Provided are a neuromorphic computing device, memory device, system, and method to maintain a spike history for neurons in a spiking neural network. A neural network spike history is generated in a memory device having an array of rows and columns of memory cells. There is one row of the rows for each of a plurality of neurons and columns for each of a plurality of time slots. Indication is made in a current column in the row of the memory cells for a firing neuron that a spike was fired. Indication is made in the current column in rows of memory cells of idle neurons that did not fire that a spike was not fired. Information in the array is used to determine a timing difference between a connected neuron and the firing neuron and to adjust a weight of the connecting synapse.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A neuromorphic computing device implementing a spiking neural network program that processes neural network information in a memory device to: generate a spike history in the memory device having an array of rows and columns of memory cells, wherein there is one row of the rows for each of a plurality of neurons of the spiking neural network, and columns for each of a plurality of time slots; in response to each of at least one firing neuron of the neurons firing a spike over a synapse at a current timeslot represented by a current column of the columns: indicate in the current column in the row of the memory cells for the firing neuron that the spike was fired in the current timeslot; indicate in the current column in rows of memory cells of idle neurons comprising neurons that did not fire that a spike was not fired; use information in the array to determine a timing difference between a connected neuron and the firing neuron to which the connected neuron connects over a synapse; and use the determined timing difference to adjust a weight of the synapse connecting the firing neuron and the connected neurons. 2. The neuromorphic computing device of claim 1, further to: generate a now pointer including an entry for each of the columns in the array, wherein entries in the now pointer correspond to time slots and include a current entry corresponding to the current column in the array and non-current entries corresponding to non-current timeslots, wherein the current entry is set to a first value and the non-current entries are set to a second value; and use the now pointer to indicate in at least one row of the at least one firing neuron the first value in the current column and the second value in the columns other than the current column, wherein the first value in one of the columns in the rows indicates firing of a spike at the time slot represented by the column in the row. 3. The neuromorphic computing device of claim 2, wherein the use the now pointer comprises: write the first value and the second values in the entries of the now pointer to corresponding columns in the at least one row of the at least one firing neuron, wherein each of the rows includes one column entry having the first value and all other of the column entries having the second value. 4. The neuromorphic computing device of claim 2, wherein the memory device includes, for each of the rows of the array, a word line through cells in the row of the memory device and, for each of the columns of the array, at least one bit line through the cells in the column of the array, further to: set all cells in the current column to ground to set to the second value using the bit line for the current column; and activate the word lines for each row of the at least one firing neuron to write entries of the now pointer to the cells of the row. 5. The neuromorphic computing device of claim 2, wherein the now pointer comprises a circular shift register, further to: in response to a new time cycle, simultaneously shift a value in a first entry through a next to last entry in the now pointer to a right and shifting a value in a last entry to the first entry. 6. The neuromorphic computing device of claim 2, wherein the determine the timing difference comprises: determine a difference of a first column location number in the array having the first value for the firing neuron and a second column location number in the array having the first value for the connected neuron; and determine a timing difference between the firing neuron and the connected neuron based on the first and second column location numbers. 7. The neuromorphic computing device of claim 2, wherein the now pointer comprises a circular shift register, wherein the determine the timing difference comprises: (a number of entries in the now pointer plus a first column location number in the array having the first value for the firing neuron minus a second column location number in the array having the first value for the connected neuron) modulo the number of entries in the now pointer. 8. The neuromorphic computing device of claim 2, further to: set reset bit lines for a reset port for the current column of the array to set all cells in the current column of the array corresponding to the current entry in the now pointer to the second value; and after setting all the entries in the current column of the array to the second value, apply the now pointer along at least one word line for the at least one firing neuron to write the first value and the second values in the entries of the now pointer to corresponding columns in the rows of the at least one firing neuron, wherein the row for each of the at least one firing neuron includes the current column having the first value and all other of the columns having the second value. 9. The neuromorphic computing device of claim 2, wherein the adjust the weight of the synapse for each of at least one pair of the firing neuron and the connected neuron comprises: calculate a weight for the synapse based on the determined timing difference in response to one of the columns in the row for the connected neuron indicating that the connected neuron did fire, wherein a weight for the synapse is unchanged in response to all the columns in the row for the connected neuron indicating that the neuron did not fire. 10. The neuromorphic computing device of claim 1, wherein in response to each of the at least one firing neuron firing the spike: determine at least one connected postsynaptic neuron connected to the firing neuron over at least one synapse; and for each of the at least one connected postsynaptic neuron, do: determine a potential based on the weight of the synapse between the connected postsynaptic neuron and the firing neuron; increment a membrane potential for the connected postsynaptic neuron by the determined potential; and fire a spike by the connected postsynaptic neuron in response to the membrane potential exceeding a threshold. 11. The neuromorphic computing device of claim 1, wherein the connected neurons connected to each of the firing neurons includes fan-in presynaptic neurons that fire spikes to the firing neuron over the synapses and fan-out postsynaptic neurons that receive the spikes fired by the firing neurons. 12. The neuromorphic computing device of claim 1, comprising at least one of a computer readable storage medium including program code implementing the spiking neural network program executed by a processor and an integrated circuit hardware device implementing hardware logic dedicated to implementing the spiking neural network program. 13. A memory device, comprising: an array of rows and columns of memory cells, wherein there is one row of the rows for each of a plurality of neurons of the spiking neural network, and columns for each of a plurality of time slots, wherein a current column of the columns represents a current timeslot, wherein the current column in the row of the memory cells for the firing neuron indicates that the spike was fired in the current timeslot, wherein the current column in rows of memory cells of idle neurons comprising neurons that did not file indicate that a spike was not fired, wherein the information in the array is used to determine a timing difference between a connected neuron and the firing neuron to which the connected neuron connects over a synapse, and wherein the determined timing difference is used to adjust a weight of the synapse connecting the firing neuron and the connected neurons. 14. The memory device of claim 13, further including: a now pointer including an entry for each of the columns in the array, wherein entries in the now pointer correspond to time slots and include a current entry corresponding to the current column in the array and non-current entries corresponding to non-current timeslots, wherein the current entry is set to a first value and the non-current entries are set to a second value, wherein the now pointer is used to indicate in at least one row of the at least one firing neuron the first value in the current column and the second value in the columns other than the current column, wherein the first value in one of the columns in the rows indicates firing of a spike at the time slot represented by the column in the row. 15. The memory device of claim 14, further comprising: for each of the rows of the array, a word line through cells in the row of the memory device and, for each of the columns of the array, at least one bit line through the cells in the column of the array, wherein all cells in the current column are set to ground to set to the second value using the bit line for the current column, and wherein the word lines for each row of the at least one firing neuron are activated to write entries of the now pointer to the cells of the row. 16. The memory device of claim 14, further comprising: reset bit lines for a reset port for the current column of the array that is set to set all cells in the current column of the array corresponding to the current entry in the now pointer to the second value; and a word line for each of the neurons, wherein after setting all the entries in the current column of the array to the second value, the now pointer is applied along at least one word line for the at least one firing neuron to write the first value and the second values in the entries of the now pointer to corresponding columns in the rows of the at least one firing neuron, wherein the row for each of the at least one firing neuron includes the current column having the first value and all other of the columns having the second value. 17. A system, comprising: a memory device; and a neuromorphic computing device implementing a spiking neural network program to: generate a spike history in the memory device having an array of rows and columns of memory cells, wherein there is one row of the rows for each of a plurality of neurons of the spiking neural network, and columns for each of a plurality of time slots; in response to each of at least one firing neuron of the neurons firing a spike over a synapse at a current timeslot represented by a current column of the columns: indicate in the current column in the row of the memory cells for the firing neuron that the spike was fired in the current timeslot; indicate in the current column in rows of memory cells of idle neurons comprising neurons that did not file that a spike was not fired; use information in the array to determine a timing difference between a connected neuron and the firing neuron to which the connected neuron connects over a synapse; and use the determined timing difference to adjust a weight of the synapse connecting the firing neuron and the connected neurons. 18. The system of claim 17, wherein the spiking neural network program is further to: generate a now pointer including an entry for each of the columns in the array, wherein entries in the now pointer correspond to time slots and include a current entry corresponding to the current column in the array and non-current entries corresponding to non-current timeslots, wherein the current entry is set to a first value and the non-current entries are set to a second value; and use the now pointer to indicate in at least one row of the at least one firing neuron the first value in the current column and the second value in the columns other than the current column, wherein the first value in one of the columns in the rows indicates firing of a spike at the time slot represented by the column in the row. 19. The system of claim 18, wherein the use the now pointer comprises: write the first value and the second values in the entries of the now pointer to corresponding columns in the at least one row of the at least one firing neuron, wherein each of the rows includes one column entry having the first value and all other of the column entries having the second value. 20. The system of claim 18, wherein the memory device includes, for each of the rows of the array, a word line through cells in the row of the memory device and, for each of the columns of the array, at least one bit line through the cells in the column of the array, further to: set all cells in the current column to ground to set to the second value using the bit line for the current column; and activate the word lines for each row of the at least one firing neuron to write entries of the now pointer to the cells of the row. 21. A method for implementing a spiking neural network program in a neuromorphic computing device, comprising: generating a spike history in a memory device having an array of rows and columns of memory cells, wherein there is one row of the rows for each of a plurality of neurons of the spiking neural network, and columns for each of a plurality of time slots; in response to each of at least one firing neuron of the neurons firing a spike over a synapse at a current timeslot represented by a current column of the columns: indicating in the current column in the row of the memory cells for the firing neuron that the spike was fired in the current timeslot; indicating in the current column in rows of memory cells of idle neurons comprising neurons that did not file that a spike was not fired; using information in the array to determine a timing difference between a connected neuron and the firing neuron to which the connected neuron connects over a synapse; and using the determined timing difference to adjust a weight of the synapse connecting the firing neuron and the connected neurons. 22. The method of claim 21, further comprising: generating a now pointer including an entry for each of the columns in the array, wherein entries in the now pointer correspond to time slots and include a current entry corresponding to the current column in the array and non-current entries corresponding to non-current timeslots, wherein the current entry is set to a first value and the non-current entries are set to a second value; and using the now pointer to indicate in at least one row of the at least one firing neuron the first value in the current column and the second value in the columns other than the current column, wherein the first value in one of the columns in the rows indicates firing of a spike at the time slot represented by the column in the row. 23. The method of claim 22, wherein the using the now pointer comprises: writing the first value and the second values in the entries of the now pointer to corresponding columns in the at least one row of the at least one firing neuron, wherein each of the rows includes one column entry having the first value and all other of the column entries having the second value. 24. The method of claim 22, wherein the memory device includes, for each of the rows of the array, a word line through cells in the row of the memory device and, for each of the columns of the array, at least one bit line through the cells in the column of the array, further comprising: setting all cells in the current column to ground to set to the second value using the bit line for the current column; and activating the word lines for each row of the at least one firing neuron to write entries of the now pointer to the cells of the row. 25. The method of claim 22, wherein the determining the timing difference comprises: determining a difference of a first column location number in the array having the first value for the firing neuron and a second column location number in the array having the first value for the connected neuron; and determining a timing difference between the firing neuron and the connected neuron based on the first and second column location numbers.