1. Fri. Sept 29 Announcements
HW/Lab 6 posted yesterday
Lab 6 has been up (unchanged since the class began)
Quiz on Module 2 (A+B) Monday
Have video lectures almost done, will edit and post today

2. General-Purpose I/O Ports
Module 2.C
General-Purpose I/O Ports
Tim Rogers 2017

3. Learning Outcome #2 Bus Timing Analysis
“An ability to interface a microcontroller to various devices”
Bus Timing Analysis
9S12C Multiplexed Bus Expansion
General-Purpose I/O Ports
Interrupt Handling
Buffered I/O
Buffered, Interrupt-Driven Printer Design Example
How?

4. Complex interfaces are useful but…
Objective
“General Purpose I/O Ports”
Why? 1
Complex interfaces are useful but…
Sometimes, you just want to read/write 0’s and 1’s manually from software.

5. Overview Hardware Software How ports are initialize/used
Basic Port Structure
PIM Block user guide:
Software
Write a simple driver for a printer, driven by GPIO (General-Purpose I/O)

6. Hardware Overview
Ports on the 9S12 (some ports padded out to pins, some not)
By default – they are used for GPIO
Ports can also be used by other modules (peripherals) – we set configuration bits to make this happen.
Can only be configured for either GPIO or the module (for example port E from lab 5).
All this IO is digital I/O (meaning you can only read/write a 0 or a 1 on each bit).

7. Hardware Port Integration Module (PIM)
PT0 + PT1:
Connected to LEDs
PAD6 + PAD7:
Connected to pushbuttons
Port P:
Not padded out on your micro
Ports A and B:
Not padded out on your micro
For 362, use PAD0-7, PT0-7 and PE0 + PE1 for GPIO.
(as long as you aren’t using them for their other purpose)

8. Read or write the actual value from here.
How to setup GPIO
If DDRT (Port T’s DDR) is b.
Will the CPU be able to write the value 70h to PTT (Port T’s GPIO value register)?
A: Yes
B: No
C: Sort of…
D: What?
If DDRT (Port T’s DDR) is b.
Will the CPU be able to write the value 70h to PTT (Port T’s GPIO value register?
A: Yes
B: No
C: Sort of… 
D: What?
If DDRT (Port T’s DDR) is b.
Will the CPU be able to write to PTT (Port T’s GPIO value register)?
A: Yes
B: No
C: Sort of…
D: What?
If DDRT (Port T’s DDR) is b.
Will the CPU be able to write to PTT (Port T’s GPIO value register)?
A: Yes
B: No
C: Sort of…
D: What?
Need to tell it to be input or output
“Data Direction Register” (DDR) specifies this. There is one of these for each port
DDR Register (8-bits)
Port Value Register (8-bits)
DDR 7-0:
0 means port is an input
1 means port is an output
PT 7-0:
Read or write the actual value from here.

9. Also a “reduced drive” register
RDR Register (8-bits)
RDR 7-0:
0 means output “full drive”
1 means output “reduced drive”
Reduced drive decreases power consumption and helps cut down on electromagnetic interference.

10. Pull Enable Bit / Polarity Select
If the pin is an input, you can specify if the 9s12 pulls the pin up or down or lets it float.
PE Register (8-bits)
PS Register (8-bits)
PE 7-0:
0 means pin float if undriven
1 means pin will be pull – look at PS to decide up or down
PS 7-0:
0 means pin be pull-up device
1 means pin will be pull-down device

11. Block Diagram of Pin Functionality
port input data (read)
dual-rank input synchronizer
port output data (write)
external pin
data direction register
Note: If module/peripheral is enabled, it “takes over” the associated port pin(s)
module/peripheral associated with port pin

12. Port timing Port data setup time = 120 ns hold time = 0 ns
Port READ Timing
Port write delay time = 40 ns
Port Write Timing

13. Printer Interface Example
Algorithm: Handshaking signals and communication protocols
HW: Interface circuit and register initializations
SW: Program-driven I/O device driver
Analysis: Examination of timing for a printer

14. Handshaking and Communication Protocol
In general, the maximum transfer rate of which an I/O device is capable does not match the maximum transfer rate of which the CPU is capable
Communication (or synchronization) between an I/O device and CPU is therefore required for reliable data transfer, using handshaking signals
signal from computer to I/O Device: DATA AVAILABLE or STROBE
signal from I/O device to computer: BUSY or READY

15. Handshaking and Communication Protocol
The sequence in which these signals are asserted and negated with respect to the data transfer is referred to as the communication protocol
Example: Handshaking signals and communication protocol for a generic (parallel port) printer
STROBE: “I have the next ASCII character available for you to print”
BUSY: “I’m currently printing the previous character you sent me (or something ‘bad’ has happened), so please don’t send me any more right now”

16. Handshaking and Communication Protocol
Data
Printer
STROBE
BUSY

17. Handshaking and Communication Protocol
STROBE 1
BUSY
Previous Character
DATA
STEP 1: Check to see if the printer is busy

18. Handshaking and Communication Protocol
STROBE 1
BUSY 2
Previous Character
Next
Character
DATA
STEP 2: Send data to printer

19. Handshaking and Communication Protocol3
STROBE 1
BUSY 2
Previous Character
Next
Character
DATA
STEP 3: CPU asserts STROBE signal

20. Handshaking and Communication Protocol3 4
STROBE 1
BUSY 2
Previous Character
Next
Character
DATA
STEP 4: Printer asserts BUSY while printing

21. Handshaking and Communication Protocol3 4
STROBE 1 5
BUSY 2
Previous Character
Next
Character
DATA
STEP 5: Printer negates BUSY when print cycle is complete

22. Printer Port Timing

23. Printer-Microcontroller Interface
…where Port X and Port Y are any available port pins
Port X
(bits 0-7)
Printer
STROBE (Port Y, bit 1)
BUSY (Port Y, bit 0)

24. Register Initializations

25. Register Initializations

26. Register Initializations

27. Register Initializations

28. Register Initializations

29. Program-Driven I/O

30. Program-Driven I/O
read device status

31. Program-Driven I/O
read device status
is device ready?

32. Program-Driven I/O
read device status
is device ready? no

33. Program-Driven I/O read device status is device ready? no yes
transfer data

34. Program-Driven Device Driver
A device driver is a low-level routine that transfers data to/from an I/O device and generates any handshaking protocol required
The printer device driver pchar functions as follows:
checks printer status and waits if the printer is busy
transfers the character passed in the A register to the printer
strobes the printer

35. Printer Initialization Routine
bmask equ $ ; BUSY bit mask
smask equ $ ; STROBE mask
pinit

36. Printer Initialization Routine
bmask equ $ ; BUSY bit mask
smask equ $ ; STROBE mask
pinit movb #$FF,ddrx

37. Printer Initialization Routine
bmask equ $ ; BUSY bit mask
smask equ $ ; STROBE mask
pinit movb #$FF,ddrx
movb #$02,ddry

38. Printer Initialization Routine
bmask equ $ ; BUSY bit mask
smask equ $ ; STROBE mask
pinit movb #$FF,ddrx
movb #$02,ddry
bset porty,smask
rts

39. Assume character to print in (A)
Printer Device Driver
bmask equ $ ; BUSY bit mask
smask equ $ ; STROBE mask
pchar pshc
pwait
staa portx
pulc
rts
Assume character to print in (A)

40. Spin loop, waits until busy is set.
Printer Device Driver
bmask equ $ ; BUSY bit mask
smask equ $ ; STROBE mask
pchar pshc
pwait brset porty,bmask,pwait
staa portx
pulc
rts
Spin loop, waits until busy is set.

41. Printer Device Driver bmask equ $01 ; BUSY bit mask
smask equ $ ; STROBE mask
pchar pshc
pwait brset porty,bmask,pwait
staa portx
bclr porty,smask ; 4 cycles
pulc
rts
Sets Strobe Low

42. Sets strobe back to high
Printer Device Driver
bmask equ $ ; BUSY bit mask
smask equ $ ; STROBE mask
pchar pshc
pwait brset porty,bmask,pwait
staa portx
bclr porty,smask ; 4 cycles
bset porty,smask ; 4 cycles
pulc
rts
Sets strobe back to high

43. Printer Port Timing Analysis
Assume 8 MHz Bus Clock  125 ns/cycle
STROBE duration = 4 cycles X 125 ns/cycle = 500 ns
STROBE asserted 4 cycles X 125 ns/cycle = 500 ns following STAA