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ECE 353Introduction to Microprocessor Systems Michael G. Morrow, P.E. Week 7
Topics • Microprocessor support circuits • Clock and reset generation • Power control • Microprocessor supervisors • I/O subsystems • GPIO pin construction • I/O port design • I/O decoding • I/O synchronization • ADuC7026 GPIO
Clocks • Clock Generation • Clock oscillators • External clocks • Phase-locked loops (PLLs) • Operation and design issues • ADuC7026 clocking • PLLCON • Power Control • ADuC7026 operating modes • POWCON ADuC7026 pin-out
Reset • The reset signal is used to force the processor into a known state from which operation can reliably be started. • On power up, the reset signal should be asserted long enough to ensure that the supply voltages are stable and the oscillator is running and stable • Reset Generation • So, how do we generate a reliable reset signal? ADuC7026 pin-out
Reset Generation • RC reset circuit operation • Shortcomings ADuC7026 pin-out
Microprocessor Supervisors • Microprocessor supervisors provide reset functionality for a variety of circumstances • Power-up reset generation • Brown-out detection • Glitches on power supply(ies) • They can also provide a number of other services • MAX807 ADuC7026 pin-out
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
I/O Port Basics • I/O subsystems allow the CPU to interact with the outside world • Basic GPIO pin requirements • Configurable as input or output • Can set value driven out on the pin • Can read the current value on the pin • Configurable vs. multiplexed pins • Unconditional I/O • The I/O device can accept or return data without delay ADuC7026 pin-out
MSI I/O Ports • Medium Scale Integration (MSI) circuits are available to construct ports • Simple byte input ports can be constructed from… • Octal buffers • Octal registers with tri-state outputs • Simple byte output ports can be constructed from octal registers
P Compatible I/O Devices • Complex I/O devices typically require more sophisticated interface and control logic • P compatible I/O devices have the necessary logic built in to the device itself • Interface designed to be reasonably compatible with many microprocessor buses • Need to add decoding/selection logic • Example • Device controllers • An organizational model commonly used to interface to complex I/O devices (serial ports, LCDs, disk drives, etc.) • Generic model • Example – Hitachi HD44780U LCD Controller
I/O Address Decoding • I/O address decoding determines the logical location of the I/O device • Isolated I/O • Memory-mapped I/O • Input vs. output ports • Same address does not guarantee same function! • Exhaustive address decoding • Partial address decoding
I/O Address Decoding (cont.) • Linear selection decoding • A single address line is used as the selection criteria for each device • Can have n input/output devices in a system with an n-bit address bus • Hazards and opportunities • Note that this idea has a very limited application space!
Conditional I/O • Conditional vs. unconditional transfers • I/O synchronization • Hardware example • Polling • Overhead • Flags / semaphores • Wait loops • Timeouts • Software exercise
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 GPIO MMRs • GPxCON • Determine which of a pin’s functions are active • This is the configuration column selection on the previous slide aduc7026.inc
ADuC7026 GPIO MMRs (cont) • GPxPAR • PARameters • Controls whether or not the internal pull-ups are used. • Does not apply to ports 2 and 4
ADuC7026 GPIO MMRs (cont) • GPxDAT • Control the pin direction • Set the output state • Read the pin value • Read the pin values that were present at reset
ADuC7026 GPIO MMRs (cont) • GPxSET • Write 1s to set the output value • 0s have no effect
ADuC7026 GPIO MMRs (cont) • GPxCLR • Write 1s to clear the output value • 0s have no effect
Wrapping Up • Homework #4 will be due on Wednesday, March 21 • Quiz #2 will be held on Thursday, March 29 at 7:15pm in 2255EH • Reading for next week • Chapter 10 • ADUC 53-60, 71-73, 75-79
Conditional I/O Exercise Write a subroutine to read data from an input device like the hardware example. Assume that the flag is a READY signal (active high). If the device does not become ready after 1 trillion polling attempts, return with R0 = -1, otherwise, return with the data in R0.
Conditional I/O Example /MS0 base address = 0x1000 0000
aduc7026.inc ;GPIO GPIO_MMR_BASE EQU 0xFFFFF400 GP0CON EQU 0x00 GP1CON EQU 0x04 GP2CON EQU 0x08 GP3CON EQU 0x0C GP4CON EQU 0x10 GP0DAT EQU 0x20 GP0SET EQU 0x24 GP0CLR EQU 0x28 GP0PAR EQU 0x2C