boards:ecb:4pio-i2c:development [RetroBrew Computers Wiki]

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====== ECB-4PIO-I2C Development ======

The original board design included:

  * 4 Zilog PIO's.
  * 8 i/o ports on 4 headers.
  * Full Z80 interrupt mode 2 support.

Updated design to include the following changes:

  * Correction for inverted ground and power plane.
  * PCF8584 I2C controller.
  * Two I2C pin header connector.
  * Reset pin header connector.
  * On board socket for 24LC512 flash.
  * Onboard socket for DS1307 RTC with battery connection header.
  * Jumpers to connect PCF8584 and DS1307 outputs to PIO #3
  * Additional power and ground connectors to i/o port headers.

Status:

  * <del>Untested, boards ordered 28-Feb-2021</del>
  * Board received (I2Cb), construction and testing commenced.
      * There are some missing silkscreen details. Corrected on version c
      * IC7 inputs (pin 2-3, 13-14) are swapped due to Kicad symbols changes. Corrected on version c
      * IC16 pin 18 should be connected to VCC not GND for correct address decoding. Corrected on version c
      * Pullups R9, R10, R11 changed from 4K7 to 1K. Corrected in version C
      * Reset connector P8 is effectively connected to /M1 not RESET.. Corrected in version c
      * /RST on PCF8584 is connect to /M1 not /RESET. Corrected in version c
      * CE-PIO1 & CE-PIO2 were swapped. Corrected in version c
      * First I2C bus output seen 17/4/2021! … but doesn't do anything yet.
      * Added pin description on silkscreen.
      * Successfully read data off 24LC512.
      * <del>Removed /WR qualification on IC15</del>
      * Allow more space for crystal, shrouded port connectors.
      * Change top layer to vcc plane for better power distribution.
      * Gate /WR with /CS to meet PCF8584 Z80 configuration requirements.
      * Version C committed including above changes.
      * Patches to version B board
        * Lift pin 1 IC15, connect lifted pin to pin 19 IC13
        * Lift pin 18 IC16, connect lifted pin 18 to pin 20
        * Lift pin 5 IC 7, connect pin 6 to pin 15
        * On solder side connect pin 19 IC13 to pin 2 IC2
        * On component side, cut track between IC16 pin 17 and IC13 pin 19.
  * 4PIO-I2Cd
      * Component placement, routing changes.
      * Initial ROMWBW HBIOS support.
      * Add jumper to connect DS1307 battery backup to backplane battery backup.
      * Add support for 3.3v I2C peripherals using logic level translator.

====== Board ======

Kicad files can be found [[https://github.com/b1ackmai1er/ECB-4PIO-I2C|here ]]but may not be the most recent.

Current revision is ECB-4PIO-I2Cd

====== Pictures ======

[[:boards:ecb:4pio-i2c:gallery|Gallery]]

{{https://www.retrobrewcomputers.org/lib/plugins/ckgedit/fckeditor/userfiles/image/boards/ecb/4pio-i2c/gallery/ecb-4pio-i2cc.png?nolink&640x376}}

====== I/O Addressing ======

The Z80 PIO I/O address range is selected by pin header block J1 and will select a 16 port address range.

Address line A7 through A4 are configurable where a jumper will select a "0" and no jumper is a "1".

Example: Selecting address block A0h-AFh

|AB7|AB6|AB5|AB4|
|OFF|ON|ON|OFF|
|1|0|1|0|

The PCF8584 I/O address range is selected by pin header J2 an will select a 2 port address range.

Address lines A7 through A2 are configurable where a jumper will select a "1" and no jumper is a "0".

**Note**  this is the reverse notation to the PIO I/O addressing.

Example: Selecting address block F0h-F1h

|AB7|AB6|AB5|AB4|AB3|AB2|AB1|
|ON|ON|ON|ON|OFF|OFF|OFF|
|1|1|1|1|0|0|0|

====== I2C Addressing ======

24LC512 devices are access on the I2C bus by first issuing a control byte. The control byte consists of a device identifier (D), the device address (A) and a bit (O) to indicate if a read or write operation is intended.

So accessing a device requires sending a bye in the following sequence: DDDDAAAO

{{:boards:ecb:4pio-i2c:development:24lc512.pdf|AT25LC512}}

For 24LC512 the device identifier D is 1010, device address is 000-111 and O is set based on a read 1 or write 0.

The onboard 24LC512 has a default device address of 000, with an alternate address 011 selectable through solder jumper JP1.

The control byte for the onboard 24LC512 is:

^24LC512^DEFAULT^ALTERNATE|
^READ|10100001 (A1)|10100111 (A7)|
^WRITE|10100000 (A0)|10100110 (A6)|

**Note **that changing the 24LC512 to the alternate address using J1 requires the 1-2 solder link to be cut otherwise a short between +5V and ground will occur.

{{:boards:ecb:4pio-i2c:development:ds1307.pdf|DS1307}}

For the DS1307 the device identifier D is 1101, device address A is 000 - only one device is supported on the bus, and O is set based on a read 0 or write 1.

The control byte for the onboard DS1307 is:

^DS1307^DEFAULT|
^READ|11010000 (D0)|
^WRITE|11010001 (D1)|

{{:boards:ecb:4pio-i2c:development:pcf8574.pdf|PCF8574}}  > {{:boards:ecb:4pio-i2c:development:hd44780_lcd_driver_datasheet.pdf|LCD}}  {{https://www.retrobrewcomputers.org/lib/exe/fetch.php?tok=0bc8fd&media=https://www.retrobrewcomputers.org/lib/exe/fetch.php?tok=842ba7&media=https://www.retrobrewcomputers.org/lib/exe/fetch.php?tok=100ca2&media=https://www.retrobrewcomputers.org/lib/exe/fetch.php?tok=10f051&media=https://www.retrobrewcomputers.org/lib/exe/fetch.php?tok=5ed9e3&media=https://www.retrobrewcomputers.org/lib/exe/fetch.php?tok=f3f91e&media=https://www.retrobrewcomputers.org/lib/exe/fetch.php?tok=f46157&media=https://www.retrobrewcomputers.org/lib/exe/fetch.php?tok=b40609&media=https://www.retrobrewcomputers.org/lib/exe/fetch.php?media=boards:ecb:4pio-i2c:development:i2c-lcd-backpack.png?linkonly|backpack}}

A PCF8574 I2C 8-bit I/O expander is commonly used to driver LCD displays.

For PCF8574 devices, the identifier D is 0100, Up to 8 devices are supported. Backpack boards usually have 3 unbridged solder jumpers giving an address 111 - A. O is set based on a read 1 or write 0.

The control byte for the onboard DS1307 is:

^PCF8574^DEFAULT|
^READ|01001111 (4F)|
^WRITE|01001110 (4E)|

====== Software & Drivers ======

__ROMWBW __
<code>

RomWBW HBIOS v3.1.1-pre.75, 2021-05-12

SBC Z80 @ 12.000MHz
0 MEM W/S, 1 I/O W/S, INT MODE 2, SBC MMU
512KB ROM, 512KB RAM

UART0: IO=0x68 16550A MODE=38400,8,N,1
I2C: IO=0xF0
DS1307: 01/01/21 00:00:01
TMS: IO=0x98 NOT PRESENT
MD: FLASH=1 1=29F040 FLASH FILE SYSTEM DISABLED
MD: UNITS=2 ROMDISK=384KB RAMDISK=256KB
FD: IO=0x36 UNITS=2
RF: IO=0xA0 WP=OFF IO=0xA4 WP=OFF DEVICES=2
PPIDE: IO=0x60
PPIDE0: LBA BLOCKS=0x003DFC20 SIZE=1983MB
PPIDE1: NO MEDIA
PPIDE: IO=0x20 PPI NOT PRESENT
PPIDE: IO=0x44 PPI NOT PRESENT

</code>

__Utilities __

RTCHB - display date and time using HBIOS functions

<code>
H>rtchb
01/01/21 00:03:19

</code>

RTCDS7 - display date and time using direct access through PCF8584.

<code>
H>rtcds7
01/01/21 00:03:35

</code>

I2CSCAN - I2C Bus Scanner

<code>
H>i2cscan
I2C Bus Scanner

    00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
00: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- 27 -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: 50 51 -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- 68 -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

</code>

====== References ======

Sample test code: [[https://www.eevblog.com/forum/projects/z80lt-gtpcf8584-i2c-interface-problem/|https://www.eevblog.com/forum/projects/z80lt-gtpcf8584-i2c-interface-problem/]]

I2C driver code: [[https://github.com/ncb85/utilis-and-examples/tree/master/cpm_i2c|https://github.com/ncb85/utilis-and-examples/tree/master/cpm_i2c]]

6809 Interface: [[https://www.aslak.net/index.php/2013/07/18/i2c-on-a-6809-computer-and-a-mini-review-of-a-logic-analyser/|https://www.aslak.net/index.php/2013/07/18/i2c-on-a-6809-computer-and-a-mini-review-of-a-logic-analyser/]]

Understanding the I2C Bus - Texas Instruments: [[https://www.ti.com/lit/an/slva704/slva704.pdf|https://www.ti.com/lit/an/slva704/slva704.pdf]]

Linux C driver: [[https://code.woboq.org/linux/linux/drivers/i2c/algos/i2c-algo-pcf.c.html|https://code.woboq.org/linux/linux/drivers/i2c/algos/i2c-algo-pcf.c.html]]

I2CScan code: [[https://groups.google.com/group/retro-comp/attach/25120f43d0386/I2CSPI.ZIP?part=0.1|https://groups.google.com/group/retro-comp/attach/25120f43d0386/I2CSPI.ZIP?part=0.1]]

====== Key Learnings ======

  * Black solder mask PCBs are terrible for development debugging. Very hard to see traces.
  * Size of pullup resistors is important on open collector outputs. There is a huge difference in rise time between 1K and 4K7 pullups.
  * Make provisions for I2C device and bus timeouts to avoid lockups.
  * Do a hard reset between each software test to ensure consistent results. Exiting in a known state is important.
  * Software control of hard reset is desirable as soft reset does guarantee consistent results.