apple2pi/pipico/bus.pio

///////////////////////////////////////////////////////////////////////////////

/*

MIT License

Copyright (c) 2022 Oliver Schmidt (https://a2retro.de/)

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

*/

///////////////////////////////////////////////////////////////////////////////

.define public gpio_addr    2   // 12 pins
.define public gpio_rw      14
.define public gpio_data    15  //  8 pins
.define public gpio_led     25
.define public gpio_enbl    26  // DEVSEL | IOSEL | IOSTRB
.define public gpio_irq     27
.define public gpio_rdy     28

.define public size_addr    13  // incl. R/W
.define public size_data    8

.define public sm_enbl      0
.define public sm_read      1   // from Apple II perspective
.define public sm_write     2   // from Apple II perspective

.define irq_write           4

/*

Implementation of the Apple II peripheral bus protocol:

- /DEVSEL, /IOSEL and /IOSTRB are supposed to combined to ENBL via an AND gate.

- On the falling edge of ENBL, the lines A0-A11 and R/W are sampled and pushed
  into the 'enable' state machine RX FIFO.

- In case of an Apple II write cycle, the lines D0-D7 are sampled ~300ns later
  and pushed into the 'write' state machine RX FIFO.

- If a byte is pushed into the 'read' state machine TX FIFO, it is driven out
  to the lines D0-D7 until the rising edge of ENBL.

*/

///////////////////////////////////////////////////////////////////////////////

.program enbl

.wrap_target

idle:
    wait 1 gpio gpio_enbl       // wait for ENBL to rise
    wait 0 gpio gpio_enbl       // wait for ENBL to fall

    in pins, size_addr          // shift A0-A11 + R/W into ISR

    jmp pin idle                // jump to idle if R/W is high

    irq irq_write               // Set 'write' IRQ

.wrap

% c-sdk {
static inline void enbl_program_init(uint offset) {
    pio_sm_config c = enbl_program_get_default_config(offset);

    // in_base:         gpio_addr
    sm_config_set_in_pins(&c, gpio_addr);

    // shift_right:     false
    // autopush:        true
    // push_threshold:  size_addr
    sm_config_set_in_shift(&c, false, true, size_addr);

    // pin:             gpio_rw
    sm_config_set_jmp_pin(&c, gpio_rw);

    // state_machine:   sm_enbl
    // initial_pc:      offset
    pio_sm_init(pio0, sm_enbl, offset, &c);

    // state_machine:   sm_enbl
    pio_sm_set_enabled(pio0, sm_enbl, true);
}
%}

///////////////////////////////////////////////////////////////////////////////

.program write

.wrap_target

    wait 1 irq irq_write [31]   // wait for 'write' IRQ to be set and clear it
                                //   [31 cycles to allow 6502 to set up D0-D7]

    in pins, size_data          // shift D0-D7 into ISR

.wrap

% c-sdk {
static inline void write_program_init(uint offset) {
    pio_sm_config c = write_program_get_default_config(offset);

    // in_base:         gpio_data
    sm_config_set_in_pins(&c, gpio_data);

    // shift_right:     false
    // autopush:        true
    // push_threshold:  size_data
    sm_config_set_in_shift(&c, false, true, size_data);

    // state_machine:   sm_write
    // initial_pc:      offset
    pio_sm_init(pio0, sm_write, offset, &c);

    // state_machine:   sm_write
    pio_sm_set_enabled(pio0, sm_write, true);
}
%}

///////////////////////////////////////////////////////////////////////////////

.program read

/*

Both set and set-side are limited to 5 pins each. So both set and side-set are
configured to set (the direction of) 4 pins. This approach allows to set the
direction of D0-D7 in one operation.

Note: The naive approach would have been
    mov osr, ~x                 // move 0xFFFFFFFF to OSR
    out pindirs, size_data      // enable output of D0-D7
    [...]
    mov osr, x                  // move 0x00000000 to OSR
    out pindirs, size_data      // disable output of D0-D7
but this would have required two operations and destroyed OSR.

*/

.side_set (size_data / 2) opt pindirs

.wrap_target

    pull block                  // pull data into OSR, block on empty FIFO

    out pins, size_data         // shift OSR out to D0-D7

    set pindirs, 15 side 15     // enable output of D0-D7

    wait 1 gpio gpio_enbl       // wait for ENBL to rise

    set pindirs, 0 side 0       // disable output of D0-D7

.wrap

% c-sdk {
static inline void read_program_init(uint offset) {
    pio_sm_config c = read_program_get_default_config(offset);

    // out_base:        gpio_data
    // out_count:       size_data
    sm_config_set_out_pins(&c, gpio_data, size_data);

    // shift_right:     true
    // autopull:        false
    // pull_threshold:  size_data
    sm_config_set_out_shift(&c, true, false, size_data);

    // set_base:        gpio_data
    // set_count:       size_data / 2
    sm_config_set_set_pins(&c, gpio_data, size_data / 2);

    // sideset_base:    gpio_data + size_data / 2
    sm_config_set_sideset_pins(&c, gpio_data + size_data / 2);

    // state_machine:   sm_read
    // initial_pc:      offset
    pio_sm_init(pio0, sm_read, offset, &c);

    // state_machine:   sm_read
    pio_sm_set_enabled(pio0, sm_read, true);
}
%}

///////////////////////////////////////////////////////////////////////////////