I need to generate a bunch of chip selects for a SPI bus that I want to use on a project. I will use a 36 position (similar to PCIExpress = cheap) card edge connector and etc.
The bus is simple :
Pin 01 -> Clock Pin 02 -> MOSI Pin 03 -> MISO Pin 04 -> CS#1 Pin 05 -> CS#2 Pin 06 -> CS#3 Pin 07 -> CS#4 Pin 08 -> CS#5 Pin 09 -> CS#6 Pin 10 -> CS#7 Pin 11 -> CS#8 Pin 12 -> CS#9 Pin 13 -> CS#10 Pin 14 -> CS#11 Pin 15 -> CS#12 Pin 16 -> CS#13 Pin 17 -> CS#14 Pin 18 -> CS#15
Other pins on the card edge connector are assorted GND and VCC pins.
I need to generate a chip select from 4 GPIO pins on the microcontroller, in order to select a SPI slave from various devices etc.
A long time ago this would be made using a simple 74LS154 device (1-of-16 decoder). The chosen microcontroller is the only 3.3 V device (EZ80), and I find it hard to find a 3.3 V equivalent to 74LS154.
Please tell me where can I find a 3.3 V 74LS154 device.
looking the datasheet from the ez80, its SPI can only do systemclock/2 speed. Its reasonable to think that the max speed at which the chip select pins change is equal to the character rate, that means (50 MHz / 2) / 8 or 3.125MHz. So there is no need for all that speed (50 MHz).