The Key Port
It’s called low-level because we interface with the keypad hardware itself instead of going through an API (that’s a pretty glamourous description of
GetKey) that does it for us. That’s why it’ll be complicated, but faaaaaaaaaaast.
OUT (n), A
- Sends a byte to port
nvia the accumulator.
OUT (C), reg
- Sends a byte to port
IN A, (n)
- Receives a byte from port
nvia the accumulator. Does not affect flags.
IN reg, (C)
Receives a byte from port
- detects parity
A port is a device that lets the CPU transfer bytes between other pieces of otherwise unconnected hardware. What’s that? You want an analogy? Okay, imagine you wanted to get a car from Yokohama to San Francisco; you couldn’t just drive it over because they’re separated by thousands of miles of open ocean. Instead, you’d take the car to the port of Yokohama, have a boat take it to the port in San Francisco, and drive off. Similarly, the CPU and the keypad have no real connection, so the port is used to interact.
The keyport on the TI-83 Plus is port #1, so we replace
n in the two instructions with 1.
The first thing to do is enable the key group we want to read from. This is done by sending the value of that group to the key port. We then read from the key port and check the value returned. You can get the values from this table.
|Group \ Key Code||FE||FD||FB||F7||EF||DF||BF||7F|
|EF||0||1||4||7||,||SIN||APPS||X, T, θ, n|
Assemble and run this program. When the program is requesting input, enter x or ->, because I said so.
ReadKey: LD A, %11111101 ; Check for [-] OUT (1), A IN A, (1) CP %11111011 JP Z, Minus LD A, %11111110 ; Check for [up] OUT (1), A IN A, (1) CP %11110111 JP Z, Up JP ReadKey Minus: LD HL, zs_minus b_call(_PutS) RET Up: LD HL, zs_up b_call(_PutS) RET zs_up: .DB "You pressed UP !", 0 zs_minus: .DB "You pressed - !", 0
Well that certainly was unexpected. You see, compared to a running program, hardware takes a very long time to react to inputs. So long in fact, that a program can easily execute several instructions before a port is ready. In the case of this program, the key port was read before it could set the correct group. In the case where x is pressed,
- Group $FD was set, which is the same group for down and
- The key port was read while it was reacting to the group switch and so garbage was read.
- Group $FE was set.
- The key port was read before it could switch from key group $FD, so the x key was stored in A.
- A was compared with the key code for up, which just so happens to also be the key code for x, and the rest is history.
To fix this, add a delay of two NOPs between setting the key group and reading the port. NOP is a do-nothing instruction that just waits for four clock cylces. It’s so pointless it doesn’t even get its own box.
You could use either CP or BIT to check the key code. To see the difference, run Program 22-2. Press and hold ALPHA, and hit LOG… nothing happens. Replace the
CP %11110111 with
BIT 3, A and reassemble. Do the same thing when running this program…
Pause until LOG is pressed.
Loop: LD A, %11011111 ; Enable group OUT (1), A IN A, (1) ; Input a key CP %11110111 ; Check if it's [LOG] RET Z ; End if so JP Loop
The reason for this behaviour lies in the way the key port reacts when multiple keys are pressed. If both key1 and key2 are pressed at the same time, the key port sends the keycode key1 AND key2.
In Program 22-2 then, the key port was sending the bitwise AND of LOG and ALPHA which is
%01110111. CP didn’t work because it was looking for the exact value
%11110111. BIT, on the other hand, will work because bit 3 is still zero.
If you want the calcultor to do something when a key is pressed, regardless of whether any other keys in the group are pressed, you should use BIT (or a shift instruction if possible). However, if you wanted a different action to be taken when two or more keys are pressed down, then you’d have to either use CP, or have a kind of a BIT chain.
Demonstration of multiple keypresses.
b_call(_RunIndicOff) LD HL, $1C23 LD (x_pos), HL DispText: b_call(_ClrLCDFull) LD HL, (x_pos) LD (PenCol), HL LD HL, string b_call(_VPutS) LD C, 1 InKey: LD A, %10111111 ; Check for [DEL] to exit OUT (C), A IN A, (C) BIT 7, A JR NZ, InArrow LD A, $FF ; Reset key port OUT (C), A RET InArrow: LD A, $FF ; Reset key port OUT (C), A LD A, %11111110 OUT (C), A IN B, (C) BIT 0, B JP Z, Down BIT 1, B JP Z, Left BIT 2, B JP Z, Right BIT 3, B JP Z, Up JP InKey Down: CALL MoveDown BIT 1, B CALL Z, MoveLeft BIT 2, B CALL Z, MoveRight JP DispText Left: ;There is no need to check for Down key anymore. CALL MoveLeft BIT 3, B CALL Z, MoveUp JP DispText Right: CALL MoveRight BIT 3, B CALL Z, MoveUp JP DispText Up: CALL MoveUp JP DispText MoveDown: LD A, (y_pos) ; Check if at bottom edge of screen CP 57 RET Z INC A ; Down one pixel LD (y_pos), A RET MoveUp: LD A, (y_pos) ; Check if at top edge of screen OR A RET Z DEC A ; Up one pixel LD (y_pos), A RET MoveLeft: LD A, (x_pos) ; Check if at left edge of screen OR A RET Z DEC A ; Left one pixel LD (x_pos), A RET MoveRight: LD A, (x_pos) ; Check if at right edge of screen CP 96-28 ; 96 - number of pixels the string takes up RET Z INC A ; Right one pixel LD (x_pos), A RET x_pos: .DB 0 y_pos: .DB 0 string: .DB "Let\'s Go!", 0