Chip select (‘CS’) multiplication in a serial peripheral interface (‘SPI’) system

1. A method of chip select (‘CS’) multiplication in a Serial Peripheral Interface (‘SPI’) system, the SPI system comprising an SPI master, a CS multiplier, a plurality of SPI slaves, and a fall time detection circuit, the SPI master coupled to the CS multiplier and the fall time detection circuit by a CS signal line, the CS multiplier comprising a plurality of CS outputs, each CS output coupled to an SPI slave, the CS multiplier configured to provide a CS signal on only one CS output at a time, the method comprising:

monitoring, by the fall time detection circuit, a voltage of a CS signal on the CS signal line, the voltage alternating between a logic high voltage and a logic low voltage, including:

receiving, from the SPI master, the CS signal on the CS signal line; and

detecting, by the fall time detection circuit, fall time of the CS signal; and

if the fall time of the CS signal is less than a predefined threshold, configuring, by the fall-time detection circuit, the CS multiplier to vary from providing a CS signal on a first CS output to providing a CS signal on a second CS output.

2. The method of claim 1 wherein the CS multiplier comprises the fall time detection circuit.

3. The method of claim 1 wherein detecting, by the fall time detection circuit, fall time of the CS signal further comprises:

detecting a first voltage of the CS signal, the first voltage being less than the logic high voltage;

starting, responsive to the detection of the first voltage, an automatically incrementing counter, the counter incrementing once for each clock period of the fall time detection circuit;

detecting a second voltage on the signal line, the second voltage being less than the first voltage and greater than the logic low voltage;

stopping, responsive to the detection of the second voltage, the automatically incrementing counter; and

calculating, in dependence upon the clock period of the fall time detection circuit and the value of the counter, a fall time for the signal line.

4. The method of claim 1 wherein:

the fall time of the signal is less than a predefined threshold responsive to the SPI master driving the signal to a logic low with an active driver; and

the fall time of the signal is not less than a predefined threshold responsive to the SPI master driving the signal to a logic low passively.

5. A fall time detection circuit for chip select (‘CS’) multiplication in a Serial Peripheral Interface (‘SPI’) system, the SPI system comprising an SPI master, a CS multiplier, and a plurality of SPI slaves, the SPI master coupled to the CS multiplier and the fall time detection circuit by a CS signal line, the CS multiplier comprising a plurality of CS outputs, each CS output coupled to an SPI slave, the CS multiplier configured to provide a CS signal on only one CS output at a time, the fall time detection circuit configured to carry out the steps of:

monitoring, by the fall time detection circuit, a voltage of a CS signal on the CS signal line, the voltage alternating between a logic high voltage and a logic low voltage, including:

receiving, from the SPI master, the CS signal on the CS signal line; and

detecting, by the fall time detection circuit, fall time of the CS signal; and

if the fall time of the CS signal is less than a predefined threshold, configuring, by the fall-time detection circuit, the CS multiplier to vary from providing a CS signal on a first CS output to providing a CS signal on a second CS output.

6. The fall time detection circuit of claim 5 wherein the CS multiplier comprises the fall time detection circuit.

7. The fall time detection circuit of claim 5 wherein detecting, by the fall time detection circuit, fall time of the CS signal further comprises:

detecting a first voltage of the CS signal, the first voltage being less than the logic high voltage;

starting, responsive to the detection of the first voltage, an automatically incrementing counter, the counter incrementing once for each clock period of the fall time detection circuit;

detecting a second voltage on the signal line, the second voltage being less than the first voltage and greater than the logic low voltage;

stopping, responsive to the detection of the second voltage, the automatically incrementing counter; and

calculating, in dependence upon the clock period of the fall time detection circuit and the value of the counter, a fall time for the signal line.

8. The fall time detection circuit of claim 5 wherein:

the fall time of the signal is less than a predefined threshold responsive to the SPI master driving the signal to a logic low with an active driver; and

the fall time of the signal is not less than a predefined threshold responsive to the SPI master driving the signal to a logic low passively.

9. A method of chip select (‘CS’) multiplication in a Serial Peripheral Interface (‘SPI’) system, the SPI system comprising an SPI master, a CS multiplier, a plurality of SPI slaves, and a fall time detection circuit, the SPI master coupled to the CS multiplier and the fall time detection circuit by a CS signal line, the CS multiplier comprising a plurality of CS outputs, each CS output coupled to an SPI slave, the CS multiplier configured to provide a CS signal on only one CS output at a time, the method comprising:

determining, by the SPI master, whether to configure the CS multiplier to vary from providing a CS signal on a first CS output to providing a CS signal on a second CS output;

if the SPI master determines to configure the CS multiplier to vary from providing a CS signal on a first CS output to providing a CS signal on a second CS output: transmitting, by the SPI master to the fall time detection circuit and the CS multiplier, a CS signal having a fall time less than a predefined threshold; and

if the SPI master determines not to configure the CS multiplier to vary from providing a CS signal on a first CS output to providing a CS signal on a second CS output: transmitting, by the SPI master to the fall time detection circuit and the CS multiplier, a CS signal having a fall time not less than the predefined threshold,

wherein the fall time detection circuit is configured to: receive the CS signal, detect fall time of the CS signal, and configure the CS multiplier to vary from providing a CS signal on a first CS output to providing a CS signal on a second CS output if the fall time of the CS signal is less than the predefined threshold.

10. The method of claim 9 wherein the CS multiplier comprises the fall time detection circuit.

11. The method of claim 9 wherein:

transmitting a CS signal having a fall time less than a predefined threshold further comprises driving the CS signal to a logic low with an active driver; and

transmitting a CS signal having a fall time not less than a predefined threshold further comprises driving the CS signal to a logic low passively.

12. An apparatus for chip select (‘CS’) multiplication in a Serial Peripheral Interface (‘SPI’) system, the SPI system comprising an SPI master, a CS multiplier, a plurality of SPI slaves, and a fall time detection circuit, the SPI master coupled to the CS multiplier and the fall time detection circuit by a CS signal line, the CS multiplier comprising a plurality of CS outputs, each CS output coupled to an SPI slave, the CS multiplier configured to provide a CS signal on only one CS output at a time, the apparatus comprising a computer processor, a computer memory operatively coupled to the computer processor, the computer memory having disposed within it computer program instructions that, when executed by the computer processor, cause the apparatus to carry out the steps of:

determining, by the SPI master, whether to configure the CS multiplier to vary from providing a CS signal on a first CS output to providing a CS signal on a second CS output;

if the SPI master determines to configure the CS multiplier to vary from providing a CS signal on a first CS output to providing a CS signal on a second CS output: transmitting, by the SPI master to the fall time detection circuit and the CS multiplier, a CS signal having a fall time less than a predefined threshold; and

if the SPI master determines not to configure the CS multiplier to vary from providing a CS signal on a first CS output to providing a CS signal on a second CS output: transmitting, by the SPI master to the fall time detection circuit and the CS multiplier, a CS signal having a fall time not less than the predefined threshold,

wherein the fall time detection circuit is configured to: receive the CS signal, detect fall time of the CS signal, and configure the CS multiplier to vary from providing a CS signal on a first CS output to providing a CS signal on a second CS output if the fall time of the CS signal is less than the predefined threshold.

13. The apparatus of claim 12 wherein the CS multiplier comprises the fall time detection circuit.

14. The apparatus of claim 12 wherein:

transmitting a CS signal having a fall time less than a predefined threshold further comprises driving the CS signal to a logic low with an active driver; and

transmitting a CS signal having a fall time not less than a predefined threshold further comprises driving the CS signal to a logic low passively.

15. A computer program product for chip select (‘CS’) multiplication in a Serial Peripheral Interface (‘SPI’) system, the SPI system comprising an SPI master, a CS multiplier, a plurality of SPI slaves, and a fall time detection circuit, the SPI master coupled to the CS multiplier and the fall time detection circuit by a CS signal line, the CS multiplier comprising a plurality of CS outputs, each CS output coupled to an SPI slave, the CS multiplier configured to provide a CS signal on only one CS output at a time, the computer program product disposed upon a computer readable medium, the computer program product comprising computer program instructions that, when executed, cause a computer to carry out the steps of:

determining, by the SPI master, whether to configure the CS multiplier to vary from providing a CS signal on a first CS output to providing a CS signal on a second CS output;

if the SPI master determines to configure the CS multiplier to vary from providing a CS signal on a first CS output to providing a CS signal on a second CS output: transmitting, by the SPI master to the fall time detection circuit and the CS multiplier, a CS signal having a fall time less than a predefined threshold; and

if the SPI master determines not to configure the CS multiplier to vary from providing a CS signal on a first CS output to providing a CS signal on a second CS output: transmitting, by the SPI master to the fall time detection circuit and the CS multiplier, a CS signal having a fall time not less than the predefined threshold,

wherein the fall time detection circuit is configured to: receive the CS signal, detect fall time of the CS signal, and configure the CS multiplier to vary from providing a CS signal on a first CS output to providing a CS signal on a second CS output if the fall time of the CS signal is less than the predefined threshold.

16. The computer program product of claim 15 wherein the CS multiplier comprises the fall time detection circuit.

17. The computer program product of claim 15 wherein:

transmitting a CS signal having a fall time less than a predefined threshold further comprises driving the CS signal to a logic low with an active driver; and

transmitting a CS signal having a fall time not less than a predefined threshold further comprises driving the CS signal to a logic low passively.

18. The computer program product of claim 15 wherein the computer readable medium comprises a signal medium.

19. The computer program product of claim 15 wherein the computer readable medium comprises a storage medium.