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ECE 353 Introduction to Microprocessor Systems. Michael J. Schulte. Week 10. Administrative. Reading for next week 10 Supplement #3, review chapter 9 in text, ADuC datasheet pages 79-82 Homework #4 returned Average of 85% Still grading Quiz #2. Topics.

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  • Reading for next week 10
    • Supplement #3, review chapter 9 in text,
    • ADuC datasheet pages 79-82
  • Homework #4 returned
    • Average of 85%
  • Still grading Quiz #2
  • ADuC7026 External Memory Interface
    • Implementation
    • Demultipexing
  • Bus Timing
    • Bus cycle timing modification
      • Wait states and more
  • Assessing timing compatibility
basic system bus operation
Basic System Bus Operation
  • Address
    • Unidirectional from CPU
  • Data
    • Bidirectional
  • Control
    • /RS or /RD – output from CPU
      • Indicates a read operation in progress
    • /WS or /WR – output from CPU
      • Indicates a write operation in progress
    • /WAIT or /READY – input to CPU
      • Used by external device to signal that it is not able to complete transfer yet
aduc7026 bus operation
ADuC7026 Bus Operation
  • The ADuC7026 external memory interface consists of
    • 16-bit multiplexed address/data bus (AD15:0)
    • High address for 8-bit operation (A16)
    • Read and write strobes (/RS, /WS)
    • Memory select signals (/MS3:0)
    • Byte enables (/BHE, /BLE)
    • Demultiplexing control signal (AE)
    • There is no WAIT/READY signal
  • Basic Read Cycle Sequence at Bus Level
    • Diagram
  • Basic Write Cycle Sequence at Bus Level
    • Diagram
aduc7026 demultiplexing
ADuC7026 Demultiplexing
  • Multiplexed Signal Timing
    • Read Cycle
  • Dealing with a multiplexed bus
    • Demultiplexing by the device
    • Demultiplexing logic to create an address bus
  • Implementation
    • Devices – latches or flip-flops
    • Connections
    • AE timing
sram timing compatibility
SRAM Timing Compatibility
  • In order to properly read and write the device, we need to ensure that the processor-to-memory interface is compatible with the memory device.
  • This is accomplished by analyzing the timing for all relevant parameters, and ensuring that the operation can be completed successfully.
  • We will work through the read cycle analysis for the ADuC7026...
assessing timing compatibility
Assessing Timing Compatibility
  • Need to know whether CPU could operate with the tAA for given device. (read cycle)
  • We designate a CPU characteristic tAVDV, which is the delay from
    • When the address becomes valid at the CPU
    • Until the data must be driven to CPU
  • This establishes an upper bound on tAA
    • tAA < tAVDV
  • Read cycle parameters (see handout)
  • Read cycle timing control (see handout)
  • Simple Example
address valid to data valid time t avdv
Address Valid to Data Valid Time (tAvdv)
  • If the fastest bus cycle is used then


  • If the bus cycle is extended then

tAVDV = 3 CLK - tDATA_SETUP + (nAW + nAH + nRDTA + nW) CLK

address wait states (nAW) – set by XMxPAR[14:12] (0-7)

address hold time (nAH) – set by XMxPAR[10] (0-1)

read turn-around time (nRDTA) – set by XMxPAR[9] (0-1)

wait states (nW) – set by XMxPAR[3:0] (0-15)

  • There is an upper bound on tAA
    • tAA < tAVDV
system timing compatibility
System Timing Compatibility
  • Need to account for all delays in a system to assess timing compatibility.
  • Consider this system.
  • Analyze the read timing with regard to:
    • tAA – address access time
    • tACS – chip enable to valid data
    • tOE – output enable to valid data
    • tDF – output hold/float time
  • Read cycle timing control (see handout)
  • Information also in Supplement 3
analyzing the address access time t aa
Analyzing the address access time (tAA)
  • Before a valid address is available at the SRAM, it must and go through the latch after ½ clock cycle . This gives

tAA < tAVDV - ½ CLK – tLATCH

  • Expanding tAVDV gives

tAA < 2.5 CLK - tDATA_SETUP - tLATCH + (nAW + nAH + nRDTA + nW) CLK

analyzing the chip enable access time t ce
Analyzing the chip enable access time (tCE)
  • The chip enable depends on /MSx, /BHE, /BHE and the address
  • The last signal to arrive at the decoder is the byte enables
  • The byte enables are asserted one cycle before the rising edge of /RS, which gives


  • If the read cycle is lengthened then


  • What happens for an 8-bit bus (no byte enables)?
analyzing the output enable access time t oe
Analyzing the output enable access time (tOE)
  • The SRAM /OE input is driven by the /RS signal
  • For the fastest bus cycle this gives


  • If the read cycle is lengthened then


analyzing the data float time t df
Analyzing the data float time (tDF)
  • How long is the data guaranteed to be valid? Will it meet the hold time?
  • The data may go invalid if the address becomes invalid, or \CS or \OE go high.
    • Which occurs first?
  • Based on the read cycle parameters, we must guarantee


  • To ensure the SRAM stops driving before the processor drives the next address, we need


aduc7026 external memory interface configuration
ADuC7026 External Memory Interface Configuration
  • The external memory interface supports four independently configured memory regions, each of which is 128kB in size.
  • In order to use the external memory interface, we need to
    • Configure the required pins (GPxCON)
    • Enable the external interface (XMCFG[0] = 1)
    • Configure for bus width and enable the region (XMxCON)
    • Configure for the desired bus timing (XMxPAR)
aduc7026 gpio ports
ADuC7026 GPIO Ports
  • The ADuC7026 has 40 pins organized as 5 ports that can be used as digital GPIO
    • All pins have multiple functions in addition being able to be used as GPIO
    • The configuration selection is set through the GPxCON MMR.
aduc7026 xmxcon
  • The XMxCON registers configure the bus width and enable the interface for the respective 128KB memory region.
    • XM0CON  0x10000000-0x1001FFFF
    • XM1CON  0x20000000-0x2001FFFF
    • XM2CON  0x30000000-0x3001FFFF
    • XM3CON  0x40000000-0x4001FFFF
aduc7026 xmxpar

Write cycle timing control

Read cycle timing control

  • The XMxPAR MMR configures the bus timing for a region
    • 0x70FF at reset
  • [14:12] – AE extend
  • [9] – implements bus turn-around
  • [8] – provides additional hold time
  • [7:4],[3:0] – extend write/read strobes
system timing compatibility1
System Timing Compatibility
  • Consider again the system.
  • Analyzing write cycle timing.
  • SRAM write characteristics
    • tWC
    • tAS, tAW, tCW
    • tWR
    • tWDS, tWDH
    • Write cycle controls
timing wrap up
Timing Wrap-Up
  • Device characteristics are just part of the total timing analysis picture
    • Line/device capacitive loading and driver slew rates
    • Transmission line effects and parasitic reactance
    • Impedance mismatch and reflections
    • Skew and physical/electrical trace length mismatch
    • Signal integrity
      • Ensuring that signals are correct in spite of all of the above issues and mutual coupling effects
wrapping up
Wrapping Up
  • Reading for next week 11 (interrupts and exceptions)
    • Textbook chapter 8
    • ADuC 74-75
    • ARM7 2.8-2.10
read cycle controls
Read Cycle Controls