My preliminary design for the digital front panel board has a PCA9535 I2C to 16 bit board. But it is more expensive at $1.65 qty one from Mouser. I could change to two PCF8574 if that would be more desirable, although they are $1.11 qty 1 from Mouser, so 2 are more expensive than the 16 bit chip. I like I2C because the firmware already has I2C routines for the Si570 and it offers almost unlimited expansion capabilities. I use the PCF8574 on my MPLPF board. The chips have an interrupt out pin that indicates a change on any of the I/O pins. The 16 bit chip allows a 4 X 4 keypad on one 8 bit port with the other 8 bits available for other functions, all under I2C control with interrupt capability. My main board layout contains the headphone jack, the microphone jack, the volume control and the key jack on the front edge. No interconnecting cables between the main board and the digital front panel will be required(that is handled by a right angle .1" X .1" connector). It has taken awhile going thru datasheets to make sure everything will line up. Better to make changes now on the computer than later on pc boards! I am also working on a BPF board that will contain up to 14 filters to cover everything from 160 meters to 70 cm. Instead of I2C expanders, I am using a small PIC(it is cheaper - less than $1). The board can be controlled either by I2C or 4 bit binary with 14 relay outputs.
Dave - WB6DHW <http://wb6dhw.com> On 2/11/2014 1:19 AM, Thomas Sarlandie wrote:
Dave, I was not sure but it looks like the pcf8574 would work in 3.3v: http://www.nxp.com/documents/data_sheet/PCF8574.pdfI vote for this option. Using the i2c bus is very elegant in my opinion. The chip is a little more expensive though (30 to 80 cents on ebay from chinese vendor and in qty of 10).thomasOn Sun, Feb 9, 2014 at 2:58 PM, Dave <dave@xxxxxxxxxx <mailto:dave@xxxxxxxxxx>> wrote:Steve: Of course if you use a I2C expander such as the PCF8574, you use 0 additional I/O lines. It connects to the I2C in parallel with the Si570. Dave - WB6DHW <http://wb6dhw.com> On 2/9/2014 2:43 PM, Steve VK2SJA wrote: Steve, this uses just two lines. Alright. I'll admit it. This muddle headed wombat is still very confused. 1 x Pin to pulse counter in 4017 decade counter to select band pass filter +1 x Pin to reset 4017 decade counter to default known start state = 2 pins 2 pins + 1 pin for controlling the low pass filter selection = 3 pins. vs 3 pins Binary Coded Decimal (BCD) to control *both* low pass filter selection and the output band pass filter selection. Two different ways to arrive at the same level of control. But is it not still 3 pins = 3 pins? So where are we saving an IO line? I'm probably being completely daft and missing the flaming obvious but I just can't see it. Help! 73, Steve.
