LCD connections, TINI bus loading - some numbers

[Bruce Boyes]

At 16:17 11/23/2000 +0000, you wrote:
>I've posted more recent versions of the component list, netlist,
>netlist converter program and PCB layout on the website.
>
>I believe the netlist is now complete, and supports the following
>features (not all of which have to be installed on the board, of
>course):
>
> general TINI support
> MP3 playback using an STA013
> any size FIFO up to 16k or so (I've tested up to 4k)
> two independent LCD or LCD-compatible modules
> two extra serial ports, one at RS232 levels
> infra-red reception and decoding using a PIC16F84-04 (uses one of the
>   extra serial ports)
> eight LEDs
> eight pushbuttons
>
>There are a couple of things I know are missing: the LCD contrast
>controls, and connections for the LCD backlights. These don't matter
>to me (I'm using a vacuum fluorescent display that just requires 5v),
>but probably matter to other people. Can anybody suggest a simple way
>to support them? (The contrast control is reasonably easy - it's the
>backlight that I've never had to drive before. There are two pins, 15
>and 16, reserved at the end of the LCD connector for the backlight,
>but I don't know what signals to provide.)

We use Vcc tied to LCD pin 15, then pin 16 pulled low through a 10 ohm
resistor and a FET driver. You can provide a jumper to bypass the 10 ohms,
since some LCDs have their own built in while others do not. If you provide
parallel pulldowns (we use the TPIC6B273 for this, 8 x FETs each sinking at
least 150 mA) then you can have a digital backlight brightness control.
more important for battery operation than this player.

For contrast, some LCDs need a negative voltage. If you avoid those for
simplicity, use a digital pot such as the DS1804. You need an opamp if you
will drive negative, plus a negative supply which could be a tap off the
charge pump of an RS232 driver, but be careful of the load.

>There's another issue I'd like to ask advice on. At the moment I am
>placing four loads on the TINI's databus: the FIFO, the FPGA, a bus
>transceiver to isolate the databus from the LCDs, and the serial
>controller. I have the option of hiding some of these loads behind the
>bus transceiver - potentially all of them except the transceiver
>itself. I'm thinking of putting the serial controller, and possibly
>the FPGA, on the 'LCD' side of the transceiver. Any thoughts?

Why not use the FPGA as a buffer? We just did that with our SBX2 board - a
XC95108 CPLD is the glue and control logic, plus the bus buffer. It
presents only one CMOS load to TINI, then. It also has an LCD interface
which does the slower LCD timing. It also includes a keypad debouncer -
maybe your FPGA already includes this.

I'd also recommend a reset circuit such as a TLC77C05 (I forget the exact
number), to put TINI and all the parts of the system in reset below 4.75V
and let them all wake up together. You don't want part of the system
running before the others are ready.

If you believe the data sheets for the C390, four CMOS loads is about the
maximum you want to place on TINI. The total capacitance matters,
bidirectional loads (memory chips, UARTS, etc) are a higher load than
unidirectional loads such as output registers.

Here's my TINI analysis:

Data bus 100 pF max, addres bus 80 pF max according to C390 data sheet:
TINI alone has 42/60 typ/max pF on data lines, 28/40 typ/max on address
1 MB TINI adds another load to address and data (10 pF max on data lines, 7
on addr)

This does not include trace capacitance. So a worst-case component TINI 1
MB now has 47pF on address (out of 80 allowed) and 70 pF on data (out of
100 allowed). PCBS add at least 10-12 pF per foot of trace as a rough
estimate, so you can see that there is not a lot of extra drive ability on
TINI. Add in 10 pF on each for board traces (TINI itself plus a socket
board) and you have 20pF available on address and 20 pF on data, 2 CMOS
loads to be conservative.

- Bruce Boyes

-----------------------------------------
           WWW.SYSTRONIX.COM
 Fast 8051s, embedded Java and much more
 new! 8x1-Wire I/O board for 1-Wire nets
  +1-801-534-1017  Salt Lake City, USA
-----------------------------------------