In its simplest form, this multi-function module supplies the main system clock signal at a nominal 7.3728MHz as well as the power-on auto reset circuitry.
If the optional additional components are added the main system clock can be any one of 10 different frequencies as well as a second clock signal which can also be any of the 10 different frequencies.
This page is for the Dual Clock and Reset Module v2.3 only. For earlier v2.0 – v2.2 see Dual Clock Module
Click image to open pdf schematic
Assembly
Assembly takes the same approach as other RC2014 Modules. Start with the lowest components and build up to the highest. Note that the ICs, diode and electrolytic capacitors need their orientation to match the the silkscreen. When fitting the two rows of header pins for the clock speed selection, adding a couple of jumpers when soldering will ensure they are parallel and at the correct spacing.
Note that the DS1233-5 should not be fitted if there is already a DS1233-5 elsewhere in the system (eg Front Panel Module)
Unless you have specific requirements, the spare pads under the ‘393 chip can all be ignored – See Useful Hacks below. If a double row header is used, prepare it as shown in this guide.
If this is being built as a single speed clock module then omit all of the components outside the lined area on the left. As a twin multispeed clock module, all components will need to be fitted.
Hardware
The heart of this module is the 7.3728 MHz crystal, couple with a pair of 22pf load capacitors, a couple of resistors and two inverter gates on the 74HCT04. This produces a 7.3728MHz square wave. There is a bit of under/overshoot, so don’t expect a perfectly square signal if probing with an oscilloscope. It is, however, square enough to keep the Z80 ticking over at a constant speed and also drive the serial UART to run at an accurate baud rate.
The DS1233-5 Reset Monitor replaces the auto reset circuit on previous clock modules. This keeps the Z80 held in reset until the power levels have stabalised, thus ensuring there is a clean power-on-reset when initially powered up. The reset button on this module, or a backplane, can also initialise a reset.
The reset signal is bought down to pin 20 on the standard bus. The clock signal, however, is bought up to both of the 2×10 headers at the top. Setting a jumper on one or both of these will put 7.3728 MHz on to CLK and / or CLK2 on the bus.
7.3728 MHz divides down nicely on 68B50 UARTs or SIO/2 serial modules, and also 16C550 based modules to give 115,200 baud serial. However, if you need to communicate with some older devices which run at this speed, you will need to lower the clock speed to give a slower baud rate.
The 74HCT74 (with 74HCT02) are wired up as a divide by 3 circuit to divide 7.3728 MHz down to 2.4576 MHz. The 74HCT393 is wired up as two divide by 2, 4, 8 and 16 circuits. One fed by 7.3728 MHz, the other by 2.4576 MHz. This gives the following options available on the headers at the top;
- 2 = 3.6864Mhz = 57600bps
- 3 = 2.4576Mhz = 38400bps
- 4 = 1.8432MHz = 28800bps
- 6 = 1.2288Mhz = 19200bps
- 8 = 0.9216Mhz = 14400bps
- 12 = 0.6144Mhz = 9600bps
- 24 = 0.3072Mhz = 4800bps
A simple 2k2 / 22uf RC (Resistor Capacitor) circuit going through a couple of NOT gates gives approximately 3Hz clock. This slow clock isn’t useful for generating a serial baud rate, and the RC2014 will run unusably low. However, it can be interesting for looking at data/address bus activity or debugging bespoke hardware.
There are 3 pins for connecting to a SPDT (Single Pole / Double Throw) switch as a manual clock. Similarly to the Slow Clock, this isn’t useful for anything productive, but for investigation, you can control exactly what clock speed the Z80 sees. Note that every instruction on the Z80 takes multiple clocks to executes. See t-cycles in the Z80 reference.
Useful Hacks
Not every speed derivative is bought out to the headers at the top. 0.406 MHz and 0.1536 MHz are both available under the ‘393. These are unlikely to be useful speeds to anybody, but can be found there if needed.
Bill Of Materials
Single Speed
1 RC2014 DUAL CLOCK PCB
1 40 pin RA Header
1 14 pin narrow DIL socket
1 74HCT04
1 7.3728 Mhz Xtal
2 22pf ceramic cap
1 100nf
1 1M resitor
1 1k resistor
1 10K resistor
1 DS1233-5+
1 RA Tactile Switch
Multispeed
1 RC2014 DUAL CLOCK PCB
1 40×2 RA Header
5 14 pin narrow DIL socket
1 74HCT04
1 74HCT00
1 74HCT393
1 74HCT74
1 74HCT02
1 7.3728 Mhz Xtal
2 22pf ceramic cap
3 100nf
1 1M resitor
1 1k resistor
1 10K resistor
3 2K2 resistor
1 DS1233-5+
1 22uf 25v electrolytic
2 jumper
1 RA Tactile Switch
4 10 pin header
1 3 pin RA header