ECB-4PIO-I2C Development

Board

Pictures

I/O Addressing

I2C Addressing

Software & Drivers

References

Key Learnings

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:

Untested, boards ordered 28-Feb-2021

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.

Removed /WR qualification on IC15

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

4PIO-I2Cd

Component placement, routing changes.

Initial ROMWBW HBIOS support.

Add jumper to connect DS1307 battery backup to backplane battery backup.

Kicad files can be found here but may not be the most recent.

Current revision is ECB-4PIO-I2Cc

Gallery

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

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

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

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:

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.

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:

PCF8574 > LCD 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:

ROMWBW

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

Utilities

RTCHB - display date and time using HBIOS functions

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

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

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

I2CSCAN - I2C Bus Scanner

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: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

Sample test code: 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

6809 Interface: 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

Linux C driver: 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

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.