Chris Z
Newbie level 2
I have a series of latches attached to a 4.2 volt power rail. These need to be driven by a standard 3.3 microcontroller, with specified Voh of Vcc - 0.4v @ 1mA. So approximately 2.9v at low currents. There are scores of traces that will be affected by my decision. If I decide to use HC series logic for the latches, I will need a minimum Vih of 0.7 * 4.2v, or 2.94v, which might just barely work. Of course, placing external pullups on the lines will bring them all into spec, but this is an awful lot of pullups, increasing cost and board space.
Alternatively, I have considered using HCT logic for the latches. This would be a cheaper solution using less board space as well as being easier to route. The problem is that HCT is only specified to work down to 4.5v.
According to this appnote by TI, the reason for this is due to noise margin on Vil. (https://www.ti.com/lit/an/scla011/scla011.pdf) However, the analysis in that paper is all done assuming 5v signalling on the CMOS side. My thoughts are that since I am using 3.3v CMOS, the induced noise will be 2/3rds of that present at 5v, thus I should be able to tolerate a reduced noise threshold. Using HCT at sub specified voltages will allow me to run and meet all voltage tolerances without expensive pullup resistors.
Has anyone else ever tried this, and what was your result? Can I count on this being reliable in the field? This design is expected to ship several hundred thousand units a year, so while I am eager to find a solution that reduces the cost, I am not eager to do it if it means lower reliability and more returns.
Thank you for any advice.
Chris
Alternatively, I have considered using HCT logic for the latches. This would be a cheaper solution using less board space as well as being easier to route. The problem is that HCT is only specified to work down to 4.5v.
According to this appnote by TI, the reason for this is due to noise margin on Vil. (https://www.ti.com/lit/an/scla011/scla011.pdf) However, the analysis in that paper is all done assuming 5v signalling on the CMOS side. My thoughts are that since I am using 3.3v CMOS, the induced noise will be 2/3rds of that present at 5v, thus I should be able to tolerate a reduced noise threshold. Using HCT at sub specified voltages will allow me to run and meet all voltage tolerances without expensive pullup resistors.
Has anyone else ever tried this, and what was your result? Can I count on this being reliable in the field? This design is expected to ship several hundred thousand units a year, so while I am eager to find a solution that reduces the cost, I am not eager to do it if it means lower reliability and more returns.
Thank you for any advice.
Chris