Z8 LED Display Note

Z8 Multiplexed 12 digit LED display controller

The Z86E04, or Z86E08 micro-controller is used in this application to multiplex a 12 digit seven segment LED display. It can drive small calculator type displays or a large dual LED per segment display. If you happen to have an old LED calculator in the garage that doesn't work any more then pull out the display part and re-use it here.

There is a restriction to the design through: the circuit can only drive seven segment type displays that can only show numbers 0 to 9. It is not a dot matrix LED display which can show an entire character set. However, the letters A to F have been implemented allowing hexadecimal data display.

Using this schematic diagram shows a LED display taken from an old Texas Instruments calculator that had stopped working years ago. I don't know the part number to it but these things are all pretty much the same. Most other parts to the system can be acquired from the given chip manufacturers or their suppliers.

I would like to add a small note on the BP5020 device though: It is produced by the company ROHM around a hybrid technology. It is a DC to DC converter using a switched mode method to manage the voltage drop and regulation. The entire package requires very little PCB real estate while very few external components. It has proved to be a useful solution to PSU problems on small electrical designs.

Contents

Design Specifications
Software Functionality
Device Pin Allocation
Device Resource Allocation
Communications Protocol
Data Sheets
Downloads

Design specifications
Input voltage	7 to 38 volts DC, or 26 volts (max) AC
Input current	500mA (Max), 120mA (Typical not including LED display)
Input clock	5volt TTL levels @ 50KHz (max), 3.15MHz when using Z8's TIMER1 prescaler set to 63.
UART bit rate	2400bps
Receive packet length	Infinite, however 12 characters or over will be ignored.
Display digit count	12 digits maximum
Current per segment	40mA (max)
Current per digit	500mA (max)

The firmware has been written in assembler code to extract the most from the microprocessor. A lot of functionality has been included into the program with two main features, besides driving the LED displays:

1	Digital inputs where four input signals have been supported - RESET, LATCH, BLANK and CLOCK.
2	RS232C 2400bps packet oriented control. Here all data delivered to the display is done so through a UART where received data is written directly to the display buffer.

The firmware also supports some pretty neat tricks to get it to do all the tasks required and still keep up with time while maintaining a flicker free display. For example it can continue to keeping count of the arriving digital clocks arriving at input CLOCK while interpreting data received through its software UART.

On the prototype a 12 digit display is refreshed 125 times per second making it fast enough to limit the strobing effect when moved suddenly. Such a scan rate for a 12 digit display translates to each digit being lit for a period of 10.4 milliseconds.

To light each digit in that time means a 20-bit data stream has to be clocked into the 20-bit driver/latch before 10.4 milliseconds is up. It actually does this in less than 300 microseconds. Even so it does not leave a lot of time to do much else. But there are tricks that can be applied in the software to extract that extra time from a very busy little microprocessor.

SOFTWARE DESCRIPTION

The software has been written to exploit a 10MHz master clock driving the system. All delay counters and UART bit times have been calibrated to use this clock speed to its optimum (10MHz).

ABOUT THE DISPLAY DEVICE

The program will control a 20 bit latch/driver which in turn controls the LED segments and cathode digit selects. The software expects a "common cathode" type LED multiplexed display to be the display device. In order to display different data in each digit requires that the digits be multiplexed. This means one digit is given its segments to light while all others are turned off. Then a little bit later the next digit gets its chance to light its segments and so on until all 12 digits have been displayed. After that it is all repeated again and again.

To set the Decimal Point on a digit, or many digits, of the display can be achieved through setting up the DP place through the UART then switching the display mode back to counter/latch mode.

In counter latch mode "leading zero suppression" is active meaning that all zeros left of the current count will be blanked out. Its similar to using a calculator. The result "1245" shows as "1245" and not "00000001245". The later is how the actual count is representation in memory so the computer does a bit of filtering for our benefit.

CLOCK/COUNTER DIGIT INPUT

The program also accepts raw digital controls into the "LATCH", "CLOCK", "RESET" and "BLANK" inputs. These control the counters in CPU memory in a serial method. The clock input directly drives the internal counter to allow counts up to 500KHz to be captured. Anything faster than this will be lost and you may have to place a prescaler circuit in the clock circuit. There is a prescaler on Timer/counter #1 which is not used here and that can be set to a maximum of 64 counts, the clock frequency can then be increased to 500K*64=32MHz before clocks begin to be lost. However, there is a limit of 10MHz demanded by the z8's counters and silicon design.

SOFTWARE UART

To change all digits quicker can be achieved by delivering data through the software UART. A special control packet is set up to ensure correct data is received and displayed. Once data begins to roll in through the serial UART then data from "CLOCK" is not shown on the display, however, the received clocks are still counted and stored away.

The UART can instruct this software to revert back to the CLOCK/COUNTER mode with a single command within the packet ('+') or a reset pulse on the "RESET" input.

Using the UART to control the display is simple, it can set any digit with any display data you like. 14 bytes are all that is required to fill out the complete display. At 2400bps this would take 40mS to complete.

Z8 (86E04/8) pin allocations are:

P0.0	UART Transmit	output
P0.1	Yellow LED	output
P0.2	Green LED	output
P2.0	20-bit Latch SBLANK control	output
P2.1	20-bit Latch CLK control	output
P2.2	20-bit Latch STROBE control	output
P2.3	20-bit Latch DATA_IN control	input
P2.4	20-bit Latch DATA_OUT control	output
P2.5	RESET input (Digital counter/latch interface)	input
P2.6	Data latch/hold input. (Digital counter/latch interface)	input
P2.7	Digit blanking control. (Digital counter/latch interface)	input
P3.1	Clock input control. (Digital counter/latch interface)	input
P3.2	Shorting Link setting input. (Operational mode select)	input
P3.3	UART receive input	input

CPU FUNCTIONAL BLOCK RESOURCE ALLOCATION

Timer-0	Used for time base generation and set to generate an interrupt once every 104 micro-seconds.
Timer-1	Used to count incoming clock pulses.
P3.1 input	Used as standard input. No interrupts generate on edge transition.
P3.2 input	Used as standard input. No interrupts generate on edge transition.
P3.3 input	Used to generate interrupt on falling edge of Receive data (Start bit).

UART COMMUNICATION PROTOCOL and CONTROL CODES

Mnemonic	Hex	ASCII	Description	Effect on digit pointer
SOH	0x01	^@	Start Of Header (packet) flag. When this character is received, even in the middle of a previous packet, will reset the packet reception pointers and display pointers then start receiving a new packet.	Reset to digit 0
CR	0x0D	^M	End of packet flag. When this character is received all received data is ignored until another SOH character is received.	No effect
DGn	0x30-0x39	'0'-'9'	Digit data to be display. These include the numerals '0', '1', '2', '3', '4', '5', '6', '7', '8' & '9'.	Increment
DGn	0x41-0x46	'A'-'F'	Digit data to be display in the letters region. These include the letters 'A', 'b', C', 'd', 'E' & 'F'.	Increment
DP	0x2E	'.'	Turn on Decimal Point for this digit.	No effect
CM	0x2C	','	Turn on Comma (VFD) for this digit.	No effect
RS	0x2B	'+'	Restore to latch/counter mode.	Reset to digit 0
ND	0x2D	'-'	Turn off Decimal Point and Comma for this digit.	No effect
BLNK	0x20	' '	Turn off this digit. Nothing displayed here.	Increment
PING	0x70	'p'	Requests control returns an answer. In this case it will be an upper case 'P'.	No effect
PINGRSP	0x50	'P'	Response from controller on ping request.	No effect

Using the communications into the device with this protocol is simple. Every communication begins with a [SOH] character and ends by a [CR] character. There does not have to be anything in-between, or there can be a million characters in-between. However, to get something meaningful shown on the display requires there to be present in the middle some control codes as shown in this table.

Remember that at the moment a [SOH] character is received the operating mode of the controller will flip over to display mode. The digital counter will have no effect on displayed data, even though the counter is still running and registering clocks.

Another point to keep in mind is that the digit pointer is always reset to position zero (0). It is the right most digit on the display so feeding numbers into the display goes in backwards. i.e.: The rightmost digit (unit) is first into the controller.

DETERMINE CONTROL RESPONSE

This packet will force the controller to say something. It can be very useful in determining in the controller is plugged in and still working. Places to use this type of feature is in systems that "auto-detect" their hardware and peripheral list.

[SOH][PING][CR]

response from controller is

[PINGRSP]

CLEAR ALL DIGITS TO BLANK

This packet will remove all displayed data from the display, accept the decimal points and commas.

[SOH][BLNK][BLNK][BLNK][BLNK][BLNK][BLNK][BLNK][BLNK][BLNK][BLNK][BLNK][BLNK][CR]

LOAD DISPLAY WITH VARIABLE LENGTH DIGIT

This packet will display the number "3210" on the display. All digits are right justified.

[SOH]['0']['1']['2']['3'][CR]

PLACE DECIMAL POINT AT SECOND DIGIT FROM RIGHT

This packet is going to set the decimal point at the second digit from the right and the flick the operating mode over to counter/latch. The data displayed at any digit is of no concern since the mode switch will lock onto the current counter value in memory.

[SOH][['0']['0'][DP][RS][CR]

CLEARING DP and COMMA settings

If your displays has filled up with randomly placed decimal points or commas then it can be cleared away using a packet like this one.

[SOH][ND][ND][ND][ND][ND][ND][ND][ND][ND][ND][ND][ND][CR]

Data Sheets for chips used in this design:

Zilog Z86E04, or Z86E08	Z8 Micro-controller data sheet (1.2MB-pdf)
Allegro A6812AS	20-bit data latch-driver (151KB-pdf) Compatible to UCN5812
Allegro UCN5812FN	20-bit data latch-driver (151KB-pdf) Compatible to A6812
Allegro UCN5801	8-bit open collector high current driver. (118KB-pdf)
Analog Devices ADM232A	Dual RS232 Transmitter/Receiver pair uses single 5volt supply (333KB-pdf)
LED display	What ever comes with your old calculator
ROHM BP5020	DC-DC switch mode PSU

Downloads

Download schematic diagram (75KB PDF)	Download the schematic diagram showing the full details of the LED display.
Download z8 firmware source code (33KB zip)	Download the ZIP archive, extract the files, and go from there. All source, assembled listings and compiled Intel HEX formatted files are present.
Download the Z8 assembler and linker tools (720KB zip)	Download the ZIP archive, unpack it using the supplied directory names and the make file for the source code will work first run. (IBM-PC executable code). These tools can also be downloaded from the Zilog web site.

The source code can be compiled using Zilog's Z8 CCP Emulator (Order code: Z86CCP00ZEM) downloadable from www.zilog.com.

To program the Z86E04, or Z86E08 you will need to get a hold of a blank chip and a programmer. Zilog also provides these in at least one form: Z8 CCP Emulator (Order code: Z86CCP00ZEM)

If you need a Z86E08 chip supplied and programmed send me an Email.