The pulse widths produced using CD54HC123 (3.3V) are about 20% shorter than calculated.
The problem was originally noticed with 143K and 2.2uF. We performed several experiments including different boards, different RC values, etc. The problem flows the CD54HC123. Whenever the CD54HC123 was used, the times were typically 20% under the calculated values. When the SN74AHC123 was used, the times matched the re-calculated* values. (* They have different K-factors)
The attached spreadsheet shows an experiment where the R&C were measured and the same circuit & RC parts tested with both timer IC’s. Again the CD54HC123 always comes-up with times that are shorter than expected. It seems the K-factor for 3.3V operation should be 0.40, but that doesn’t agree with the datasheet.
| Timing Tes Results for 100k*0.1uF time constant : | ||||||
| K factor is taken as 0.47 for CD74HC123 & 0.97 for SN74AHC123 Vcc=3.3V | ||||||
| Device SN74AHC123 | Device CD74HC123 | |||||
| Set1 Reading | Theoretical Value | Measured valueValue | Set1 Reading | Theoretical Value | Measured valueValue | |
| R791 | 1.00E+05 | 9.96E+04 | R791 | 1.00E+05 | 9.96E+04 | |
| C520 | 1.00E-07 | 9.66E-08 | C520 | 1.00E-07 | 9.66E-08 | |
| Reset pulse with SN74AHC123 | 9.70E-03 | 9.33E-03 | Reset pulse with CD74HC123 | 4.70E-03 | 4.52E-03 | |
| Practical Observed Value (CRO) | 9.78E-03 | Practical Observed Value (CRO) | 3.93E-03 | |||
| Variation | 5% | Variation with measured | -15% | |||
| Variation with theoretical | 1% | Variation with theoretical | -20% | |||
| Set2 Reading | Theoretical Value | Measured valueValue | Set2 Reading | Theoretical Value | Measured valueValue | |
| R791 | 1.00E+05 | 9.96E+04 | R791 | 1.00E+05 | 9.96E+04 | |
| C520 | 1.00E-07 | 9.06E-08 | C520 | 1.00E-07 | 9.06E-08 | |
| Reset pulse with SN74AHC123 | 9.70E-03 | 8.75E-03 | Reset pulse with CD74HC123 | 4.70E-03 | 4.24E-03 | |
| Practical Observed Value (CRO) | 9.26E-03 | Practical Observed Value (CRO) | 3.88E-03 | |||
| Variation | 5% | Variation | -9% | |||
| Variation with theoretical | -5% | Variation with theoretical | -21% | |||
