Sunday, 23 October 2011

29. Manual Up/Down Counter

I modified the circuit in the previous post by removing the 555 timer clock input and adding two switches so I could manually increment or decrement the value. These sent a signal to a 556 Timer chip (dual 555s) setup in monostable fashion to send a distinct pulse to the "clock-up/down" signals of the 74193. I also wired the "borrow" output of the first 74193 to the "clock-down" input of the second 74193 so it could count backwards. As inactive inputs to the clock up/down should be high when not in use I sent the 556 outputs via a 7404 hex inverter.

On the monostable circuits I used 8.2K resistors and a 10μF capacitors. I tried a 47μF capacitors but the pulse length was a little too long. With the 10μF capacitors I am getting the occasional double clock pulse, so it would sometimes skip 2 counts. I hooked up my oscilloscope and it looks like I have some noise on the output pulse when triggered. I'm not sure what's causing this so any ideas would be much appreciated.

Update: the circuit noise I have read is due to the switch contacts bouncing and to fix this I need a debouncing circuit.

Thursday, 20 October 2011

28. Here are my 2 digits

Giving my PIC16F505s programmed as Hex-to-7-segment-display-drivers a work out by making a Hex Counter. I wanted to have 4 digits but the breadboard wasn't big enough. I might work on this later

For now, check out my little video:

27. Traffic Lights with PIC12F508s

I thought I'd have some fun with the PICs, so I made this 3 way traffic light system.

1. +5V
2. Transmit
3. Receive
4. Master/Slave
5. Green
6. Amber
7. Red
8. Ground

PIC code can be found here.

Friday, 7 October 2011

26. No Logic In using PICs

I considered using PIC chips for memory logic, so they could select whether an address was for RAM or ROM. Programmatically this is not a problem, the code can be written to do this quite easily. The problem is that they cant do it fast enough.

Most Z80 instructions work over 4 clock cycles, T1, T2, T3 and T4. During T1, the address pins A0-A15 are set to the value of PC (Program Counter) and half a cycle later, MREQ (Memory Request) and RD (Read) pins go active. By the end of T2 the data is fetched from the data bus on D0-D7.

If the Z80 is running at 1MHz then each cycle is 1μs (1 micro second), so fetching an instruction takes at most 2μs.

Now consider a PIC microcontroller running at 4MHz. Again each instruction takes 4 cycles, so the instruction to read the pins on a PIC such as MOVF GPIO,W will take 1μs (4Mhz / 4 cycles), so at most we could only perform 2 instructions on the PIC while the Z80 is trying to fetch the next instruction or operand on the data bus. A branch instruction on the PIC takes 8 cycles, so we could not even fit a loop into the time frame.

The only it could work with a PIC is if the Z80 was running at a much slower clock rate. However this Z80 can go at 2.5MHz and the Z80B can go at 4MHz, and it'd be a shame not to run at full speed.

You can clock some of the PICs with an external crystal to 20MHz and the more expensive PICS to 60MHz and this would allow 5 or 15 instructions in 1μs respectively, but probably still not fast enough at higher Z80 speeds.

Time to bring out the classic TTL chips.

Saturday, 1 October 2011

25. Need a clock? Take your PIC!

Having discovered PIC microcontrollers, I think I will use them quite a lot on this project. One use is as a clock. The smaller PICs have 4MHZ internal oscillators and the larger ones I think are 8MHz. One of the pins can be configured to be a CLKOUTand will send a signal 1/4 of the clock speed. This is effectively 1 pulse for every instruction. Using the internal oscillator of 4MHz this should give a 1MHz clock signal.

I'm using one my PIC16F505s for this, although a PIC12F508 would be (a) cheaper and (b) has less pins [update: and it doesn't externalize its clock]. We only need to connect up +5V and ground, plus the clock pin. On the PIC16F505 its pin 3 (RB4/OSC2/CLKOUT).

The code would look like this:
include "" ;
        __config _IntRC_OSC_CLKOUTEN
        org     000
loop    nop
        goto    loop

I tried it on the Z80. I think it works. I measured it on my small oscilloscope but I don't think its fast enough to keep up with the signal change. Instead I measured the signal on the A15 pin. I did get a signal but there was a lot of noise when the pin went low. Its 4.705Hz, so multiply this by 65536 (the Z80 is executing NOP and the PC will loop every 65536 instructions) and then by 4 (number of clock signals to execute NOP) should give the clock speed. It gives 1,233,387.52, just slightly over 1MHz. Peak voltage is 2.040V, low is 0.480V. At this resolution I am not sure these voltages are accurate. I checked the MREQ pin and this looks like its working but the voltage delta is 800mV. Is this enough to give a true high/low difference? I'm not sure at this stage.

There was a wierd effect after a few mins where the higher address pins such as A15 seemed to be static rather than blinking. I put a decoupling capacitor (don't ask me why I just guessed), on the PIC chip and that seems to have solved the issue. Its been running for over an hour now and seems to be ok.

Time to see what else these babies can do...