06-02-2016, 06:44 PM
Decided to change direction again.
I've ordered a couple of these modules: http://pldkit.com/xilinx/xm2c2 Just under £30 the pair including postage from Estonia.
I have some panels with sets of "Schadow" push switches which are just right for the job. The top row of 10 switches is for preview, the 3 sets of 6 are for BBC1, ITV, BBC2. i have an assortment of orange and black presses which are easily interchanged. Some have a slot for a label, some don't. I'll find a sensible looking set of presses.
I think that the CPLD module will happily do a multiplexed LED drive without any external buffers. So I'll fit a ribbon cable from one side to drive LEDs and buttons. Plus a ribbon from the other side to drive the router. Power at 5V will come from the router.
One slight unknown is whether the 3.3V outputs from the CPLD will drive the MC14504 level shifters on the routers. These need 5V CMOS levels which are only just about reached by 3.3V signals. The MC14504s can be programmed to work with TTL levels, simply by changing the voltage on pin 13. 3.3V CMOS will easily drive 5V TTL. This is likely the simplest solution. Alternatively I could lower the Vcc on the MC14504s to 3.3V. A bit more complex and not covered in the data sheet though it would surely work.
Another minor unknown is what sort of series resistors I will need to set the LED current. I'll be multiplexing 4 LEDs so need 4x normal LED currrent. The LEDs would typically be run at about 5mA so 20mA drive needed in a x4 mux. The CPLD outputs will deliver this easily, the LED voltage drop is likely to approach 2V at this sort of current so (3.3-2)/0.02 = 60R. So I'll start with 68R or 56R and see what happens.
The logic design will be some pretty simple VHDL. I have the Xilinx tools and have designed for this class of device before so I don't anticipate any problems. Correcting any design errors is a lot easier in VHDL than hardwired CMOS/TTL.
You can see mods I've made to the top switch panel. The pairs of BNCs and phonos are for 2 aux AV inputs. The toggle switch is to select off-air or testcards in standby mode. There will be a coaxial low voltage power socket on the back to take power from a wallwart. This will signal whether the system is running or in standby.
I've ordered a couple of these modules: http://pldkit.com/xilinx/xm2c2 Just under £30 the pair including postage from Estonia.
I have some panels with sets of "Schadow" push switches which are just right for the job. The top row of 10 switches is for preview, the 3 sets of 6 are for BBC1, ITV, BBC2. i have an assortment of orange and black presses which are easily interchanged. Some have a slot for a label, some don't. I'll find a sensible looking set of presses.
I think that the CPLD module will happily do a multiplexed LED drive without any external buffers. So I'll fit a ribbon cable from one side to drive LEDs and buttons. Plus a ribbon from the other side to drive the router. Power at 5V will come from the router.
One slight unknown is whether the 3.3V outputs from the CPLD will drive the MC14504 level shifters on the routers. These need 5V CMOS levels which are only just about reached by 3.3V signals. The MC14504s can be programmed to work with TTL levels, simply by changing the voltage on pin 13. 3.3V CMOS will easily drive 5V TTL. This is likely the simplest solution. Alternatively I could lower the Vcc on the MC14504s to 3.3V. A bit more complex and not covered in the data sheet though it would surely work.
Another minor unknown is what sort of series resistors I will need to set the LED current. I'll be multiplexing 4 LEDs so need 4x normal LED currrent. The LEDs would typically be run at about 5mA so 20mA drive needed in a x4 mux. The CPLD outputs will deliver this easily, the LED voltage drop is likely to approach 2V at this sort of current so (3.3-2)/0.02 = 60R. So I'll start with 68R or 56R and see what happens.
The logic design will be some pretty simple VHDL. I have the Xilinx tools and have designed for this class of device before so I don't anticipate any problems. Correcting any design errors is a lot easier in VHDL than hardwired CMOS/TTL.
You can see mods I've made to the top switch panel. The pairs of BNCs and phonos are for 2 aux AV inputs. The toggle switch is to select off-air or testcards in standby mode. There will be a coaxial low voltage power socket on the back to take power from a wallwart. This will signal whether the system is running or in standby.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
