, Registers, ALU, and the CU)

• BIU—the bus interface unit handles the read/write access to memory (see Figure 4.9)

Bus Interface Unit

• Bus signals are divided into 3 groups: control, address, and data

• Control: E (the clock that controls the timing of each bus cycle) and R/W.)

• Address: 16 signals ( the memory address for the current bus cycle.)

• Data: the EAR (Effective Address Register) contains data address for current instruction (set during third phase of execution.)

4.1.6 Processor Architecture (cont.)

• Registers—temporary storage elements with a usage defined by instructions.– Condition Code Register (Figure 4.10)

• S—Stop disable• X—XIRQ Interrupt mask• H—Half carry from bit 3 (used for BCD addition)• I—Interrupt mask• N—Negative• Z—Zero• V—signed oVerflow• C--Carry/borrow or unsigned overflow

– A,B,D,X,Y,SP,PC Registers (Figure 4.9)

4.1.6 Processor Architecture (cont.)

• Arithmetic Logic Unit (ALU) —performs arithmetic and logic operations.

• Control Unit (CU) —orchestrates the sequence of operations in the processor (recall page 34) with the opcode of the current instruction located in the instruction register (IR).

Checkpoint

• Checkpoint 4.4: For what do the acronyms CU BIU and ALU stand?

Phases (page 117)

• Phase 1 Opcode and operand fetch

• Phase 2 Decode instruction

• Phase 3 Evaluate address

• Phase 4 Data read

• Phase 5 Free cycle

• Phase 6 Data write

Phase 1: Op code and Operand Fetch

• The op code is fetched and placed in the instruction register (IR).– Inherent mode –no additional bytes– Immediate addressing mode—1 or 2 bytes– Direct addressing mode—1 byte (used to calculate

the effective address).– Extended Addressing mode—2 bytes (used to

calculate the effective address)– Indexed Addressing Mode—1,2,or 3 bytes (used to

calculate the effective address)– PC relative addressing—1 or 2 bytes (rel=target-

currrent-n where n is the number of bytes in the instruction.)

Phase 2: Decode Instruction

• Requires no extra bus cycles—processor determines instruction to be executed.

Phase 3 Evaluate Address

• Usually does not require any bus cycles—processor calculates the effective address for the EAR (note: some indirect modes require addition cycles).

Phase 4: Data Read

• If data is needed from memory, the contents of the EAR will be used to fetch 1 or 2 bytes from memory.

• These cycles will either be fetches or stack pulls.

Phase 5: Free Cycles

• Null cycles or free cycles will be generated if additional time is needed for the result of the ALU and the setting of the CCR as needed (simulation will not show these cycles, but will count them as needed.)

Phase 6: Data Write

• If needed, data is stored in memory (1 or 2 bytes.)

• These cycles will either be data writes or stack pushes.

Assembly Example (TExaS)

• movw $3800,$240– Assume the location of the instruction is

$F000.– 180438000240 contains the opcode and

operands– [6] indicates the number of cycles– {ORPWPO} explains the details of the cycle .– See page 118.

4.1.7 I/O Port Architecture

• Some ports are input only (PE1 and PEO on the 9S12.)

• See figure 4.11 , page 118.• There are no latched input ports on the

9S12 (see Figure 4.12 for an example).• Most of the 9S12 port pins can be inputs

or outputs—Freescale uses a direction register —see Figure 4.14 for an illustration of a bidirectional port.

Example 4.1

• Design an I/O driver for a single output pin.

• Solution: (page 120-121)—initialize, set and clear (using the instructions bset and bclr.

4.2 Understanding Software Execution at the Bus Cycle Level

• More details on bus cycles.

4.3 9S12 Architecture Details• All 9S12 microcontrollers have a 16-bit central processing unit (HCS12CPU).• System Integration Manual (SIM)• Random Access Memory (RAM)• Electronic Erasable Programmable Memory (EEPROM)• PLL (Phase-locked Loop)• Asynchronous serial communication interface (SCI)• Serial Peripheral Interface (SPI)• Inter-integrated circuit (I2C)• Key wakeup• 16-bit timer• Pulse Width Modulation (PWM)• 8-bit digital-to-analog converter (DAC)• Liquid Crystal Display (LCD)• Controller area network (CAN 2.0)• Universal serial bus (USB2.0) interface• Ethernet (MAC FEC 10/100) interface• Memory Expansion Logic

4.3.1 9S12C32

• See Figure 4.16

• Smaller and lower cost of the 9S12’s.

• RAM—2 Kbytes

• EEPROM—32 Kbytes

• Pin packages—(Table 4.5, page128)– Also, see Fig. 4.17 (NC12C32 Nanocore 12).

• TExaS does not simulate the external data bus, SPI, PWM, or CAN.

4.3.2 OS12DP512

• 112 pin module with 91 I/O pins.

• EEPROM—512 Kbytes– Only 48 K bytes is directly addressable.– Paged Memory is use the access the rest.

4.3.3 9S12E128

• 8 K Bytes of RAM• 128 K bytes of EEPROM.• 12 input capture/output compare timer pins• 12 pulse-width modulated output pins• 16 ADC inputs• Two DAC outputs• One SPI module• Three SCI modules • One I2C module• Two sizes: 80 pins and 112 pins.

4.3.4 Operating Modes

• 8 different modes

• BKGD,MODA, and MODB are used to select the mode (0,0,0—1,1,1).

• Table 4.10, page 134 shows the three most common.– Normal single chip– Normal expanded narrow– Normal expanded wide

Single—Chip Mode

• Mode used in the Valvano text.

• All ports are available for input/output.

• RAM—2Kbytes to 32Kbytes

• If more RAM is needed, then expanded narrow and expanded wide modes are used to interface larger external memory.

4.3.5 Phase-Locked Loop

• Execution speeds are normally determined by an external clock.

• A slower clock requires less power.

• Some MC9S12C32 boards have an 8 MHz crystal creating a 4-MHz clock.

• The Phase-Locked Loop allows the software to adjust the execution speed.

4.4 The Stack

• Two basic operations: Push and Pull or Push and Pop.• Push—saves data on the top of the stack (decrement SP

then store at SP.)• Pull—removes data (read at SP then increment the SP).• Instructions psha and pula use Register A.• These instructions use inherent addressing and do not

change the CCR.• Last in first out (LIFO).• See page 135.• See Figure 4.21.

Checkpoints

• Checkpoint 4.7: After a psha instruction, how many copies exist of the data being pushed?

• Checkpoint 4.8: After a pula instruction, how many copies exist of the daata being pulled?

• Checkpoint 4.9: Assume you have two 8-bit global variables M and N. Write assembly code that switches the values in M and N using just the ldaa staa psha and pula instructions.

4.5 16-Bit Timer

• The 16 bit timer on the 9S12 is called TCNT.• It is a counter that increments at a fixed rate and can be

used to create pulses, squarewaves, and pulse-width modulated waves.

• It can also be used to measure the period, pulse-width, or frequency of an input signal.

• Table 4.12 shows how PR2,PR1 and PR0 (in the TSCR2 register) are used to set the rate of the counter.

• Bit 7 of the TSCR1 register must be set in order to use TCNT.

• Program 4.5 (page 138 of text) illustrates how to set a time delay.

4.6 Memory Allocation

• The memory of a PC-compatible computer is configured as a linear array—segments are used by the programmer.

• Embedded systems use segmentation.

• Segments could have the following groups: global variables, heap, local variables fixed constants, and machine instructions.

4.7 Performance Debugging

• 4.7.1 Instrumentation– A prescaler placed between the E clock and

the TCNT counter can be used to measure timing (with a little intrusiveness.)

– Program 4.7 (page 142 shows how Port T, bit 6, can also be used to measure timing, when it is attached to an oscilloscope—jsr statements at “strategic” places can then be used to mearue the timing.

4.7.2 Measurement of Dynamic Efficiency

1. Count bus cycles using assembly listing (only useful for short programs).

2. The internal timer TNT could be used (program 4.9, page 144 of text).

3. Oscilloscope can be attached to an used pin. (program 4.10, page 144)—looks similar to 4.7.1.