Re: Choice between CMOS 4099 or 74HC259
- From: "antedeluvian" <akagan@xxxxxxxxxxxx>
- Date: Sat, 29 Oct 2005 17:49:07 -0500
MM
For the PIC, do you need an A/D or not?
AS far as the 4099s are concerned, they cannot drive much current. Make
sure you use low current LEDs.
-Aubrey
>antedeluvian wrote:
>> MM,
>>
>> The way I understand your question is that you want to replace a
74HC259
>> with a 4099. If this is the case, then you should know that aside from
>> anything else, they are not pin compatible. Secondly the Clear
function
>> appears to be inverse between the devices.
>>
>> Digikey seems to have plenty 4099s, so you don't need a circuit
redesign.
>>
>> Generally you need to consider several things when replacing one
technolgy
>> with another. Firstly the setup times and write times are normally
much
>> longer in 4000 series CMOS. The 74HC series is quicker so you probably
>> could make the substitution (if pin equivalent), but you do need to
make
>> sure the timing is OK.
>>
>> Driving 4000 series from any TTL output normally requires a pull up
>> resistor to ensure logic high level compatibilty (provided they are
both
>> run from 5V) unless the 4000 series device has Schmitt trigger inputs
>> (which the 4099 does not). Drive levels of the 4000 series into TTL
can
>> prove problematic. You need to investigate the output drive capabilty
of
>> the CMOS part.
>>
>> The 4000 CMOS series can operate with a supply voltage from 3V to 15V.
>> This can be advantageous in some circumstances, but obviously
interfacing
>> with 5V systems would need extra attention.
>>
>> -Aubrey
>
>Thanks for that, Aubrey. Okay, I now have found Cricklewood Electronics
>that also do the 4099. Like I said, I'm a novice so when some of the
>main catalogues didn't stock it I assumed it had become outdated and
>little sought after.
>
>By the way, the 4099s will be driving LEDs, one per pin. This is in
>conjunction with MIDI.
>
>Since I'm here, what's the next best choice to the 16F628(A)? I was
>intending to use it in preference to a 16F84 because the former comes
>with an inbuilt USART. However, that utilises RB1 and RB2 as RX/TX
>respectively, which thus lose me those pins as outputs. Pin 8 (RB2/TX)
>apparently cannot be configured as a bog-standard output when the USART
>is invoked, even if TX is not required. And all the other pins that
>could be outputs are already used up.
>
>Therefore I am now considering the 16F877 instead, which has many more
>pins to play with. However, this PIC is twice the price and has a lot
>of additional functionality/capacity that I don't need. Is there
>something in between? For example, do any of the 28-pin PICs have a
>USART?
>
>MM
>
>
.
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