1. Parallel I/O

2. Introduction This section focuses on performing parallel input and output operations on the 68HC11 3 operation types – Simple, blind data transfers – Strobed transfers – Transfers with handshaking Contd

3. SEGMENT code Look at examples of each transfer operation Reading: – Text, Chap 9: » Read: 9.1, 9.4-9.8.1 » Scan: 9.2, 9.3 – E9: Section 6

4. Recall the block diagram of the 68HC11

5. Simple I/O operations These I/O operations take place under the direction of the processor Operations are performed without regard to the status of the I/O port – the port is always assumed to be "ready“ contd

6. Since the current status of the port is not known, it is possible to – Read the same value in more than once on input operations – Overwrite the current value in an output port that is waiting to be transferred "Good" examples of use: – Input initialization values from a port -- port is in known state as a result of system reset – I/O when overwrites or multiple reads don't matter -- writing to the hex display ’ s port in the labs, for example

7. Simple I/O example -- keypad interface – Consider the simple 12-key keypad shown below

8. High-level procedure to detect key closures » Drive Row 1 input low » Read Columns -- any =0? » Repeat this process for all remaining rows until “see” a column=0 » If no column=0, then no key has been depressed CONTD

9. Knowing which row and column were low permits you to calculate the key number that was pressed » Can use a simple equation (similar to the approach of Listing 9.5) – Key# = col# + (3*row#) -3 – Assumes row and col numbers start with 1 – Listing 9.5 is for 2-of-7 keyboard! » Or use a lookup table (similar to the approach in Listing 9.10) – Assumes row and col use same port (port C

10. – Key debounce » Need to avoid the multiple make/break closures associated with key bounce » “Simple” approach is ID the key closed and then wait for a period of time to see if the key is still closed (therefore not bouncing) » Then and only then, return the key value » The waiting period will vary from switch to switch -- 10s to 100s of ms CONTD

11. – How will the system react if you hold a key down for a long time? » Should an action (in response to a key closure) be repeated over and over despite there being only 1 switch closure “action” » Should the action take place only once and then wait for the key “break” before recognizing another closure? » Consider routines to recognize key makes and breaks

12. Strobed I/O This method of performing I/O operations uses a control line, the strobe, to notify the receiving unit of the availability of data at its (input) port CONTD

13. Strobed I/O Contd… The process: – Device performing the write places data onto data bus (its output port) – Strobe signal is asserted (for 2 cycles in 68HC11)

14. – Strobe signal causes an I/O interrupt to occur or a flag to be set -- in either case the receiving device is signaled that new data has arrived – It is up to the receiving device to read the new data at its input port in a "timely" fashion

15. Problem – If a second data item arrives at the input port and is strobed in (latched) before the input device has read the first item, the first item will be overwritten and lost

16. 68HC11 register support for strobed I/O – DDRC -- data direction register for port C – PORTCL -- port C input latch -- data is latched on STRA edge – PORTC -- input pins for port C -- not latched – PORTB -- latched output data port B -- outputs data using STRB – PIOC -- parallel I/O control register – PORTD -- bits 6 and 7 are STRA and STRB

18. Strobed input operations using port C – Data is placed at the input pins of port C – STRA is asserted by peripheral device, causing » Data to be latched into PORTCL » STAF flag to be asserted » Interrupt initiated, if interrupts are enabled – Data is read into to processor from PORTCL – To clear STAF, read PIOC first then PORTCL

19. Strobed Output – Peripheral device is connected to Port B – When the MCU writes to Port B,... » Data is placed on Port B pins » STRB is asserted for 2 clock cycles – Peripheral device should use STRB to latch the data – STRB can be configured as active-high or active-low

20. Handshaking I/O  In this mode, the sending and receiving devices exchange positive sent/received signals to one another – Insures that each transmitted word is received before the next word is transmitted

21. SHR Transfers can occur in either the pulsed or the interlocked method – Pulsed input operations » Peripheral pulses STRA to indicate that data is present » When 68HC11 reads the data (from PORTCL), it automatically generates an acknowledgment strobe on STRB for 2 Cycles CONTD

22. SAR » This mode is selected by initializing the PLS bit (bit 2 of the PIOC) to 1 – Interlocked input operations » Here, STRB acts as a READY signal » Asserted = 68HC11 ready to receive data » Negated = 68HC11 is not ready -- do not send data now

24. Output operations with handshaking – Port C is used for output handshake operations, along with STRB and STRA – STRB is the output "data available" strobe – STRA is the acknowledgment / input ready strobe line – PIOC bit 3, OIN, set to 1 for output operations

25. – Pulsed operations » 68HC11 writes data to PORTCL and automatically asserts STRB for 2 cycles » Peripheral device reads data upon receipt of the STRB strobe » Peripheral asserts its READY line (68HC11's input STRA line) to signal receipt of data » PIOC bit 2, PLS set to 1 for pulsed mode – Interlocked mode » Upon writing data to PORTCL, STRB is asserted » STRB negated only upon ACK

27. Handshaking example: Centronics parallel printer interface – The "Centronics" definition of a printer port interface has become the standard parallel printer interface

28. – Using the 68HC11 as the output device (to the printer) requires a software interface (note the errors in the text!)

31. ; I/O service routines to control "Centronics" interface ; part of printer control system. Also demonstrates ; software handshaking because port C configured ; for simple input strobe. Main program would call ; service routines to store input data in a RAM print ; buffer for later output to print mechanism. CONTD

32. ;;; CONNECTIONS ; Computer Printer Port MCU as Printer Controller ; DATA -----------------> Port C ; STB -----------------> STRA ; ACK <----------------- PD2 ; BUSY <----------------- PD CONTD

33. PIOC CONFIGURATION DETAILS ; INVB=x STRB not used ; EGA=0 STRA/ACK active on falling edge ; PLS=x Pulsed/Interlocked not used ; OIN=x Output/input handshake not used ; HNDS=0 Simple strobe mode ; CWOM=0 Normal CMOS outputs ; STAI=0 Disable interrupt ; STAF=x Sets on falling STRA line ; To clear STAF, read PIOC, then read PORTCL

34. ORG $100 ; Subroutine INIT_INTRF ; Initializes parallel interface part ; of printer upon power-up reset. ; Calling Registers: ; IX = Address of register block ; No Return Registers except CCR affected CONTD

35. INIT_INTRF: psha ; preserve registers ldaa PIOC,X ; clear STAF if set ldaa PORTCL,X bset PORTD,X $0C ; PD2, 3 output and bset DDRD,X $0C ; BUSY, ACK high ldaa #00 ; configure PIOC staa PIOC,X pula ; restore registers rts ; and return

36. ; Subroutine INPUT ; Reads parallel port to get byte sent ; by an external device ; Calling registers ; IX = Address of register block ; Return registers ; ACCA = input data byte ; CCR affected

37. INPUT: bclr PORTD,X $0C ; BUSY low and pulse ACK nop ; for approx 5 us (E=2MHz) bset PORTD,X $04 ; set ACK high again CIN: brclr PIOC,X $80 CIN ; wait for STB pulse ldaa PORTCL,X ; get input and clear STAF bset PORTD,X $08 ; set BUSY high rts ; return

38. Parallel subsystem summary Ports B and C are available for I/O only in the single chip mode -- can be replaced by the PRU when in expanded mode Port A – 3 input, 3 output, 2 bi-directional pins – Bits DDRA7 and DDRA3 in PACTL set direction for A7 and A3

39. Port C – Data register is PORTC – Latched register is PORTCL – Each bit is bi-directional » Direction set using register DDRC Port D – 6 bi-directional pins, directions set by DDRD – Pin 6 = STRA, Pin 7 = STRB » These pins become AS and R/W* in expanded multiplexed mode

40. Port E – 8-bit input only Conditions on reset – All data direction bits set to 0 (input) – Output port bits set to 0 – Input port bits high impedance

41. Strobed I/O – Strobed output via Port B, strobed input via Port C – Detected edge on STRA causes input data to be latched in PORTCL and flag set (and interrupt, if enabled) – Writing data to Port B also pulses STRB Handshake I/O – Port C used for either input or output operation – Input: read data from PORTCL – Output: write data to PORTCL