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Z8
Multiplexed 16 digit VFD |
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The Z86E04, or Z86E08 micro-controller is used in this application to multiplex a 16 digit fourteen segment Vacuum Fluorescent Display (VFD) display. The type chosen here is a Futaba 16-ST-26ZA (I think). Building the circuit as per this schematic diagram shows a 16 digit VFD controller. The Z8 is translate 2400bps UART data into SPI format and feed directly to the Micrel Display Controller. It also supports digital pulse counters which are immediately reflected on the display and a software driven UART which allow much faster and more flexible display data control. The VFD's filament, which must be of special concern, should be driven at the correct voltage, and current requirements stated by the manufacturer. In this situation this is achieved by a Zener based voltage regulator and current limiting series resistor. Also the filament drive method used is AC transformer coupled. The VFD segments (anodes) and digit selects (grid) are both controller by a Micrel display controller. VFD's require both grid and anode to be at a higher voltage than ground to allow segment illumination. This means that VFD's are not the same as LED's to control. LED's (common cathode types) require digit selects to be ground and segment positive via current limiting resistors. VFD's, on the other hand, require both: lighted segments, and digit selects to be positive with respect to ground. The negative part of the device is the filament (heater). There are two very good application notes on VFD control circuits that I have found so far. It is useful reading if you intend to understand the operation of these devices in detail: Contents
The firmware has been written in assembler code to extract the most from the microprocessor. A lot of work load is taken up by the Micrel display controller but some functionality has been included into the program with two main features, besides driving the VFD displays:
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 presented at input CLOCK while interpreting data received through its software UART. 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 Micrel display controller which in turn controls the VFD segments and digit selects. The software expects a VFD multiplexed display to be the display device. 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 50KHz 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 63 counts, the clock frequency can then be increased to 50K*64=3.15MHz 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. 18 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:
CPU FUNCTIONAL BLOCK RESOURCE ALLOCATION
UART COMMUNICATION PROTOCOL and CONTROL CODES
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 usefull 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:
The source code can be compiled using Zilog's Z8 CCP Emulator (Order code: Z86CCP00ZEM) downloadable from www.zilog.com. To program the 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)
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This page was last updated: Friday, February 07, 2003 14:44 Au EST.
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