SP8790 Modulus Extender


SP8790 ÷4 Extender for 2-Modulus Counters Plessey


The SP8790 is a divide-by-four counter designed for use with 2-modulus counters. It increases the minimum division ratio of the 2-modulus counter while retaining the same difference in division ratios. Thus a divide-by-10 or 11 with the SP8790 becomes a divide-by-40 or 41; a divide by 5 or 6 becomes a divide by 20 or 21.
The function is especially useful in low power frequency synthesisers because it can bring the output frequency of the combined 2-modulus counter and SP8790 into the region where CMOS or low power TTL can control the divider. The power-saving advantages are obvious. 
The device interfaces easily to the SP8690 range of divide by 10 or 11. The control inputs are TTL and CMOS compatible, and the output is a free collector which, with the addition of a pull-up resistor, interfaces to CMOS and TTL. 
The SP8790 is available in three temperature grades: 0°C to +70°C (SP8790B) -55°C to +125°C (SP8790A)
The SP8790 requires supplies of 0V and +5V ±0.25V. 


  • Ultra-Low Power: 40mW 
  • Full Military Temperature Range 
  • I/P and O/P Interface Direct to CMOS/TTL 

Quick Reference Data

  • Supply Voltage: +5V ±5%
  • Max Frequency: 60MHz
  • Operating Temperature Range: -55°C to +125°C

Operating Notes

The SP8790 extends the division ratio of 2-modulus counters while retaining the same 2-modulus resolution. 
A typical application to give a ÷40/41 function is shown in Fig. 1. In this basic form, however, the devices will self-oscillate if no input signal source is present. This may be prevented by using one of the arrangements shown in Fig. 2.
The maximum frequency of operation is over 60MHz when the SP8790 is used as a prescaler. The limitation on this maximum frequency is the saturating O/P stage. When the SP8790 is used as a controller, its internal delays do not permit operation at frequencies above 40MHz. 
The device usually is driven from a 2-modulus divider which has fast output edges. Hence there is normally no input slew rate problem. 
TTL devices require a pull-up resistor to ensure the necessary minimum of 3.5V. Note that the device can interface from 10V CMOS with no additional components.
SP8790 Typical Application Plessey

Fig. 1

SP8790 Method to avoid Oscillations - Plessey

Fig. 2